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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd"> <html xmlns="http://www.w3.org/1999/xhtml"> <head> <meta http-equiv="X-UA-Compatible" content="IE=Edge" /> <meta http-equiv="Content-Type" content="text/html; charset=utf-8" /> <title>3. An Informal Introduction to Python — Python 2.7.18 documentation</title> <link rel="stylesheet" href="../_static/classic.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <script type="text/javascript" id="documentation_options" data-url_root="../" src="../_static/documentation_options.js"></script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/language_data.js"></script> <script type="text/javascript" src="../_static/sidebar.js"></script> <link rel="search" type="application/opensearchdescription+xml" title="Search within Python 2.7.18 documentation" href="../_static/opensearch.xml"/> <link rel="author" title="About these documents" href="../about.html" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="copyright" title="Copyright" href="../copyright.html" /> <link rel="next" title="4. More Control Flow Tools" href="controlflow.html" /> <link rel="prev" title="2. Using the Python Interpreter" href="interpreter.html" /> <link rel="shortcut icon" type="image/png" href="../_static/py.png" /> <link rel="canonical" href="file:///usr/share/doc/python2.7/html/tutorial/introduction.html" /> <script type="text/javascript" src="../_static/copybutton.js"></script> </head><body> <div id="outdated-warning" style="padding: .5em; text-align: center; background-color: #FFBABA; color: #6A0E0E;"> This document is for an old version of Python that is <a href="https://devguide.python.org/devcycle/#end-of-life-branches">no longer supported</a>. You should install the python3 and python3-doc packages and read the <a href="file:///usr/share/doc/python3-doc/html/tutorial/introduction.html"> Python documentation for the Python3 version packaged in this release</a>. </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" accesskey="I">index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="controlflow.html" title="4. More Control Flow Tools" accesskey="N">next</a> |</li> <li class="right" > <a href="interpreter.html" title="2. Using the Python Interpreter" accesskey="P">previous</a> |</li> <li><img src="../_static/py.png" alt="" style="vertical-align: middle; margin-top: -1px"/></li> <li><a href="https://www.python.org/">Python</a> »</li> <li> <a href="../index.html">Python 2.7.18 documentation</a> » </li> <li class="nav-item nav-item-1"><a href="index.html" accesskey="U">The Python Tutorial</a> »</li> </ul> </div> <div class="document"> <div class="documentwrapper"> <div class="bodywrapper"> <div class="body" role="main"> <div class="section" id="an-informal-introduction-to-python"> <span id="tut-informal"></span><h1>3. An Informal Introduction to Python<a class="headerlink" href="#an-informal-introduction-to-python" title="Permalink to this headline">¶</a></h1> <p>In the following examples, input and output are distinguished by the presence or absence of prompts (<a class="reference internal" href="../glossary.html#term"><span class="xref std std-term">>>></span></a> and <a class="reference internal" href="../glossary.html#term-1"><span class="xref std std-term">…</span></a>): to repeat the example, you must type everything after the prompt, when the prompt appears; lines that do not begin with a prompt are output from the interpreter. Note that a secondary prompt on a line by itself in an example means you must type a blank line; this is used to end a multi-line command.</p> <p>Many of the examples in this manual, even those entered at the interactive prompt, include comments. Comments in Python start with the hash character, <code class="docutils literal notranslate"><span class="pre">#</span></code>, and extend to the end of the physical line. A comment may appear at the start of a line or following whitespace or code, but not within a string literal. A hash character within a string literal is just a hash character. Since comments are to clarify code and are not interpreted by Python, they may be omitted when typing in examples.</p> <p>Some examples:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="c1"># this is the first comment</span> <span class="n">spam</span> <span class="o">=</span> <span class="mi">1</span> <span class="c1"># and this is the second comment</span> <span class="c1"># ... and now a third!</span> <span class="n">text</span> <span class="o">=</span> <span class="s2">"# This is not a comment because it's inside quotes."</span> </pre></div> </div> <div class="section" id="using-python-as-a-calculator"> <span id="tut-calculator"></span><h2>3.1. Using Python as a Calculator<a class="headerlink" href="#using-python-as-a-calculator" title="Permalink to this headline">¶</a></h2> <p>Let’s try some simple Python commands. Start the interpreter and wait for the primary prompt, <code class="docutils literal notranslate"><span class="pre">>>></span></code>. (It shouldn’t take long.)</p> <div class="section" id="numbers"> <span id="tut-numbers"></span><h3>3.1.1. Numbers<a class="headerlink" href="#numbers" title="Permalink to this headline">¶</a></h3> <p>The interpreter acts as a simple calculator: you can type an expression at it and it will write the value. Expression syntax is straightforward: the operators <code class="docutils literal notranslate"><span class="pre">+</span></code>, <code class="docutils literal notranslate"><span class="pre">-</span></code>, <code class="docutils literal notranslate"><span class="pre">*</span></code> and <code class="docutils literal notranslate"><span class="pre">/</span></code> work just like in most other languages (for example, Pascal or C); parentheses (<code class="docutils literal notranslate"><span class="pre">()</span></code>) can be used for grouping. For example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">2</span> <span class="o">+</span> <span class="mi">2</span> <span class="go">4</span> <span class="gp">>>> </span><span class="mi">50</span> <span class="o">-</span> <span class="mi">5</span><span class="o">*</span><span class="mi">6</span> <span class="go">20</span> <span class="gp">>>> </span><span class="p">(</span><span class="mi">50</span> <span class="o">-</span> <span class="mf">5.0</span><span class="o">*</span><span class="mi">6</span><span class="p">)</span> <span class="o">/</span> <span class="mi">4</span> <span class="go">5.0</span> <span class="gp">>>> </span><span class="mi">8</span> <span class="o">/</span> <span class="mf">5.0</span> <span class="go">1.6</span> </pre></div> </div> <p>The integer numbers (e.g. <code class="docutils literal notranslate"><span class="pre">2</span></code>, <code class="docutils literal notranslate"><span class="pre">4</span></code>, <code class="docutils literal notranslate"><span class="pre">20</span></code>) have type <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>, the ones with a fractional part (e.g. <code class="docutils literal notranslate"><span class="pre">5.0</span></code>, <code class="docutils literal notranslate"><span class="pre">1.6</span></code>) have type <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a>. We will see more about numeric types later in the tutorial.