^^&&^^

Edit File: controlflow.html

<!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="Content-Type" content="text/html; charset=utf-8" />
 
 <title>4. More Control Flow Tools &mdash; Python 2.7.5 documentation</title>
 
 <link rel="stylesheet" href="../_static/default.css" type="text/css" />
 <link rel="stylesheet" href="../_static/pygments.css" type="text/css" />
 
 <script type="text/javascript">
 var DOCUMENTATION_OPTIONS = {
 URL_ROOT: '../',
 VERSION: '2.7.5',
 COLLAPSE_INDEX: false,
 FILE_SUFFIX: '.html',
 HAS_SOURCE: true
 };
 </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/sidebar.js"></script>
 <link rel="search" type="application/opensearchdescription+xml"
 title="Search within Python 2.7.5 documentation"
 href="../_static/opensearch.xml"/>
 <link rel="author" title="About these documents" href="../about.html" />
 <link rel="copyright" title="Copyright" href="../copyright.html" />
 <link rel="top" title="Python 2.7.5 documentation" href="../index.html" />
 <link rel="up" title="The Python Tutorial" href="index.html" />
 <link rel="next" title="5. Data Structures" href="datastructures.html" />
 <link rel="prev" title="3. An Informal Introduction to Python" href="introduction.html" />
 <link rel="shortcut icon" type="image/png" href="../_static/py.png" />
 <script type="text/javascript" src="../_static/copybutton.js"></script>

</head>
 <body>
 <div class="related">
 <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="datastructures.html" title="5. Data Structures"
 accesskey="N">next</a> |</li>
 <li class="right" >
 <a href="introduction.html" title="3. An Informal Introduction to Python"
 accesskey="P">previous</a> |</li>
 <li><img src="../_static/py.png" alt=""
 style="vertical-align: middle; margin-top: -1px"/></li>
 <li><a href="http://www.python.org/">Python</a> &raquo;</li>
 <li>
 <a href="../index.html">Python 2.7.5 documentation</a> &raquo;
 </li>

<li><a href="index.html" accesskey="U">The Python Tutorial</a> &raquo;</li> 
 </ul>
 </div>

<div class="document">
 <div class="documentwrapper">
 <div class="bodywrapper">
 <div class="body">
 
