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<H1><A NAME="SECTION000400000000000000000">
3 PEP 289: Generator Expressions</A>
</H1>

<P>
The iterator feature introduced in Python 2.2 and the
<tt class="module">itertools</tt> module make it easier to write programs that loop
through large data sets without having the entire data set in memory
at one time.  List comprehensions don't fit into this picture very
well because they produce a Python list object containing all of the
items.  This unavoidably pulls all of the objects into memory, which
can be a problem if your data set is very large.  When trying to write
a functionally-styled program, it would be natural to write something
like:

<P>
<div class="verbatim"><pre>
links = [link for link in get_all_links() if not link.followed]
for link in links:
    ...
</pre></div>

<P>
instead of 

<P>
<div class="verbatim"><pre>
for link in get_all_links():
    if link.followed:
        continue
    ...
</pre></div>

<P>
The first form is more concise and perhaps more readable, but if
you're dealing with a large number of link objects you'd have to write
the second form to avoid having all link objects in memory at the same
time.

<P>
Generator expressions work similarly to list comprehensions but don't
materialize the entire list; instead they create a generator that will
return elements one by one.  The above example could be written as:

<P>
<div class="verbatim"><pre>
links = (link for link in get_all_links() if not link.followed)
for link in links:
    ...
</pre></div>

<P>
Generator expressions always have to be written inside parentheses, as
in the above example.  The parentheses signalling a function call also
count, so if you want to create a iterator that will be immediately
passed to a function you could write:

<P>
<div class="verbatim"><pre>
print sum(obj.count for obj in list_all_objects())
</pre></div>

<P>
Generator expressions differ from list comprehensions in various small
ways.  Most notably, the loop variable (<var>obj</var> in the above
example) is not accessible outside of the generator expression.  List
comprehensions leave the variable assigned to its last value; future
versions of Python will change this, making list comprehensions match
generator expressions in this respect.

<P>
<div class="seealso">
  <p class="heading">See Also:</p>

<dl compact="compact" class="seerfc">
    <dt><a href="http://www.python.org/peps/pep-0289.html"
        title="Generator Expressions"
        >PEP 289, <em>Generator Expressions</em></a>
    <dd>Proposed by Raymond Hettinger and
implemented by Jiwon Seo with early efforts steered by Hye-Shik Chang.
  </dl>
</div>

<P>

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	52
	53	<H1><A NAME="SECTION000400000000000000000">
	54	3 PEP 289: Generator Expressions</A>
	55	</H1>
	56
	57	<P>
	58	The iterator feature introduced in Python 2.2 and the
	59	<tt class="module">itertools</tt> module make it easier to write programs that loop
	60	through large data sets without having the entire data set in memory
	61	at one time. List comprehensions don't fit into this picture very
	62	well because they produce a Python list object containing all of the
	63	items. This unavoidably pulls all of the objects into memory, which
	64	can be a problem if your data set is very large. When trying to write
65	a functionally-styled program, it would be natural to write something
66	like:
67
68	<P>
69	<div class="verbatim"><pre>
70	links = [link for link in get_all_links() if not link.followed]
71	for link in links:
72	...
73	</pre></div>
74
75	<P>
76	instead of
77
78	<P>
79	<div class="verbatim"><pre>
80	for link in get_all_links():
81	if link.followed:
82	continue
83	...
84	</pre></div>
85
86	<P>
87	The first form is more concise and perhaps more readable, but if
88	you're dealing with a large number of link objects you'd have to write
89	the second form to avoid having all link objects in memory at the same
90	time.
91
92	<P>
93	Generator expressions work similarly to list comprehensions but don't
94	materialize the entire list; instead they create a generator that will
95	return elements one by one. The above example could be written as:
96
97	<P>
98	<div class="verbatim"><pre>
99	links = (link for link in get_all_links() if not link.followed)
100	for link in links:
101	...
102	</pre></div>
103
104	<P>
105	Generator expressions always have to be written inside parentheses, as
106	in the above example. The parentheses signalling a function call also
107	count, so if you want to create a iterator that will be immediately
108	passed to a function you could write:
109
110	<P>
111	<div class="verbatim"><pre>
112	print sum(obj.count for obj in list_all_objects())
113	</pre></div>
114
115	<P>
116	Generator expressions differ from list comprehensions in various small
117	ways. Most notably, the loop variable (<var>obj</var> in the above
118	example) is not accessible outside of the generator expression. List
119	comprehensions leave the variable assigned to its last value; future
120	versions of Python will change this, making list comprehensions match
121	generator expressions in this respect.
122
123	<P>
124	<div class="seealso">
125	<p class="heading">See Also:</p>
126
127	<dl compact="compact" class="seerfc">
128	<dt><a href="http://www.python.org/peps/pep-0289.html"
129	title="Generator Expressions"
130	>PEP 289, <em>Generator Expressions</em></a>
131	<dd>Proposed by Raymond Hettinger and
132	implemented by Jiwon Seo with early efforts steered by Hye-Shik Chang.
133	</dl>
134	</div>
135
136	<P>
137
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