</p> <p>The return type of a division (<code class="docutils literal notranslate"><span class="pre">/</span></code>) operation depends on its operands. If both operands are of type <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a>, <a class="reference internal" href="../glossary.html#term-floor-division"><span class="xref std std-term">floor division</span></a> is performed and an <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a> is returned. If either operand is a <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a>, classic division is performed and a <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a> is returned. The <code class="docutils literal notranslate"><span class="pre">//</span></code> operator is also provided for doing floor division no matter what the operands are. The remainder can be calculated with the <code class="docutils literal notranslate"><span class="pre">%</span></code> operator:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">17</span> <span class="o">/</span> <span class="mi">3</span> <span class="c1"># int / int -> int</span> <span class="go">5</span> <span class="gp">>>> </span><span class="mi">17</span> <span class="o">/</span> <span class="mf">3.0</span> <span class="c1"># int / float -> float</span> <span class="go">5.666666666666667</span> <span class="gp">>>> </span><span class="mi">17</span> <span class="o">//</span> <span class="mf">3.0</span> <span class="c1"># explicit floor division discards the fractional part</span> <span class="go">5.0</span> <span class="gp">>>> </span><span class="mi">17</span> <span class="o">%</span> <span class="mi">3</span> <span class="c1"># the % operator returns the remainder of the division</span> <span class="go">2</span> <span class="gp">>>> </span><span class="mi">5</span> <span class="o">*</span> <span class="mi">3</span> <span class="o">+</span> <span class="mi">2</span> <span class="c1"># result * divisor + remainder</span> <span class="go">17</span> </pre></div> </div> <p>With Python, it is possible to use the <code class="docutils literal notranslate"><span class="pre">**</span></code> operator to calculate powers <a class="footnote-reference" href="#id3" id="id1">[1]</a>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">5</span> <span class="o">**</span> <span class="mi">2</span> <span class="c1"># 5 squared</span> <span class="go">25</span> <span class="gp">>>> </span><span class="mi">2</span> <span class="o">**</span> <span class="mi">7</span> <span class="c1"># 2 to the power of 7</span> <span class="go">128</span> </pre></div> </div> <p>The equal sign (<code class="docutils literal notranslate"><span class="pre">=</span></code>) is used to assign a value to a variable. Afterwards, no result is displayed before the next interactive prompt:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">width</span> <span class="o">=</span> <span class="mi">20</span> <span class="gp">>>> </span><span class="n">height</span> <span class="o">=</span> <span class="mi">5</span> <span class="o">*</span> <span class="mi">9</span> <span class="gp">>>> </span><span class="n">width</span> <span class="o">*</span> <span class="n">height</span> <span class="go">900</span> </pre></div> </div> <p>If a variable is not “defined” (assigned a value), trying to use it will give you an error:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">n</span> <span class="c1"># try to access an undefined variable</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">NameError</span>: <span class="n">name 'n' is not defined</span> </pre></div> </div> <p>There is full support for floating point; operators with mixed type operands convert the integer operand to floating point:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="mi">3</span> <span class="o">*</span> <span class="mf">3.75</span> <span class="o">/</span> <span class="mf">1.5</span> <span class="go">7.5</span> <span class="gp">>>> </span><span class="mf">7.0</span> <span class="o">/</span> <span class="mi">2</span> <span class="go">3.5</span> </pre></div> </div> <p>In interactive mode, the last printed expression is assigned to the variable <code class="docutils literal notranslate"><span class="pre">_</span></code>. This means that when you are using Python as a desk calculator, it is somewhat easier to continue calculations, for example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">tax</span> <span class="o">=</span> <span class="mf">12.5</span> <span class="o">/</span> <span class="mi">100</span> <span class="gp">>>> </span><span class="n">price</span> <span class="o">=</span> <span class="mf">100.50</span> <span class="gp">>>> </span><span class="n">price</span> <span class="o">*</span> <span class="n">tax</span> <span class="go">12.5625</span> <span class="gp">>>> </span><span class="n">price</span> <span class="o">+</span> <span class="n">_</span> <span class="go">113.0625</span> <span class="gp">>>> </span><span class="nb">round</span><span class="p">(</span><span class="n">_</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span> <span class="go">113.06</span> </pre></div> </div> <p>This variable should be treated as read-only by the user. Don’t explicitly assign a value to it — you would create an independent local variable with the same name masking the built-in variable with its magic behavior.</p> <p>In addition to <a class="reference internal" href="../library/functions.html#int" title="int"><code class="xref py py-class docutils literal notranslate"><span class="pre">int</span></code></a> and <a class="reference internal" href="../library/functions.html#float" title="float"><code class="xref py py-class docutils literal notranslate"><span class="pre">float</span></code></a>, Python supports other types of numbers, such as <a class="reference internal" href="../library/decimal.html#decimal.Decimal" title="decimal.Decimal"><code class="xref py py-class docutils literal notranslate"><span class="pre">Decimal</span></code></a> and <a class="reference internal" href="../library/fractions.html#fractions.Fraction" title="fractions.Fraction"><code class="xref py py-class docutils literal notranslate"><span class="pre">Fraction</span></code></a>. Python also has built-in support for <a class="reference internal" href="../library/stdtypes.html#typesnumeric"><span class="std std-ref">complex numbers</span></a>, and uses the <code class="docutils literal notranslate"><span class="pre">j</span></code> or <code class="docutils literal notranslate"><span class="pre">J</span></code> suffix to indicate the imaginary part (e.g. <code class="docutils literal notranslate"><span class="pre">3+5j</span></code>).