 <div class="section" id="more-control-flow-tools">
<h1>4. More Control Flow Tools<a class="headerlink" href="#more-control-flow-tools" title="Permalink to this headline">¶</a></h1>
Besides the <a class="reference internal" href="../reference/compound_stmts.html#while"><tt class="xref std std-keyword docutils literal">while</tt></a> statement just introduced, Python knows the usual
control flow statements known from other languages, with some twists.
<div class="section" id="if-statements">
<h2>4.1. <a class="reference internal" href="../reference/compound_stmts.html#if"><tt class="xref std std-keyword docutils literal">if</tt></a> Statements<a class="headerlink" href="#if-statements" title="Permalink to this headline">¶</a></h2>
Perhaps the most well-known statement type is the <a class="reference internal" href="../reference/compound_stmts.html#if"><tt class="xref std std-keyword docutils literal">if</tt></a> statement. For
example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; x = int(raw_input(&quot;Please enter an integer: &quot;))
Please enter an integer: 42
&gt;&gt;&gt; if x &lt; 0:
... x = 0
... print &#39;Negative changed to zero&#39;
... elif x == 0:
... print &#39;Zero&#39;
... elif x == 1:
... print &#39;Single&#39;
... else:
... print &#39;More&#39;
...
More
</pre></div>
</div>
There can be zero or more <a class="reference internal" href="../reference/compound_stmts.html#elif"><tt class="xref std std-keyword docutils literal">elif</tt></a> parts, and the <a class="reference internal" href="../reference/compound_stmts.html#else"><tt class="xref std std-keyword docutils literal">else</tt></a> part is
optional. The keyword &#8216;<a class="reference internal" href="../reference/compound_stmts.html#elif"><tt class="xref std std-keyword docutils literal">elif</tt></a>&#8216; is short for &#8216;else if&#8217;, and is useful
to avoid excessive indentation. An <a class="reference internal" href="../reference/compound_stmts.html#if"><tt class="xref std std-keyword docutils literal">if</tt></a> ... <a class="reference internal" href="../reference/compound_stmts.html#elif"><tt class="xref std std-keyword docutils literal">elif</tt></a> ...
<a class="reference internal" href="../reference/compound_stmts.html#elif"><tt class="xref std std-keyword docutils literal">elif</tt></a> ... sequence is a substitute for the <tt class="docutils literal">switch</tt> or
<tt class="docutils literal">case</tt> statements found in other languages.
</div>
<div class="section" id="for-statements">
<h2>4.2. <a class="reference internal" href="../reference/compound_stmts.html#for"><tt class="xref std std-keyword docutils literal">for</tt></a> Statements<a class="headerlink" href="#for-statements" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="../reference/compound_stmts.html#for"><tt class="xref std std-keyword docutils literal">for</tt></a> statement in Python differs a bit from what you may be used
to in C or Pascal. Rather than always iterating over an arithmetic progression
of numbers (like in Pascal), or giving the user the ability to define both the
iteration step and halting condition (as C), Python&#8217;s <a class="reference internal" href="../reference/compound_stmts.html#for"><tt class="xref std std-keyword docutils literal">for</tt></a> statement
iterates over the items of any sequence (a list or a string), in the order that
they appear in the sequence. For example (no pun intended):
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; # Measure some strings:
... words = [&#39;cat&#39;, &#39;window&#39;, &#39;defenestrate&#39;]
&gt;&gt;&gt; for w in words:
... print w, len(w)
...
cat 3
window 6
defenestrate 12
</pre></div>
</div>
If you need to modify the sequence you are iterating over while inside the loop
(for example to duplicate selected items), it is recommended that you first
make a copy. Iterating over a sequence does not implicitly make a copy. The
slice notation makes this especially convenient:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for w in words[:]: # Loop over a slice copy of the entire list.
... if len(w) &gt; 6:
... words.insert(0, w)
...
&gt;&gt;&gt; words
[&#39;defenestrate&#39;, &#39;cat&#39;, &#39;window&#39;, &#39;defenestrate&#39;]
</pre></div>
</div>
</div>
<div class="section" id="the-range-function">
<h2>4.3. The <a class="reference internal" href="../library/functions.html#range" title="range"><tt class="xref py py-func docutils literal">range()</tt></a> Function<a class="headerlink" href="#the-range-function" title="Permalink to this headline">¶</a></h2>
If you do need to iterate over a sequence of numbers, the built-in function
<a class="reference internal" href="../library/functions.html#range" title="range"><tt class="xref py py-func docutils literal">range()</tt></a> comes in handy. It generates lists containing arithmetic
progressions:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
</pre></div>
</div>
The given end point is never part of the generated list; <tt class="docutils literal">range(10)</tt> generates
a list of 10 values, the legal indices for items of a sequence of length 10. It
is possible to let the range start at another number, or to specify a different
increment (even negative; sometimes this is called the &#8216;step&#8217;):
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; range(5, 10)
[5, 6, 7, 8, 9]
&gt;&gt;&gt; range(0, 10, 3)
[0, 3, 6, 9]
&gt;&gt;&gt; range(-10, -100, -30)
[-10, -40, -70]
</pre></div>
</div>
To iterate over the indices of a sequence, you can combine <a class="reference internal" href="../library/functions.html#range" title="range"><tt class="xref py py-func docutils literal">range()</tt></a> and
<a class="reference internal" href="../library/functions.html#len" title="len"><tt class="xref py py-func docutils literal">len()</tt></a> as follows:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; a = [&#39;Mary&#39;, &#39;had&#39;, &#39;a&#39;, &#39;little&#39;, &#39;lamb&#39;]
&gt;&gt;&gt; for i in range(len(a)):
... print i, a[i]
...
0 Mary
1 had
2 a
3 little
4 lamb
</pre></div>
</div>
In most such cases, however, it is convenient to use the <a class="reference internal" href="../library/functions.html#enumerate" title="enumerate"><tt class="xref py py-func docutils literal">enumerate()</tt></a>
function, see <a class="reference internal" href="datastructures.html#tut-loopidioms">Looping Techniques</a>.
</div>
<div class="section" id="break-and-continue-statements-and-else-clauses-on-loops">
