Merged revisions 59488-59511 via svnmerge from
svn+ssh://pythondev@svn.python.org/python/trunk
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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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diff --git a/Doc/tutorial/datastructures.rst b/Doc/tutorial/datastructures.rst
index 54d1922..cf9fea3 100644
--- a/Doc/tutorial/datastructures.rst
+++ b/Doc/tutorial/datastructures.rst
@@ -277,6 +277,48 @@
['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