</p> </div> <div class="section" id="strings"> <span id="tut-strings"></span><h3>3.1.2. Strings<a class="headerlink" href="#strings" title="Permalink to this headline">¶</a></h3> <p>Besides numbers, Python can also manipulate strings, which can be expressed in several ways. They can be enclosed in single quotes (<code class="docutils literal notranslate"><span class="pre">'...'</span></code>) or double quotes (<code class="docutils literal notranslate"><span class="pre">"..."</span></code>) with the same result <a class="footnote-reference" href="#id4" id="id2">[2]</a>. <code class="docutils literal notranslate"><span class="pre">\</span></code> can be used to escape quotes:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="s1">'spam eggs'</span> <span class="c1"># single quotes</span> <span class="go">'spam eggs'</span> <span class="gp">>>> </span><span class="s1">'doesn</span><span class="se">\'</span><span class="s1">t'</span> <span class="c1"># use \' to escape the single quote...</span> <span class="go">"doesn't"</span> <span class="gp">>>> </span><span class="s2">"doesn't"</span> <span class="c1"># ...or use double quotes instead</span> <span class="go">"doesn't"</span> <span class="gp">>>> </span><span class="s1">'"Yes," they said.'</span> <span class="go">'"Yes," they said.'</span> <span class="gp">>>> </span><span class="s2">"</span><span class="se">\"</span><span class="s2">Yes,</span><span class="se">\"</span><span class="s2"> they said."</span> <span class="go">'"Yes," they said.'</span> <span class="gp">>>> </span><span class="s1">'"Isn</span><span class="se">\'</span><span class="s1">t," they said.'</span> <span class="go">'"Isn\'t," they said.'</span> </pre></div> </div> <p>In the interactive interpreter, the output string is enclosed in quotes and special characters are escaped with backslashes. While this might sometimes look different from the input (the enclosing quotes could change), the two strings are equivalent. The string is enclosed in double quotes if the string contains a single quote and no double quotes, otherwise it is enclosed in single quotes. The <a class="reference internal" href="../reference/simple_stmts.html#print"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">print</span></code></a> statement produces a more readable output, by omitting the enclosing quotes and by printing escaped and special characters:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="s1">'"Isn</span><span class="se">\'</span><span class="s1">t," they said.'</span> <span class="go">'"Isn\'t," they said.'</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="s1">'"Isn</span><span class="se">\'</span><span class="s1">t," they said.'</span> <span class="go">"Isn't," they said.</span> <span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="s1">'First line.</span><span class="se">\n</span><span class="s1">Second line.'</span> <span class="c1"># \n means newline</span> <span class="gp">>>> </span><span class="n">s</span> <span class="c1"># without print, \n is included in the output</span> <span class="go">'First line.\nSecond line.'</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="n">s</span> <span class="c1"># with print, \n produces a new line</span> <span class="go">First line.</span> <span class="go">Second line.</span> </pre></div> </div> <p>If you don’t want characters prefaced by <code class="docutils literal notranslate"><span class="pre">\</span></code> to be interpreted as special characters, you can use <em>raw strings</em> by adding an <code class="docutils literal notranslate"><span class="pre">r</span></code> before the first quote:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="nb">print</span> <span class="s1">'C:\some</span><span class="se">\n</span><span class="s1">ame'</span> <span class="c1"># here \n means newline!</span> <span class="go">C:\some</span> <span class="go">ame</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="sa">r</span><span class="s1">'C:\some\name'</span> <span class="c1"># note the r before the quote</span> <span class="go">C:\some\name</span> </pre></div> </div> <p>String literals can span multiple lines. One way is using triple-quotes: <code class="docutils literal notranslate"><span class="pre">"""..."""</span></code> or <code class="docutils literal notranslate"><span class="pre">'''...'''</span></code>. End of lines are automatically included in the string, but it’s possible to prevent this by adding a <code class="docutils literal notranslate"><span class="pre">\</span></code> at the end of the line. The following example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="nb">print</span> <span class="s2">"""</span><span class="se">\</span> <span class="s2">Usage: thingy [OPTIONS]</span> <span class="s2"> -h Display this usage message</span> <span class="s2"> -H hostname Hostname to connect to</span> <span class="s2">"""</span> </pre></div> </div> <p>produces the following output (note that the initial newline is not included):</p> <div class="highlight-text notranslate"><div class="highlight"><pre><span></span>Usage: thingy [OPTIONS] -h Display this usage message -H hostname Hostname to connect to </pre></div> </div> <p>Strings can be concatenated (glued together) with the <code class="docutils literal notranslate"><span class="pre">+</span></code> operator, and repeated with <code class="docutils literal notranslate"><span class="pre">*</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="c1"># 3 times 'un', followed by 'ium'</span> <span class="gp">>>> </span><span class="mi">3</span> <span class="o">*</span> <span class="s1">'un'</span> <span class="o">+</span> <span class="s1">'ium'</span> <span class="go">'unununium'</span> </pre></div> </div> <p>Two or more <em>string literals</em> (i.e. the ones enclosed between quotes) next to each other are automatically concatenated.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="s1">'Py'</span> <span class="s1">'thon'</span> <span class="go">'Python'</span> </pre></div> </div> <p>This feature is particularly useful when you want to break long strings:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">text</span> <span class="o">=</span> <span class="p">(</span><span class="s1">'Put several strings within parentheses '</span> <span class="gp">... </span> <span class="s1">'to have them joined together.'</span><span class="p">)</span> <span class="gp">>>> </span><span class="n">text</span> <span class="go">'Put several strings within parentheses to have them joined together.'</span> </pre></div> </div> <p>This only works with two literals though, not with variables or expressions:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">prefix</span> <span class="o">=</span> <span class="s1">'Py'</span> <span class="gp">>>> </span><span class="n">prefix</span> <span class="s1">'thon'</span> <span class="c1"># can't concatenate a variable and a string literal</span> <span class="go"> ...