<h2>4.4. <a class="reference internal" href="../reference/simple_stmts.html#break"><tt class="xref std std-keyword docutils literal">break</tt></a> and <a class="reference internal" href="../reference/simple_stmts.html#continue"><tt class="xref std std-keyword docutils literal">continue</tt></a> Statements, and <a class="reference internal" href="../reference/compound_stmts.html#else"><tt class="xref std std-keyword docutils literal">else</tt></a> Clauses on Loops<a class="headerlink" href="#break-and-continue-statements-and-else-clauses-on-loops" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="../reference/simple_stmts.html#break"><tt class="xref std std-keyword docutils literal">break</tt></a> statement, like in C, breaks out of the smallest enclosing
<a class="reference internal" href="../reference/compound_stmts.html#for"><tt class="xref std std-keyword docutils literal">for</tt></a> or <a class="reference internal" href="../reference/compound_stmts.html#while"><tt class="xref std std-keyword docutils literal">while</tt></a> loop.
Loop statements may have an <tt class="docutils literal">else</tt> clause; it is executed when the loop
terminates through exhaustion of the list (with <a class="reference internal" href="../reference/compound_stmts.html#for"><tt class="xref std std-keyword docutils literal">for</tt></a>) or when the
condition becomes false (with <a class="reference internal" href="../reference/compound_stmts.html#while"><tt class="xref std std-keyword docutils literal">while</tt></a>), but not when the loop is
terminated by a <a class="reference internal" href="../reference/simple_stmts.html#break"><tt class="xref std std-keyword docutils literal">break</tt></a> statement. This is exemplified by the
following loop, which searches for prime numbers:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for n in range(2, 10):
... for x in range(2, n):
... if n % x == 0:
... print n, &#39;equals&#39;, x, &#39;*&#39;, n/x
... break
... else:
... # loop fell through without finding a factor
... print n, &#39;is a prime number&#39;
...
2 is a prime number
3 is a prime number
4 equals 2 * 2
5 is a prime number
6 equals 2 * 3
7 is a prime number
8 equals 2 * 4
9 equals 3 * 3
</pre></div>
</div>
(Yes, this is the correct code. Look closely: the <tt class="docutils literal">else</tt> clause belongs to
the <a class="reference internal" href="../reference/compound_stmts.html#for"><tt class="xref std std-keyword docutils literal">for</tt></a> loop, not the <a class="reference internal" href="../reference/compound_stmts.html#if"><tt class="xref std std-keyword docutils literal">if</tt></a> statement.)
When used with a loop, the <tt class="docutils literal">else</tt> clause has more in common with the
<tt class="docutils literal">else</tt> clause of a <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement than it does that of
<a class="reference internal" href="../reference/compound_stmts.html#if"><tt class="xref std std-keyword docutils literal">if</tt></a> statements: a <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement&#8217;s <tt class="docutils literal">else</tt> clause runs
when no exception occurs, and a loop&#8217;s <tt class="docutils literal">else</tt> clause runs when no <tt class="docutils literal">break</tt>
occurs. For more on the <a class="reference internal" href="../reference/compound_stmts.html#try"><tt class="xref std std-keyword docutils literal">try</tt></a> statement and exceptions, see
<a class="reference internal" href="errors.html#tut-handling">Handling Exceptions</a>.
The <a class="reference internal" href="../reference/simple_stmts.html#continue"><tt class="xref std std-keyword docutils literal">continue</tt></a> statement, also borrowed from C, continues with the next
iteration of the loop:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; for num in range(2, 10):
... if num % 2 == 0:
... print &quot;Found an even number&quot;, num
... continue
... print &quot;Found a number&quot;, num
Found an even number 2
Found a number 3
Found an even number 4
Found a number 5
Found an even number 6
Found a number 7
Found an even number 8
Found a number 9
</pre></div>
</div>
</div>
<div class="section" id="pass-statements">
<h2>4.5. <a class="reference internal" href="../reference/simple_stmts.html#pass"><tt class="xref std std-keyword docutils literal">pass</tt></a> Statements<a class="headerlink" href="#pass-statements" title="Permalink to this headline">¶</a></h2>
The <a class="reference internal" href="../reference/simple_stmts.html#pass"><tt class="xref std std-keyword docutils literal">pass</tt></a> statement does nothing. It can be used when a statement is
required syntactically but the program requires no action. For example:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; while True:
... pass # Busy-wait for keyboard interrupt (Ctrl+C)
...
</pre></div>
</div>
This is commonly used for creating minimal classes:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; class MyEmptyClass:
... pass
...
</pre></div>
</div>
Another place <a class="reference internal" href="../reference/simple_stmts.html#pass"><tt class="xref std std-keyword docutils literal">pass</tt></a> can be used is as a place-holder for a function or
conditional body when you are working on new code, allowing you to keep thinking
at a more abstract level. The <a class="reference internal" href="../reference/simple_stmts.html#pass"><tt class="xref std std-keyword docutils literal">pass</tt></a> is silently ignored:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def initlog(*args):
... pass # Remember to implement this!
...
</pre></div>
</div>
</div>
<div class="section" id="defining-functions">
<h2>4.6. Defining Functions<a class="headerlink" href="#defining-functions" title="Permalink to this headline">¶</a></h2>
We can create a function that writes the Fibonacci series to an arbitrary
boundary:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def fib(n): # write Fibonacci series up to n
... &quot;&quot;&quot;Print a Fibonacci series up to n.&quot;&quot;&quot;
... a, b = 0, 1
... while a &lt; n:
... print a,
... a, b = b, a+b
...
&gt;&gt;&gt; # Now call the function we just defined:
... fib(2000)
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597
</pre></div>
</div>
The keyword <a class="reference internal" href="../reference/compound_stmts.html#def"><tt class="xref std std-keyword docutils literal">def</tt></a> introduces a function definition. It must be
followed by the function name and the parenthesized list of formal parameters.
The statements that form the body of the function start at the next line, and
must be indented.
The first statement of the function body can optionally be a string literal;
this string literal is the function&#8217;s documentation string, or docstring.
(More about docstrings can be found in the section <a class="reference internal" href="#tut-docstrings">Documentation Strings</a>.)
There are tools which use docstrings to automatically produce online or printed
documentation, or to let the user interactively browse through code; it&#8217;s good
