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<title>5.2.5 Generator expressions</title>
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<H2><A NAME="SECTION007250000000000000000"></A><A NAME="genexpr"></A>
<BR>
5.2.5 Generator expressions
</H2>
<a id='l2h-349' xml:id='l2h-349'></a>
<P>
A generator expression is a compact generator notation in parentheses:

<P>
<dl><dd class="grammar">
<div class="productions">
<table>
<tr>
    <td><a id='tok-generator_expression' xml:id='tok-generator_expression'>generator_expression</a></td>
    <td>::=</td>
    <td>"(" <a class='grammartoken' href="lists.html#tok-test">test</a> <a class='grammartoken' href="genexpr.html#tok-genexpr_for">genexpr_for</a> ")"</td></tr>
  <tr>
    <td><a id='tok-genexpr_for' xml:id='tok-genexpr_for'>genexpr_for</a></td>
    <td>::=</td>
    <td>"for" <a class='grammartoken' href="exprlists.html#tok-expression_list">expression_list</a> "in" <a class='grammartoken' href="lists.html#tok-test">test</a>
              [<a class='grammartoken' href="genexpr.html#tok-genexpr_iter">genexpr_iter</a>]</td></tr>
  <tr>
    <td><a id='tok-genexpr_iter' xml:id='tok-genexpr_iter'>genexpr_iter</a></td>
    <td>::=</td>
    <td><a class='grammartoken' href="genexpr.html#tok-genexpr_for">genexpr_for</a> | <a class='grammartoken' href="genexpr.html#tok-genexpr_if">genexpr_if</a></td></tr>
  <tr>
    <td><a id='tok-genexpr_if' xml:id='tok-genexpr_if'>genexpr_if</a></td>
    <td>::=</td>
    <td>"if" <a class='grammartoken' href="lists.html#tok-test">test</a> [<a class='grammartoken' href="genexpr.html#tok-genexpr_iter">genexpr_iter</a>]</td></tr>
</table>
</div>
<a class="grammar-footer"
  href="grammar.txt" type="text/plain"
  >Download entire grammar as text.</a>
</dd></dl>

<P>
A generator expression yields a new generator object.
<a id='l2h-350' xml:id='l2h-350'></a><a id='l2h-351' xml:id='l2h-351'></a>It consists of a single expression followed by at least one
<tt class="keyword">for</tt> clause and zero or more <tt class="keyword">for</tt> or <tt class="keyword">if</tt>
clauses.  The iterating values of the new generator are those that
would be produced by considering each of the <tt class="keyword">for</tt> or
<tt class="keyword">if</tt> clauses a block, nesting from left to right, and
evaluating the expression to yield a value that is reached the
innermost block for each iteration.

<P>
Variables used in the generator expression are evaluated lazily
when the <tt class="method">next()</tt> method is called for generator object
(in the same fashion as normal generators). However, the leftmost
<tt class="keyword">for</tt> clause is immediately evaluated so that error produced
by it can be seen before any other possible error in the code that
handles the generator expression.
Subsequent <tt class="keyword">for</tt> clauses cannot be evaluated immediately since
they may depend on the previous <tt class="keyword">for</tt> loop.
For example: "<tt class="samp">(x*y for x in range(10) for y in bar(x))</tt>".

<P>
The parentheses can be omitted on calls with only one argument.
See section <A href="calls.html#calls">5.3.4</A> for the detail.

<P>

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<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
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