</span> <span class="go">SyntaxError: invalid syntax</span> <span class="gp">>>> </span><span class="p">(</span><span class="s1">'un'</span> <span class="o">*</span> <span class="mi">3</span><span class="p">)</span> <span class="s1">'ium'</span> <span class="go"> ...</span> <span class="go">SyntaxError: invalid syntax</span> </pre></div> </div> <p>If you want to concatenate variables or a variable and a literal, use <code class="docutils literal notranslate"><span class="pre">+</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">prefix</span> <span class="o">+</span> <span class="s1">'thon'</span> <span class="go">'Python'</span> </pre></div> </div> <p>Strings can be <em>indexed</em> (subscripted), with the first character having index 0. There is no separate character type; a character is simply a string of size one:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span> <span class="o">=</span> <span class="s1">'Python'</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="c1"># character in position 0</span> <span class="go">'P'</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">5</span><span class="p">]</span> <span class="c1"># character in position 5</span> <span class="go">'n'</span> </pre></div> </div> <p>Indices may also be negative numbers, to start counting from the right:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span> <span class="c1"># last character</span> <span class="go">'n'</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="o">-</span><span class="mi">2</span><span class="p">]</span> <span class="c1"># second-last character</span> <span class="go">'o'</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="o">-</span><span class="mi">6</span><span class="p">]</span> <span class="go">'P'</span> </pre></div> </div> <p>Note that since -0 is the same as 0, negative indices start from -1.</p> <p>In addition to indexing, <em>slicing</em> is also supported. While indexing is used to obtain individual characters, <em>slicing</em> allows you to obtain a substring:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">0</span><span class="p">:</span><span class="mi">2</span><span class="p">]</span> <span class="c1"># characters from position 0 (included) to 2 (excluded)</span> <span class="go">'Py'</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">2</span><span class="p">:</span><span class="mi">5</span><span class="p">]</span> <span class="c1"># characters from position 2 (included) to 5 (excluded)</span> <span class="go">'tho'</span> </pre></div> </div> <p>Note how the start is always included, and the end always excluded. This makes sure that <code class="docutils literal notranslate"><span class="pre">s[:i]</span> <span class="pre">+</span> <span class="pre">s[i:]</span></code> is always equal to <code class="docutils literal notranslate"><span class="pre">s</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[:</span><span class="mi">2</span><span class="p">]</span> <span class="o">+</span> <span class="n">word</span><span class="p">[</span><span class="mi">2</span><span class="p">:]</span> <span class="go">'Python'</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[:</span><span class="mi">4</span><span class="p">]</span> <span class="o">+</span> <span class="n">word</span><span class="p">[</span><span class="mi">4</span><span class="p">:]</span> <span class="go">'Python'</span> </pre></div> </div> <p>Slice indices have useful defaults; an omitted first index defaults to zero, an omitted second index defaults to the size of the string being sliced.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[:</span><span class="mi">2</span><span class="p">]</span> <span class="c1"># character from the beginning to position 2 (excluded)</span> <span class="go">'Py'</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">4</span><span class="p">:]</span> <span class="c1"># characters from position 4 (included) to the end</span> <span class="go">'on'</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="o">-</span><span class="mi">2</span><span class="p">:]</span> <span class="c1"># characters from the second-last (included) to the end</span> <span class="go">'on'</span> </pre></div> </div> <p>One way to remember how slices work is to think of the indices as pointing <em>between</em> characters, with the left edge of the first character numbered 0. Then the right edge of the last character of a string of <em>n</em> characters has index <em>n</em>, for example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span> <span class="o">+---+---+---+---+---+---+</span> <span class="o">|</span> <span class="n">P</span> <span class="o">|</span> <span class="n">y</span> <span class="o">|</span> <span class="n">t</span> <span class="o">|</span> <span class="n">h</span> <span class="o">|</span> <span class="n">o</span> <span class="o">|</span> <span class="n">n</span> <span class="o">|</span> <span class="o">+---+---+---+---+---+---+</span> <span class="mi">0</span> <span class="mi">1</span> <span class="mi">2</span> <span class="mi">3</span> <span class="mi">4</span> <span class="mi">5</span> <span class="mi">6</span> <span class="o">-</span><span class="mi">6</span> <span class="o">-</span><span class="mi">5</span> <span class="o">-</span><span class="mi">4</span> <span class="o">-</span><span class="mi">3</span> <span class="o">-</span><span class="mi">2</span> <span class="o">-</span><span class="mi">1</span> </pre></div> </div> <p>The first row of numbers gives the position of the indices 0…6 in the string; the second row gives the corresponding negative indices. The slice from <em>i</em> to <em>j</em> consists of all characters between the edges labeled <em>i</em> and <em>j</em>, respectively.</p> <p>For non-negative indices, the length of a slice is the difference of the indices, if both are within bounds. For example, the length of <code class="docutils literal notranslate"><span class="pre">word[1:3]</span></code> is 2.</p> <p>Attempting to use an index that is too large will result in an error:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">42</span><span class="p">]</span> <span class="c1"># the word only has 6 characters</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n"><module></span> <span class="gr">IndexError</span>: <span class="n">string index out of range</span> </pre></div> </div> <p>However, out of range slice indexes are handled gracefully when used for slicing:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">4</span><span class="p">:</span><span class="mi">42</span><span class="p">]</span> <span class="go">'on'</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">42</span><span class="p">:]</span> <span class="go">''</span> </pre></div> </div> <p>Python strings cannot be changed — they are <a class="reference internal" href="../glossary.html#term-immutable"><span class="xref std std-term">immutable</span></a>. Therefore, assigning to an indexed position in the string results in an error:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="o">=</span> <span class="s1">'J'</span> <span class="go"> ...