practice to include docstrings in code that you write, so make a habit of it.
The execution of a function introduces a new symbol table used for the local
variables of the function. More precisely, all variable assignments in a
function store the value in the local symbol table; whereas variable references
first look in the local symbol table, then in the local symbol tables of
enclosing functions, then in the global symbol table, and finally in the table
of built-in names. Thus, global variables cannot be directly assigned a value
within a function (unless named in a <a class="reference internal" href="../reference/simple_stmts.html#global"><tt class="xref std std-keyword docutils literal">global</tt></a> statement), although they
may be referenced.
The actual parameters (arguments) to a function call are introduced in the local
symbol table of the called function when it is called; thus, arguments are
passed using call by value (where the value is always an object reference,
not the value of the object). <a class="footnote-reference" href="#id2" id="id1">[1]</a> When a function calls another function, a new
local symbol table is created for that call.
A function definition introduces the function name in the current symbol table.
The value of the function name has a type that is recognized by the interpreter
as a user-defined function. This value can be assigned to another name which
can then also be used as a function. This serves as a general renaming
mechanism:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; fib
&lt;function fib at 10042ed0&gt;
&gt;&gt;&gt; f = fib
&gt;&gt;&gt; f(100)
0 1 1 2 3 5 8 13 21 34 55 89
</pre></div>
</div>
Coming from other languages, you might object that <tt class="docutils literal">fib</tt> is not a function but
a procedure since it doesn&#8217;t return a value. In fact, even functions without a
<a class="reference internal" href="../reference/simple_stmts.html#return"><tt class="xref std std-keyword docutils literal">return</tt></a> statement do return a value, albeit a rather boring one. This
value is called <tt class="docutils literal">None</tt> (it&#8217;s a built-in name). Writing the value <tt class="docutils literal">None</tt> is
normally suppressed by the interpreter if it would be the only value written.
You can see it if you really want to using <a class="reference internal" href="../reference/simple_stmts.html#print"><tt class="xref std std-keyword docutils literal">print</tt></a>:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; fib(0)
&gt;&gt;&gt; print fib(0)
None
</pre></div>
</div>
It is simple to write a function that returns a list of the numbers of the
Fibonacci series, instead of printing it:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def fib2(n): # return Fibonacci series up to n
... &quot;&quot;&quot;Return a list containing the Fibonacci series up to n.&quot;&quot;&quot;
... result = []
... a, b = 0, 1
... while a &lt; n:
... result.append(a) # see below
... a, b = b, a+b
... return result
...
&gt;&gt;&gt; f100 = fib2(100) # call it
&gt;&gt;&gt; f100 # write the result
[0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]
</pre></div>
</div>
This example, as usual, demonstrates some new Python features:
<ul class="simple">
<li>The <a class="reference internal" href="../reference/simple_stmts.html#return"><tt class="xref std std-keyword docutils literal">return</tt></a> statement returns with a value from a function.
<a class="reference internal" href="../reference/simple_stmts.html#return"><tt class="xref std std-keyword docutils literal">return</tt></a> without an expression argument returns <tt class="docutils literal">None</tt>. Falling off
the end of a function also returns <tt class="docutils literal">None</tt>.</li>
<li>The statement <tt class="docutils literal">result.append(a)</tt> calls a method of the list object
<tt class="docutils literal">result</tt>. A method is a function that &#8216;belongs&#8217; to an object and is named
<tt class="docutils literal">obj.methodname</tt>, where <tt class="docutils literal">obj</tt> is some object (this may be an expression),
and <tt class="docutils literal">methodname</tt> is the name of a method that is defined by the object&#8217;s type.
Different types define different methods. Methods of different types may have
the same name without causing ambiguity. (It is possible to define your own
object types and methods, using classes, see <a class="reference internal" href="classes.html#tut-classes">Classes</a>)
The method <tt class="xref py py-meth docutils literal">append()</tt> shown in the example is defined for list objects; it
adds a new element at the end of the list. In this example it is equivalent to
<tt class="docutils literal">result = result + [a]</tt>, but more efficient.</li>
</ul>
</div>
<div class="section" id="more-on-defining-functions">
<h2>4.7. More on Defining Functions<a class="headerlink" href="#more-on-defining-functions" title="Permalink to this headline">¶</a></h2>
It is also possible to define functions with a variable number of arguments.
There are three forms, which can be combined.
<div class="section" id="default-argument-values">
<h3>4.7.1. Default Argument Values<a class="headerlink" href="#default-argument-values" title="Permalink to this headline">¶</a></h3>
The most useful form is to specify a default value for one or more arguments.
This creates a function that can be called with fewer arguments than it is
defined to allow. For example:
<div class="highlight-python"><div class="highlight"><pre>def ask_ok(prompt, retries=4, complaint=&#39;Yes or no, please!&#39;):
 while True:
 ok = raw_input(prompt)
 if ok in (&#39;y&#39;, &#39;ye&#39;, &#39;yes&#39;):
 return True
 if ok in (&#39;n&#39;, &#39;no&#39;, &#39;nop&#39;, &#39;nope&#39;):
 return False
 retries = retries - 1
 if retries &lt; 0:
 raise IOError(&#39;refusenik user&#39;)
 print complaint
</pre></div>
</div>
This function can be called in several ways:
<ul class="simple">
<li>giving only the mandatory argument:
<tt class="docutils literal">ask_ok('Do you really want to quit?')</tt></li>
<li>giving one of the optional arguments:
<tt class="docutils literal">ask_ok('OK to overwrite the file?', 2)</tt></li>
<li>or even giving all arguments:
<tt class="docutils literal">ask_ok('OK to overwrite the file?', 2, 'Come on, only yes or no!')</tt></li>
</ul>
This example also introduces the <a class="reference internal" href="../reference/expressions.html#in"><tt class="xref std std-keyword docutils literal">in</tt></a> keyword. This tests whether or
not a sequence contains a certain value.
The default values are evaluated at the point of function definition in the
defining scope, so that
<div class="highlight-python"><div class="highlight"><pre>i = 5

