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Merged revisions 59488-59511 via svnmerge from

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  r59489 | christian.heimes | 2007-12-14 03:33:57 +0100 (Fri, 14 Dec 2007) | 1 line

  Silence a warning about an unsed variable in debug builds
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  r59490 | christian.heimes | 2007-12-14 03:35:23 +0100 (Fri, 14 Dec 2007) | 2 lines

  Fixed bug #1620: New @spam.getter property syntax modifies the property in place.
  I added also the feature that a @prop.getter decorator does not overwrite the doc string of the property if it was given as an argument to property().
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  r59491 | raymond.hettinger | 2007-12-14 03:49:47 +0100 (Fri, 14 Dec 2007) | 1 line

  Cleaner method naming convention
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  r59492 | christian.heimes | 2007-12-14 04:02:34 +0100 (Fri, 14 Dec 2007) | 1 line

  Fixed a warning in _codecs_iso2022.c and some non C89 conform // comments.
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  r59493 | christian.heimes | 2007-12-14 05:38:13 +0100 (Fri, 14 Dec 2007) | 1 line

  Fixed warning in ssl module
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  r59500 | raymond.hettinger | 2007-12-14 19:08:20 +0100 (Fri, 14 Dec 2007) | 1 line

  Add line spacing for readability
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  r59501 | raymond.hettinger | 2007-12-14 19:12:21 +0100 (Fri, 14 Dec 2007) | 3 lines

  Update method names for named tuples.
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  r59503 | georg.brandl | 2007-12-14 20:03:36 +0100 (Fri, 14 Dec 2007) | 3 lines

  Add a section about nested listcomps to the tutorial.
  Thanks to Ian Bruntlett and Robert Lehmann.
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  r59504 | raymond.hettinger | 2007-12-14 20:19:59 +0100 (Fri, 14 Dec 2007) | 1 line

  Faster and simpler _replace() method
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  r59505 | raymond.hettinger | 2007-12-14 22:51:50 +0100 (Fri, 14 Dec 2007) | 1 line

  Add usage note
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  r59507 | andrew.kuchling | 2007-12-14 23:41:18 +0100 (Fri, 14 Dec 2007) | 1 line

  Remove warning about URL
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  r59510 | andrew.kuchling | 2007-12-14 23:52:36 +0100 (Fri, 14 Dec 2007) | 1 line

  Bump the version number, and make a few small edits
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  r59511 | christian.heimes | 2007-12-15 00:42:36 +0100 (Sat, 15 Dec 2007) | 2 lines

  Fixed bug #1628
  The detection now works on Unix with Makefile, Makefile with VPATH and on Windows.
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This commit is contained in:
Christian Heimes 2007-12-15 01:27:15 +00:00
parent 54cc54c1fe
commit 0449f63f53
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42
Doc/tutorial/datastructures.rst
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@ -277,6 +277,48 @@ List comprehensions can be applied to complex expressions and nested functions::
['3.1', '3.14', '3.142', '3.1416', '3.14159']
Nested List Comprehensions
--------------------------
If you've got the stomach for it, list comprehensions can be nested. They are a
powerful tool but -- like all powerful tools -- they need to be used carefully,
if at all.
Consider the following example of a 3x3 matrix held as a list containing three
lists, one list per row::
>>> mat = [
... [1, 2, 3],
... [4, 5, 6],
... [7, 8, 9],
... ]
Now, if you wanted to swap rows and columns, you could use a list
comprehension::
>>> print [[row[i] for row in mat] for i in [0, 1, 2]]
[[1, 4, 7], [2, 5, 8], [3, 6, 9]]
Special care has to be taken for the *nested* list comprehension:
To avoid apprehension when nesting list comprehensions, read from right to
left.
A more verbose version of this snippet shows the flow explicitly::
for i in [0, 1, 2]:
for row in mat:
print row[i],
print
In real world, you should prefer builtin functions to complex flow statements.
The :func:`zip` function would do a great job for this use case::
>>> zip(*mat)
[(1, 4, 7), (2, 5, 8), (3, 6, 9)]
See :ref:`tut-unpacking-arguments` for details on the asterisk in this line.
.. _tut-del:
The :keyword:`del` statement