</span> <span class="go">TypeError: 'str' object does not support item assignment</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[</span><span class="mi">2</span><span class="p">:]</span> <span class="o">=</span> <span class="s1">'py'</span> <span class="go"> ...</span> <span class="go">TypeError: 'str' object does not support item assignment</span> </pre></div> </div> <p>If you need a different string, you should create a new one:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="s1">'J'</span> <span class="o">+</span> <span class="n">word</span><span class="p">[</span><span class="mi">1</span><span class="p">:]</span> <span class="go">'Jython'</span> <span class="gp">>>> </span><span class="n">word</span><span class="p">[:</span><span class="mi">2</span><span class="p">]</span> <span class="o">+</span> <span class="s1">'py'</span> <span class="go">'Pypy'</span> </pre></div> </div> <p>The built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> returns the length of a string:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">s</span> <span class="o">=</span> <span class="s1">'supercalifragilisticexpialidocious'</span> <span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">s</span><span class="p">)</span> <span class="go">34</span> </pre></div> </div> <div class="admonition seealso"> <p class="first admonition-title">See also</p> <dl class="last docutils"> <dt><a class="reference internal" href="../library/stdtypes.html#typesseq"><span class="std std-ref">Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange</span></a></dt> <dd>Strings, and the Unicode strings described in the next section, are examples of <em>sequence types</em>, and support the common operations supported by such types.</dd> <dt><a class="reference internal" href="../library/stdtypes.html#string-methods"><span class="std std-ref">String Methods</span></a></dt> <dd>Both strings and Unicode strings support a large number of methods for basic transformations and searching.</dd> <dt><a class="reference internal" href="../library/string.html#formatstrings"><span class="std std-ref">Format String Syntax</span></a></dt> <dd>Information about string formatting with <a class="reference internal" href="../library/stdtypes.html#str.format" title="str.format"><code class="xref py py-meth docutils literal notranslate"><span class="pre">str.format()</span></code></a>.</dd> <dt><a class="reference internal" href="../library/stdtypes.html#string-formatting"><span class="std std-ref">String Formatting Operations</span></a></dt> <dd>The old formatting operations invoked when strings and Unicode strings are the left operand of the <code class="docutils literal notranslate"><span class="pre">%</span></code> operator are described in more detail here.</dd> </dl> </div> </div> <div class="section" id="unicode-strings"> <span id="tut-unicodestrings"></span><h3>3.1.3. Unicode Strings<a class="headerlink" href="#unicode-strings" title="Permalink to this headline">¶</a></h3> <p>Starting with Python 2.0 a new data type for storing text data is available to the programmer: the Unicode object. It can be used to store and manipulate Unicode data (see <a class="reference external" href="http://www.unicode.org/">http://www.unicode.org/</a>) and integrates well with the existing string objects, providing auto-conversions where necessary.</p> <p>Unicode has the advantage of providing one ordinal for every character in every script used in modern and ancient texts. Previously, there were only 256 possible ordinals for script characters. Texts were typically bound to a code page which mapped the ordinals to script characters. This lead to very much confusion especially with respect to internationalization (usually written as <code class="docutils literal notranslate"><span class="pre">i18n</span></code> — <code class="docutils literal notranslate"><span class="pre">'i'</span></code> + 18 characters + <code class="docutils literal notranslate"><span class="pre">'n'</span></code>) of software. Unicode solves these problems by defining one code page for all scripts.</p> <p>Creating Unicode strings in Python is just as simple as creating normal strings:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="sa">u</span><span class="s1">'Hello World !'</span> <span class="go">u'Hello World !'</span> </pre></div> </div> <p>The small <code class="docutils literal notranslate"><span class="pre">'u'</span></code> in front of the quote indicates that a Unicode string is supposed to be created. If you want to include special characters in the string, you can do so by using the Python <em>Unicode-Escape</em> encoding. The following example shows how:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="sa">u</span><span class="s1">'Hello</span><span class="se">\u0020</span><span class="s1">World !'</span> <span class="go">u'Hello World !'</span> </pre></div> </div> <p>The escape sequence <code class="docutils literal notranslate"><span class="pre">\u0020</span></code> indicates to insert the Unicode character with the ordinal value 0x0020 (the space character) at the given position.</p> <p>Other characters are interpreted by using their respective ordinal values directly as Unicode ordinals. If you have literal strings in the standard Latin-1 encoding that is used in many Western countries, you will find it convenient that the lower 256 characters of Unicode are the same as the 256 characters of Latin-1.</p> <p>For experts, there is also a raw mode just like the one for normal strings. You have to prefix the opening quote with ‘ur’ to have Python use the <em>Raw-Unicode-Escape</em> encoding. It will only apply the above <code class="docutils literal notranslate"><span class="pre">\uXXXX</span></code> conversion if there is an uneven number of backslashes in front of the small ‘u’.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="sa">ur</span><span class="s1">'Hello\u0020World !'</span> <span class="go">u'Hello World !'</span> <span class="gp">>>> </span><span class="sa">ur</span><span class="s1">'Hello</span><span class="se">\\</span><span class="s1">u0020World !'</span> <span class="go">u'Hello\\\\u0020World !'</span> </pre></div> </div> <p>The raw mode is most useful when you have to enter lots of backslashes, as can be necessary in regular expressions.</p> <p>Apart from these standard encodings, Python provides a whole set of other ways of creating Unicode strings on the basis of a known encoding.