def f(arg=i):
 print arg

i = 6
f()
</pre></div>
</div>
will print <tt class="docutils literal">5</tt>.
Important warning: The default value is evaluated only once. This makes a
difference when the default is a mutable object such as a list, dictionary, or
instances of most classes. For example, the following function accumulates the
arguments passed to it on subsequent calls:
<div class="highlight-python"><div class="highlight"><pre>def f(a, L=[]):
 L.append(a)
 return L

print f(1)
print f(2)
print f(3)
</pre></div>
</div>
This will print
<div class="highlight-python"><div class="highlight"><pre>[1]
[1, 2]
[1, 2, 3]
</pre></div>
</div>
If you don&#8217;t want the default to be shared between subsequent calls, you can
write the function like this instead:
<div class="highlight-python"><div class="highlight"><pre>def f(a, L=None):
 if L is None:
 L = []
 L.append(a)
 return L
</pre></div>
</div>
</div>
<div class="section" id="keyword-arguments">
<h3>4.7.2. Keyword Arguments<a class="headerlink" href="#keyword-arguments" title="Permalink to this headline">¶</a></h3>
Functions can also be called using <a class="reference internal" href="../glossary.html#term-keyword-argument">keyword arguments</a>
of the form <tt class="docutils literal">kwarg=value</tt>. For instance, the following function:
<div class="highlight-python"><div class="highlight"><pre>def parrot(voltage, state=&#39;a stiff&#39;, action=&#39;voom&#39;, type=&#39;Norwegian Blue&#39;):
 print &quot;-- This parrot wouldn&#39;t&quot;, action,
 print &quot;if you put&quot;, voltage, &quot;volts through it.&quot;
 print &quot;-- Lovely plumage, the&quot;, type
 print &quot;-- It&#39;s&quot;, state, &quot;!&quot;
</pre></div>
</div>
accepts one required argument (<tt class="docutils literal">voltage</tt>) and three optional arguments
(<tt class="docutils literal">state</tt>, <tt class="docutils literal">action</tt>, and <tt class="docutils literal">type</tt>). This function can be called in any
of the following ways:
<div class="highlight-python"><div class="highlight"><pre>parrot(1000) # 1 positional argument
parrot(voltage=1000) # 1 keyword argument
parrot(voltage=1000000, action=&#39;VOOOOOM&#39;) # 2 keyword arguments
parrot(action=&#39;VOOOOOM&#39;, voltage=1000000) # 2 keyword arguments
parrot(&#39;a million&#39;, &#39;bereft of life&#39;, &#39;jump&#39;) # 3 positional arguments
parrot(&#39;a thousand&#39;, state=&#39;pushing up the daisies&#39;) # 1 positional, 1 keyword
</pre></div>
</div>
but all the following calls would be invalid:
<div class="highlight-python"><div class="highlight"><pre>parrot() # required argument missing
parrot(voltage=5.0, &#39;dead&#39;) # non-keyword argument after a keyword argument
parrot(110, voltage=220) # duplicate value for the same argument
parrot(actor=&#39;John Cleese&#39;) # unknown keyword argument
</pre></div>
</div>
In a function call, keyword arguments must follow positional arguments.
All the keyword arguments passed must match one of the arguments
accepted by the function (e.g. <tt class="docutils literal">actor</tt> is not a valid argument for the
<tt class="docutils literal">parrot</tt> function), and their order is not important. This also includes
non-optional arguments (e.g. <tt class="docutils literal">parrot(voltage=1000)</tt> is valid too).
No argument may receive a value more than once.
Here&#8217;s an example that fails due to this restriction:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def function(a):
... pass
...
&gt;&gt;&gt; function(0, a=0)
Traceback (most recent call last):
 File &quot;&lt;stdin&gt;&quot;, line 1, in ?
TypeError: function() got multiple values for keyword argument &#39;a&#39;
</pre></div>
</div>
When a final formal parameter of the form <tt class="docutils literal">**name</tt> is present, it receives a
dictionary (see <a class="reference internal" href="../library/stdtypes.html#typesmapping">Mapping Types &#8212; dict</a>) containing all keyword arguments except for
those corresponding to a formal parameter. This may be combined with a formal
parameter of the form <tt class="docutils literal">*name</tt> (described in the next subsection) which
receives a tuple containing the positional arguments beyond the formal parameter
list. (<tt class="docutils literal">*name</tt> must occur before <tt class="docutils literal">**name</tt>.) For example, if we define a
function like this:
<div class="highlight-python"><div class="highlight"><pre>def cheeseshop(kind, *arguments, **keywords):
 print &quot;-- Do you have any&quot;, kind, &quot;?&quot;
 print &quot;-- I&#39;m sorry, we&#39;re all out of&quot;, kind
 for arg in arguments:
 print arg
 print &quot;-&quot; * 40
 keys = sorted(keywords.keys())
 for kw in keys:
 print kw, &quot;:&quot;, keywords[kw]
</pre></div>
</div>
It could be called like this:
<div class="highlight-python"><div class="highlight"><pre>cheeseshop(&quot;Limburger&quot;, &quot;It&#39;s very runny, sir.&quot;,
 &quot;It&#39;s really very, VERY runny, sir.&quot;,
 shopkeeper=&#39;Michael Palin&#39;,
 client=&quot;John Cleese&quot;,
 sketch=&quot;Cheese Shop Sketch&quot;)
</pre></div>
</div>
and of course it would print:
<div class="highlight-python"><pre>-- Do you have any Limburger ?
-- I'm sorry, we're all out of Limburger
It's very runny, sir.
It's really very, VERY runny, sir.
----------------------------------------
client : John Cleese
shopkeeper : Michael Palin
sketch : Cheese Shop Sketch</pre>
</div>
Note that the list of keyword argument names is created by sorting the result
of the keywords dictionary&#8217;s <tt class="docutils literal">keys()</tt> method before printing its contents;
