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Improved wording for generator expressions.

This commit is contained in:
Raymond Hettinger 2004-05-19 22:30:25 +00:00
parent 059e170c1d
commit 6e1fd2f208
1 changed files with 15 additions and 22 deletions
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Doc/whatsnew/whatsnew24.tex
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@ -14,8 +14,8 @@
\maketitle
\tableofcontents
This article explains the new features in Python 2.4. No release date
for Python 2.4 has been set; expect that this will happen mid-2004.
This article explains the new features in Python 2.4. The release
date is expected to be around September 2004.
While Python 2.3 was primarily a library development release, Python
2.4 may extend the core language and interpreter in
@ -91,12 +91,12 @@ XXX write this.
%======================================================================
\section{PEP 229: Generator Expressions}
Generator expressions create in-line generators using a syntax similar
to list comprehensions but with parenthesis instead of the surrounding
brackets.
Genexps allow simple generators to be constructed without a separate function
definition. Writing:
Now, simple generators can be coded succinctly as expressions using a syntax
like list comprehensions but with parentheses instead of brackets. These
expressions are designed for situations where the generator is used right
away by an enclosing function. Generator expressions are more compact but
less versatile than full generator definitions and the tend to be more memory
friendly than equivalent list comprehensions.
\begin{verbatim}
g = (tgtexp for var1 in exp1 for var2 in exp2 if exp3)
@ -121,11 +121,9 @@ a whole list is memory all at once. Applications using memory
friendly generator expressions may scale-up to high volumes of data
more readily than with list comprehensions.
Generator expressions are intended to be used inside functions
such as \function{sum()}, \function{min()}, \function{set()}, and
\function{dict()}. These functions consume their data all at once
and would not benefit from having a full list instead of a generator
as an input:
Generator expressions are best used in functions that consume their
data all at once and would not benefit from having a full list instead
of a generator as an input:
\begin{verbatim}
>>> sum(i*i for i in range(10))
@ -149,20 +147,15 @@ as an input:
\end{verbatim}
These examples show the intended use for generator expressions
in situations where the values get consumed immediately after the
generator is created. In these situations, they operate like
memory efficient versions of list comprehensions.
For more complex uses of generators, it is strongly recommended that
the traditional full generator definitions be used instead. In a
generator expression, the first for-loop expression is evaluated
as soon as the expression is defined while the other expressions do
not get evaluated until the generator is run. This nuance is never
an issue when the generator is used immediately. If it is not used
right away, then it is better to write a full generator definition
which more clearly reveals when the expressions are evaluated and is
more obvious about the visibility and lifetime of its looping variables.
an issue when the generator is used immediately; however, if it is not
used right away, a full generator definition would be much more clear
about when the sub-expressions are evaluated and would be more obvious
about the visibility and lifetime of the variables.
\begin{seealso}
\seepep{289}{Generator Expressions}{Proposed by Raymond Hettinger and