</p> <p id="index-0">The built-in function <a class="reference internal" href="../library/functions.html#unicode" title="unicode"><code class="xref py py-func docutils literal notranslate"><span class="pre">unicode()</span></code></a> provides access to all registered Unicode codecs (COders and DECoders). Some of the more well known encodings which these codecs can convert are <em>Latin-1</em>, <em>ASCII</em>, <em>UTF-8</em>, and <em>UTF-16</em>. The latter two are variable-length encodings that store each Unicode character in one or more bytes. The default encoding is normally set to ASCII, which passes through characters in the range 0 to 127 and rejects any other characters with an error. When a Unicode string is printed, written to a file, or converted with <a class="reference internal" href="../library/functions.html#str" title="str"><code class="xref py py-func docutils literal notranslate"><span class="pre">str()</span></code></a>, conversion takes place using this default encoding.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="sa">u</span><span class="s2">"abc"</span> <span class="go">u'abc'</span> <span class="gp">>>> </span><span class="nb">str</span><span class="p">(</span><span class="sa">u</span><span class="s2">"abc"</span><span class="p">)</span> <span class="go">'abc'</span> <span class="gp">>>> </span><span class="sa">u</span><span class="s2">"äöü"</span> <span class="go">u'\xe4\xf6\xfc'</span> <span class="gp">>>> </span><span class="nb">str</span><span class="p">(</span><span class="sa">u</span><span class="s2">"äöü"</span><span class="p">)</span> <span class="gt">Traceback (most recent call last):</span> File <span class="nb">"<stdin>"</span>, line <span class="m">1</span>, in <span class="n">?</span> <span class="gr">UnicodeEncodeError</span>: <span class="n">'ascii' codec can't encode characters in position 0-2: ordinal not in range(128)</span> </pre></div> </div> <p>To convert a Unicode string into an 8-bit string using a specific encoding, Unicode objects provide an <code class="xref py py-func docutils literal notranslate"><span class="pre">encode()</span></code> method that takes one argument, the name of the encoding. Lowercase names for encodings are preferred.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="sa">u</span><span class="s2">"äöü"</span><span class="o">.</span><span class="n">encode</span><span class="p">(</span><span class="s1">'utf-8'</span><span class="p">)</span> <span class="go">'\xc3\xa4\xc3\xb6\xc3\xbc'</span> </pre></div> </div> <p>If you have data in a specific encoding and want to produce a corresponding Unicode string from it, you can use the <a class="reference internal" href="../library/functions.html#unicode" title="unicode"><code class="xref py py-func docutils literal notranslate"><span class="pre">unicode()</span></code></a> function with the encoding name as the second argument.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">unicode</span><span class="p">(</span><span class="s1">'</span><span class="se">\xc3\xa4\xc3\xb6\xc3\xbc</span><span class="s1">'</span><span class="p">,</span> <span class="s1">'utf-8'</span><span class="p">)</span> <span class="go">u'\xe4\xf6\xfc'</span> </pre></div> </div> </div> <div class="section" id="lists"> <span id="tut-lists"></span><h3>3.1.4. Lists<a class="headerlink" href="#lists" title="Permalink to this headline">¶</a></h3> <p>Python knows a number of <em>compound</em> data types, used to group together other values. The most versatile is the <em>list</em>, which can be written as a list of comma-separated values (items) between square brackets. Lists might contain items of different types, but usually the items all have the same type.</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">9</span><span class="p">,</span> <span class="mi">16</span><span class="p">,</span> <span class="mi">25</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">squares</span> <span class="go">[1, 4, 9, 16, 25]</span> </pre></div> </div> <p>Like strings (and all other built-in <a class="reference internal" href="../glossary.html#term-sequence"><span class="xref std std-term">sequence</span></a> type), lists can be indexed and sliced:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="c1"># indexing returns the item</span> <span class="go">1</span> <span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="o">-</span><span class="mi">1</span><span class="p">]</span> <span class="go">25</span> <span class="gp">>>> </span><span class="n">squares</span><span class="p">[</span><span class="o">-</span><span class="mi">3</span><span class="p">:]</span> <span class="c1"># slicing returns a new list</span> <span class="go">[9, 16, 25]</span> </pre></div> </div> <p>All slice operations return a new list containing the requested elements. This means that the following slice returns a new (shallow) copy of the list:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span><span class="p">[:]</span> <span class="go">[1, 4, 9, 16, 25]</span> </pre></div> </div> <p>Lists also supports operations like concatenation:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">squares</span> <span class="o">+</span> <span class="p">[</span><span class="mi">36</span><span class="p">,</span> <span class="mi">49</span><span class="p">,</span> <span class="mi">64</span><span class="p">,</span> <span class="mi">81</span><span class="p">,</span> <span class="mi">100</span><span class="p">]</span> <span class="go">[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]</span> </pre></div> </div> <p>Unlike strings, which are <a class="reference internal" href="../glossary.html#term-immutable"><span class="xref std std-term">immutable</span></a>, lists are a <a class="reference internal" href="../glossary.html#term-mutable"><span class="xref std std-term">mutable</span></a> type, i.e. it is possible to change their content:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">cubes</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">8</span><span class="p">,</span> <span class="mi">27</span><span class="p">,</span> <span class="mi">65</span><span class="p">,</span> <span class="mi">125</span><span class="p">]</span> <span class="c1"># something's wrong here</span> <span class="gp">>>> </span><span class="mi">4</span> <span class="o">**</span> <span class="mi">3</span> <span class="c1"># the cube of 4 is 64, not 65!</span> <span class="go">64</span> <span class="gp">>>> </span><span class="n">cubes</span><span class="p">[</span><span class="mi">3</span><span class="p">]</span> <span class="o">=</span> <span class="mi">64</span> <span class="c1"># replace the wrong value</span> <span class="gp">>>> </span><span class="n">cubes</span> <span class="go">[1, 8, 27, 64, 125]</span> </pre></div> </div> <p>You can also add new items at the end of the list, by using the <code class="xref py py-meth docutils literal notranslate"><span class="pre">append()</span></code> <em>method</em> (we will see more about methods later):</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">cubes</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">216</span><span class="p">)</span> <span class="c1"># add the cube of 6</span> <span class="gp">>>> </span><span class="n">cubes</span><span class="o">.