if this is not done, the order in which the arguments are printed is undefined.
</div>
<div class="section" id="arbitrary-argument-lists">
<h3>4.7.3. Arbitrary Argument Lists<a class="headerlink" href="#arbitrary-argument-lists" title="Permalink to this headline">¶</a></h3>
Finally, the least frequently used option is to specify that a function can be
called with an arbitrary number of arguments. These arguments will be wrapped
up in a tuple (see <a class="reference internal" href="datastructures.html#tut-tuples">Tuples and Sequences</a>). Before the variable number of arguments,
zero or more normal arguments may occur.
<div class="highlight-python"><div class="highlight"><pre>def write_multiple_items(file, separator, *args):
 file.write(separator.join(args))
</pre></div>
</div>
</div>
<div class="section" id="unpacking-argument-lists">
<h3>4.7.4. Unpacking Argument Lists<a class="headerlink" href="#unpacking-argument-lists" title="Permalink to this headline">¶</a></h3>
The reverse situation occurs when the arguments are already in a list or tuple
but need to be unpacked for a function call requiring separate positional
arguments. For instance, the built-in <a class="reference internal" href="../library/functions.html#range" title="range"><tt class="xref py py-func docutils literal">range()</tt></a> function expects separate
start and stop arguments. If they are not available separately, write the
function call with the <tt class="docutils literal">*</tt>-operator to unpack the arguments out of a list
or tuple:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; range(3, 6) # normal call with separate arguments
[3, 4, 5]
&gt;&gt;&gt; args = [3, 6]
&gt;&gt;&gt; range(*args) # call with arguments unpacked from a list
[3, 4, 5]
</pre></div>
</div>
In the same fashion, dictionaries can deliver keyword arguments with the <tt class="docutils literal">**</tt>-operator:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def parrot(voltage, state=&#39;a stiff&#39;, action=&#39;voom&#39;):
... print &quot;-- This parrot wouldn&#39;t&quot;, action,
... print &quot;if you put&quot;, voltage, &quot;volts through it.&quot;,
... print &quot;E&#39;s&quot;, state, &quot;!&quot;
...
&gt;&gt;&gt; d = {&quot;voltage&quot;: &quot;four million&quot;, &quot;state&quot;: &quot;bleedin&#39; demised&quot;, &quot;action&quot;: &quot;VOOM&quot;}
&gt;&gt;&gt; parrot(**d)
-- This parrot wouldn&#39;t VOOM if you put four million volts through it. E&#39;s bleedin&#39; demised !
</pre></div>
</div>
</div>
<div class="section" id="lambda-forms">
<h3>4.7.5. Lambda Forms<a class="headerlink" href="#lambda-forms" title="Permalink to this headline">¶</a></h3>
By popular demand, a few features commonly found in functional programming
languages like Lisp have been added to Python. With the <a class="reference internal" href="../reference/expressions.html#lambda"><tt class="xref std std-keyword docutils literal">lambda</tt></a>
keyword, small anonymous functions can be created. Here&#8217;s a function that
returns the sum of its two arguments: <tt class="docutils literal">lambda a, b: a+b</tt>. Lambda forms can be
used wherever function objects are required. They are syntactically restricted
to a single expression. Semantically, they are just syntactic sugar for a
normal function definition. Like nested function definitions, lambda forms can
reference variables from the containing scope:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def make_incrementor(n):
... return lambda x: x + n
...
&gt;&gt;&gt; f = make_incrementor(42)
&gt;&gt;&gt; f(0)
42
&gt;&gt;&gt; f(1)
43
</pre></div>
</div>
</div>
<div class="section" id="documentation-strings">
<h3>4.7.6. Documentation Strings<a class="headerlink" href="#documentation-strings" title="Permalink to this headline">¶</a></h3>
There are emerging conventions about the content and formatting of documentation
strings.
The first line should always be a short, concise summary of the object&#8217;s
purpose. For brevity, it should not explicitly state the object&#8217;s name or type,
since these are available by other means (except if the name happens to be a
verb describing a function&#8217;s operation). This line should begin with a capital
letter and end with a period.
If there are more lines in the documentation string, the second line should be
blank, visually separating the summary from the rest of the description. The
following lines should be one or more paragraphs describing the object&#8217;s calling
conventions, its side effects, etc.
The Python parser does not strip indentation from multi-line string literals in
Python, so tools that process documentation have to strip indentation if
desired. This is done using the following convention. The first non-blank line
after the first line of the string determines the amount of indentation for
the entire documentation string. (We can&#8217;t use the first line since it is
generally adjacent to the string&#8217;s opening quotes so its indentation is not
apparent in the string literal.) Whitespace &#8220;equivalent&#8221; to this indentation is
then stripped from the start of all lines of the string. Lines that are
indented less should not occur, but if they occur all their leading whitespace
should be stripped. Equivalence of whitespace should be tested after expansion
of tabs (to 8 spaces, normally).
Here is an example of a multi-line docstring:
<div class="highlight-python"><div class="highlight"><pre>&gt;&gt;&gt; def my_function():
... &quot;&quot;&quot;Do nothing, but document it.
...
... No, really, it doesn&#39;t do anything.
... &quot;&quot;&quot;
... pass
...
&gt;&gt;&gt; print my_function.__doc__
Do nothing, but document it.

