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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> | |
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