</span><span class="n">append</span><span class="p">(</span><span class="mi">7</span> <span class="o">**</span> <span class="mi">3</span><span class="p">)</span> <span class="c1"># and the cube of 7</span> <span class="gp">>>> </span><span class="n">cubes</span> <span class="go">[1, 8, 27, 64, 125, 216, 343]</span> </pre></div> </div> <p>Assignment to slices is also possible, and this can even change the size of the list or clear it entirely:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">letters</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'a'</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">,</span> <span class="s1">'c'</span><span class="p">,</span> <span class="s1">'d'</span><span class="p">,</span> <span class="s1">'e'</span><span class="p">,</span> <span class="s1">'f'</span><span class="p">,</span> <span class="s1">'g'</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">letters</span> <span class="go">['a', 'b', 'c', 'd', 'e', 'f', 'g']</span> <span class="gp">>>> </span><span class="c1"># replace some values</span> <span class="gp">>>> </span><span class="n">letters</span><span class="p">[</span><span class="mi">2</span><span class="p">:</span><span class="mi">5</span><span class="p">]</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'C'</span><span class="p">,</span> <span class="s1">'D'</span><span class="p">,</span> <span class="s1">'E'</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">letters</span> <span class="go">['a', 'b', 'C', 'D', 'E', 'f', 'g']</span> <span class="gp">>>> </span><span class="c1"># now remove them</span> <span class="gp">>>> </span><span class="n">letters</span><span class="p">[</span><span class="mi">2</span><span class="p">:</span><span class="mi">5</span><span class="p">]</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </span><span class="n">letters</span> <span class="go">['a', 'b', 'f', 'g']</span> <span class="gp">>>> </span><span class="c1"># clear the list by replacing all the elements with an empty list</span> <span class="gp">>>> </span><span class="n">letters</span><span class="p">[:]</span> <span class="o">=</span> <span class="p">[]</span> <span class="gp">>>> </span><span class="n">letters</span> <span class="go">[]</span> </pre></div> </div> <p>The built-in function <a class="reference internal" href="../library/functions.html#len" title="len"><code class="xref py py-func docutils literal notranslate"><span class="pre">len()</span></code></a> also applies to lists:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">letters</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'a'</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">,</span> <span class="s1">'c'</span><span class="p">,</span> <span class="s1">'d'</span><span class="p">]</span> <span class="gp">>>> </span><span class="nb">len</span><span class="p">(</span><span class="n">letters</span><span class="p">)</span> <span class="go">4</span> </pre></div> </div> <p>It is possible to nest lists (create lists containing other lists), for example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span> <span class="o">=</span> <span class="p">[</span><span class="s1">'a'</span><span class="p">,</span> <span class="s1">'b'</span><span class="p">,</span> <span class="s1">'c'</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">n</span> <span class="o">=</span> <span class="p">[</span><span class="mi">1</span><span class="p">,</span> <span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">x</span> <span class="o">=</span> <span class="p">[</span><span class="n">a</span><span class="p">,</span> <span class="n">n</span><span class="p">]</span> <span class="gp">>>> </span><span class="n">x</span> <span class="go">[['a', 'b', 'c'], [1, 2, 3]]</span> <span class="gp">>>> </span><span class="n">x</span><span class="p">[</span><span class="mi">0</span><span class="p">]</span> <span class="go">['a', 'b', 'c']</span> <span class="gp">>>> </span><span class="n">x</span><span class="p">[</span><span class="mi">0</span><span class="p">][</span><span class="mi">1</span><span class="p">]</span> <span class="go">'b'</span> </pre></div> </div> </div> </div> <div class="section" id="first-steps-towards-programming"> <span id="tut-firststeps"></span><h2>3.2. First Steps Towards Programming<a class="headerlink" href="#first-steps-towards-programming" title="Permalink to this headline">¶</a></h2> <p>Of course, we can use Python for more complicated tasks than adding two and two together. For instance, we can write an initial sub-sequence of the <em>Fibonacci</em> series as follows:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="c1"># Fibonacci series:</span> <span class="gp">... </span><span class="c1"># the sum of two elements defines the next</span> <span class="gp">... </span><span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span> <span class="gp">>>> </span><span class="k">while</span> <span class="n">b</span> <span class="o"><</span> <span class="mi">10</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span> <span class="n">b</span> <span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span> <span class="gp">...</span> <span class="go">1</span> <span class="go">1</span> <span class="go">2</span> <span class="go">3</span> <span class="go">5</span> <span class="go">8</span> </pre></div> </div> <p>This example introduces several new features.</p> <ul> <li><p class="first">The first line contains a <em>multiple assignment</em>: the variables <code class="docutils literal notranslate"><span class="pre">a</span></code> and <code class="docutils literal notranslate"><span class="pre">b</span></code> simultaneously get the new values 0 and 1. On the last line this is used again, demonstrating that the expressions on the right-hand side are all evaluated first before any of the assignments take place. The right-hand side expressions are evaluated from the left to the right.</p> </li> <li><p class="first">The <a class="reference internal" href="../reference/compound_stmts.html#while"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">while</span></code></a> loop executes as long as the condition (here: <code class="docutils literal notranslate"><span class="pre">b</span> <span class="pre"><</span> <span class="pre">10</span></code>) remains true. In Python, like in C, any non-zero integer value is true; zero is false. The condition may also be a string or list value, in fact any sequence; anything with a non-zero length is true, empty sequences are false. The test used in the example is a simple comparison. The standard comparison operators are written the same as in C: <code class="docutils literal notranslate"><span class="pre"><</span></code> (less than), <code class="docutils literal notranslate"><span class="pre">></span></code> (greater than), <code class="docutils literal notranslate"><span class="pre">==</span></code> (equal to), <code class="docutils literal notranslate"><span class="pre"><=</span></code> (less than or equal to), <code class="docutils literal notranslate"><span class="pre">>=</span></code> (greater than or equal to) and <code class="docutils literal notranslate"><span class="pre">!