No, really, it doesn&#39;t do anything.
</pre></div>
</div>
</div>
</div>
<div class="section" id="intermezzo-coding-style">
<h2>4.8. Intermezzo: Coding Style<a class="headerlink" href="#intermezzo-coding-style" title="Permalink to this headline">¶</a></h2>
Now that you are about to write longer, more complex pieces of Python, it is a
good time to talk about coding style. Most languages can be written (or more
concise, formatted) in different styles; some are more readable than others.
Making it easy for others to read your code is always a good idea, and adopting
a nice coding style helps tremendously for that.
For Python, <a class="pep reference external" href="http://www.python.org/dev/peps/pep-0008">PEP 8</a> has emerged as the style guide that most projects adhere to;
it promotes a very readable and eye-pleasing coding style. Every Python
developer should read it at some point; here are the most important points
extracted for you:
<ul>
<li>Use 4-space indentation, and no tabs.
4 spaces are a good compromise between small indentation (allows greater
nesting depth) and large indentation (easier to read). Tabs introduce
confusion, and are best left out.
</li>
<li>Wrap lines so that they don&#8217;t exceed 79 characters.
This helps users with small displays and makes it possible to have several
code files side-by-side on larger displays.
</li>
<li>Use blank lines to separate functions and classes, and larger blocks of
code inside functions.
</li>
<li>When possible, put comments on a line of their own.
</li>
<li>Use docstrings.
</li>
<li>Use spaces around operators and after commas, but not directly inside
bracketing constructs: <tt class="docutils literal">a = f(1, 2) + g(3, 4)</tt>.
</li>
<li>Name your classes and functions consistently; the convention is to use
<tt class="docutils literal">CamelCase</tt> for classes and <tt class="docutils literal">lower_case_with_underscores</tt> for functions
and methods. Always use <tt class="docutils literal">self</tt> as the name for the first method argument
(see <a class="reference internal" href="classes.html#tut-firstclasses">A First Look at Classes</a> for more on classes and methods).
</li>
<li>Don&#8217;t use fancy encodings if your code is meant to be used in international
environments. Plain ASCII works best in any case.
</li>
</ul>
Footnotes
<table class="docutils footnote" frame="void" id="id2" 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>Actually, call by object reference would be a better description,
since if a mutable object is passed, the caller will see any changes the
callee makes to it (items inserted into a list).</td></tr>
</tbody>
</table>
</div>
</div>

</div>
 </div>
 </div>
 <div class="sphinxsidebar">
 <div class="sphinxsidebarwrapper">
 <h3><a href="../contents.html">Table Of Contents</a></h3>
 <ul>
<li><a class="reference internal" href="#">4. More Control Flow Tools</a><ul>
<li><a class="reference internal" href="#if-statements">4.1. <tt class="docutils literal">if</tt> Statements</a></li>
<li><a class="reference internal" href="#for-statements">4.2. <tt class="docutils literal">for</tt> Statements</a></li>
<li><a class="reference internal" href="#the-range-function">4.3. The <tt class="docutils literal">range()</tt> Function</a></li>
<li><a class="reference internal" href="#break-and-continue-statements-and-else-clauses-on-loops">4.4. <tt class="docutils literal">break</tt> and <tt class="docutils literal">continue</tt> Statements, and <tt class="docutils literal">else</tt> Clauses on Loops</a></li>
<li><a class="reference internal" href="#pass-statements">4.5. <tt class="docutils literal">pass</tt> Statements</a></li>
<li><a class="reference internal" href="#defining-functions">4.6. Defining Functions</a></li>
<li><a class="reference internal" href="#more-on-defining-functions">4.7. More on Defining Functions</a><ul>
<li><a class="reference internal" href="#default-argument-values">4.7.1. Default Argument Values</a></li>
<li><a class="reference internal" href="#keyword-arguments">4.7.2. Keyword Arguments</a></li>
<li><a class="reference internal" href="#arbitrary-argument-lists">4.7.3. Arbitrary Argument Lists</a></li>
<li><a class="reference internal" href="#unpacking-argument-lists">4.7.4. Unpacking Argument Lists</a></li>
<li><a class="reference internal" href="#lambda-forms">4.7.5. Lambda Forms</a></li>
<li><a class="reference internal" href="#documentation-strings">4.7.6. Documentation Strings</a></li>
</ul>
</li>
<li><a class="reference internal" href="#intermezzo-coding-style">4.8. Intermezzo: Coding Style</a></li>
</ul>
</li>
</ul>

<h4>Previous topic</h4>
 <a href="introduction.html"
 title="previous chapter">3. An Informal Introduction to Python</a>
 <h4>Next topic</h4>
 <a href="datastructures.html"
 title="next chapter">5. Data Structures</a>
<h3>This Page</h3>
<ul class="this-page-menu">
 <li><a href="../bugs.html">Report a Bug</a></li>
 <li><a href="../_sources/tutorial/controlflow.txt"
 rel="nofollow">Show Source</a></li>
</ul>

<div id="searchbox" style="display: none">
 <h3>Quick search</h3>
 <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>
 
 Enter search terms or a module, class or function name.
 
</div>
<script type="text/javascript">$('#searchbox').show(0);</script>
 </div>
 </div>
 <div class="clearer"></div>
 </div>
 <div class="related">
 <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="datastructures.html" title="5. Data Structures"
 >next</a> |</li>
 <li class="right" >
 <a href="introduction.html" title="3. An Informal Introduction to Python"
 >previous</a> |</li>
 <li><img src="../_static/py.png" alt=""
 style="vertical-align: middle; margin-top: -1px"/></li>
 <li><a href="http://www.python.org/">Python</a> &raquo;</li>
 <li>
 <a href="../index.html">Python 2.7.5 documentation</a> &raquo;
 </li>

<li><a href="index.html" >The Python Tutorial</a> &raquo;</li> 
 </ul>
 </div>
 <div class="footer">
 &copy; <a href="../copyright.html">Copyright</a> 1990-2020, Python Software Foundation.
 
 The Python Software Foundation is a non-profit corporation.
 <a href="http://www.python.org/psf/donations/">Please donate.</a>
 
 Last updated on Oct 13, 2020.
 <a href="../bugs.html">Found a bug</a>?
 
 Created using <a href="http://sphinx.pocoo.org/">Sphinx</a> 1.1.3.
 </div>

</body>
</html>