=</span></code> (not equal to).</p> </li> <li><p class="first">The <em>body</em> of the loop is <em>indented</em>: indentation is Python’s way of grouping statements. At the interactive prompt, you have to type a tab or space(s) for each indented line. In practice you will prepare more complicated input for Python with a text editor; all decent text editors have an auto-indent facility. When a compound statement is entered interactively, it must be followed by a blank line to indicate completion (since the parser cannot guess when you have typed the last line). Note that each line within a basic block must be indented by the same amount.</p> </li> <li><p class="first">The <a class="reference internal" href="../reference/simple_stmts.html#print"><code class="xref std std-keyword docutils literal notranslate"><span class="pre">print</span></code></a> statement writes the value of the expression(s) it is given. It differs from just writing the expression you want to write (as we did earlier in the calculator examples) in the way it handles multiple expressions and strings. Strings are printed without quotes, and a space is inserted between items, so you can format things nicely, like this:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">i</span> <span class="o">=</span> <span class="mi">256</span><span class="o">*</span><span class="mi">256</span> <span class="gp">>>> </span><span class="nb">print</span> <span class="s1">'The value of i is'</span><span class="p">,</span> <span class="n">i</span> <span class="go">The value of i is 65536</span> </pre></div> </div> <p>A trailing comma avoids the newline after the output:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="gp">>>> </span><span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="mi">0</span><span class="p">,</span> <span class="mi">1</span> <span class="gp">>>> </span><span class="k">while</span> <span class="n">b</span> <span class="o"><</span> <span class="mi">1000</span><span class="p">:</span> <span class="gp">... </span> <span class="nb">print</span> <span class="n">b</span><span class="p">,</span> <span class="gp">... </span> <span class="n">a</span><span class="p">,</span> <span class="n">b</span> <span class="o">=</span> <span class="n">b</span><span class="p">,</span> <span class="n">a</span><span class="o">+</span><span class="n">b</span> <span class="gp">...</span> <span class="go">1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987</span> </pre></div> </div> <p>Note that the interpreter inserts a newline before it prints the next prompt if the last line was not completed.</p> </li> </ul> <p class="rubric">Footnotes</p> <table class="docutils footnote" frame="void" id="id3" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id1">[1]</a></td><td>Since <code class="docutils literal notranslate"><span class="pre">**</span></code> has higher precedence than <code class="docutils literal notranslate"><span class="pre">-</span></code>, <code class="docutils literal notranslate"><span class="pre">-3**2</span></code> will be interpreted as <code class="docutils literal notranslate"><span class="pre">-(3**2)</span></code> and thus result in <code class="docutils literal notranslate"><span class="pre">-9</span></code>. To avoid this and get <code class="docutils literal notranslate"><span class="pre">9</span></code>, you can use <code class="docutils literal notranslate"><span class="pre">(-3)**2</span></code>.</td></tr> </tbody> </table> <table class="docutils footnote" frame="void" id="id4" rules="none"> <colgroup><col class="label" /><col /></colgroup> <tbody valign="top"> <tr><td class="label"><a class="fn-backref" href="#id2">[2]</a></td><td>Unlike other languages, special characters such as <code class="docutils literal notranslate"><span class="pre">\n</span></code> have the same meaning with both single (<code class="docutils literal notranslate"><span class="pre">'...'</span></code>) and double (<code class="docutils literal notranslate"><span class="pre">"..."</span></code>) quotes. The only difference between the two is that within single quotes you don’t need to escape <code class="docutils literal notranslate"><span class="pre">"</span></code> (but you have to escape <code class="docutils literal notranslate"><span class="pre">\'</span></code>) and vice versa.</td></tr> </tbody> </table> </div> </div> </div> </div> </div> <div class="sphinxsidebar" role="navigation" aria-label="main navigation"> <div class="sphinxsidebarwrapper"> <h3><a href="../contents.html">Table of Contents</a></h3> <ul> <li><a class="reference internal" href="#">3. An Informal Introduction to Python</a><ul> <li><a class="reference internal" href="#using-python-as-a-calculator">3.1. Using Python as a Calculator</a><ul> <li><a class="reference internal" href="#numbers">3.1.1. Numbers</a></li> <li><a class="reference internal" href="#strings">3.1.2. Strings</a></li> <li><a class="reference internal" href="#unicode-strings">3.1.3. Unicode Strings</a></li> <li><a class="reference internal" href="#lists">3.1.4. Lists</a></li> </ul> </li> <li><a class="reference internal" href="#first-steps-towards-programming">3.2. First Steps Towards Programming</a></li> </ul> </li> </ul> <h4>Previous topic</h4> <p class="topless"><a href="interpreter.html" title="previous chapter">2. Using the Python Interpreter</a></p> <h4>Next topic</h4> <p class="topless"><a href="controlflow.html" title="next chapter">4. More Control Flow Tools</a></p> <div role="note" aria-label="source link"> <h3>This Page</h3> <ul class="this-page-menu"> <li><a href="../_sources/tutorial/introduction.rst.txt" rel="nofollow">Show Source</a></li> </ul> </div> <div id="searchbox" style="display: none" role="search"> <h3>Quick search</h3> <div class="searchformwrapper"> <form class="search" action="../search.html" method="get"> <input type="text" name="q" /> <input type="submit" value="Go" /> <input type="hidden" name="check_keywords" value="yes" /> <input type="hidden" name="area" value="default" /> </form> </div> </div> <script type="text/javascript">$('#searchbox').show(0);</script> </div> </div> <div class="clearer"></div> </div> <div class="related" role="navigation" aria-label="related navigation"> <h3>Navigation</h3> <ul> <li class="right" style="margin-right: 10px"> <a href="../genindex.html" title="General Index" >index</a></li> <li class="right" > <a href="../py-modindex.html" title="Python Module Index" >modules</a> |</li> <li class="right" > <a href="controlflow.html" title="4. 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