[Python-checkins] cpython (merge 3.2 -> default): #13549: merge with 3.2.
ezio.melotti
python-checkins at python.org
Tue Dec 13 14:38:33 CET 2011
http://hg.python.org/cpython/rev/37094a1454ab
changeset: 73953:37094a1454ab
parent: 73950:6f097ff9ac04
parent: 73952:ad5c70296c7b
user: Ezio Melotti <ezio.melotti at gmail.com>
date: Tue Dec 13 15:38:13 2011 +0200
summary:
#13549: merge with 3.2.
files:
Doc/tutorial/datastructures.rst | 166 +++++++++++--------
1 files changed, 98 insertions(+), 68 deletions(-)
diff --git a/Doc/tutorial/datastructures.rst b/Doc/tutorial/datastructures.rst
--- a/Doc/tutorial/datastructures.rst
+++ b/Doc/tutorial/datastructures.rst
@@ -168,107 +168,137 @@
List Comprehensions
-------------------
-List comprehensions provide a concise way to create lists from sequences.
-Common applications are to make lists where each element is the result of
-some operations applied to each member of the sequence, or to create a
-subsequence of those elements that satisfy a certain condition.
+List comprehensions provide a concise way to create lists.
+Common applications are to make new lists where each element is the result of
+some operations applied to each member of another sequence or iterable, or to
+create a subsequence of those elements that satisfy a certain condition.
+
+For example, assume we want to create a list of squares, like::
+
+ >>> squares = []
+ >>> for x in range(10):
+ ... squares.append(x**2)
+ ...
+ >>> squares
+ [0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
+
+We can obtain the same result with::
+
+ squares = [x**2 for x in range(10)]
+
+This is also equivalent to ``squares = map(lambda x: x**2, range(10))``,
+but it's more concise and readable.
A list comprehension consists of brackets containing an expression followed
by a :keyword:`for` clause, then zero or more :keyword:`for` or :keyword:`if`
-clauses. The result will be a list resulting from evaluating the expression in
-the context of the :keyword:`for` and :keyword:`if` clauses which follow it. If
-the expression would evaluate to a tuple, it must be parenthesized.
+clauses. The result will be a new list resulting from evaluating the expression
+in the context of the :keyword:`for` and :keyword:`if` clauses which follow it.
+For example, this listcomp combines the elements of two lists if they are not
+equal::
-Here we take a list of numbers and return a list of three times each number::
+ >>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
+ [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
- >>> vec = [2, 4, 6]
- >>> [3*x for x in vec]
- [6, 12, 18]
+and it's equivalent to::
-Now we get a little fancier::
+ >>> combs = []
+ >>> for x in [1,2,3]:
+ ... for y in [3,1,4]:
+ ... if x != y:
+ ... combs.append((x, y))
+ ...
+ >>> combs
+ [(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
- >>> [[x, x**2] for x in vec]
- [[2, 4], [4, 16], [6, 36]]
+Note how the order of the :keyword:`for` and :keyword:`if` statements is the
+same in both these snippets.
-Here we apply a method call to each item in a sequence::
+If the expression is a tuple (e.g. the ``(x, y)`` in the previous example),
+it must be parenthesized. ::
+ >>> vec = [-4, -2, 0, 2, 4]
+ >>> # create a new list with the values doubled
+ >>> [x*2 for x in vec]
+ [-8, -4, 0, 4, 8]
+ >>> # filter the list to exclude negative numbers
+ >>> [x for x in vec if x >= 0]
+ [0, 2, 4]
+ >>> # apply a function to all the elements
+ >>> [abs(x) for x in vec]
+ [4, 2, 0, 2, 4]
+ >>> # call a method on each element
>>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
>>> [weapon.strip() for weapon in freshfruit]
['banana', 'loganberry', 'passion fruit']
-
-Using the :keyword:`if` clause we can filter the stream::
-
- >>> [3*x for x in vec if x > 3]
- [12, 18]
- >>> [3*x for x in vec if x < 2]
- []
-
-Tuples can often be created without their parentheses, but not here::
-
- >>> [x, x**2 for x in vec] # error - parens required for tuples
+ >>> # create a list of 2-tuples like (number, square)
+ >>> [(x, x**2) for x in range(6)]
+ [(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]
+ >>> # the tuple must be parenthesized, otherwise an error is raised
+ >>> [x, x**2 for x in range(6)]
File "<stdin>", line 1, in ?
- [x, x**2 for x in vec]
+ [x, x**2 for x in range(6)]
^
SyntaxError: invalid syntax
- >>> [(x, x**2) for x in vec]
- [(2, 4), (4, 16), (6, 36)]
+ >>> # flatten a list using a listcomp with two 'for'
+ >>> vec = [[1,2,3], [4,5,6], [7,8,9]]
+ >>> [num for elem in vec for num in elem]
+ [1, 2, 3, 4, 5, 6, 7, 8, 9]
-Here are some nested for loops and other fancy behavior::
+List comprehensions can contain complex expressions and nested functions::
- >>> vec1 = [2, 4, 6]
- >>> vec2 = [4, 3, -9]
- >>> [x*y for x in vec1 for y in vec2]
- [8, 6, -18, 16, 12, -36, 24, 18, -54]
- >>> [x+y for x in vec1 for y in vec2]
- [6, 5, -7, 8, 7, -5, 10, 9, -3]
- >>> [vec1[i]*vec2[i] for i in range(len(vec1))]
- [8, 12, -54]
-
-List comprehensions can be applied to complex expressions and nested functions::
-
- >>> [str(round(355/113, i)) for i in range(1, 6)]
+ >>> from math import pi
+ >>> [str(round(pi, i)) for i in range(1, 6)]
['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.
+The initial expression in a list comprehension can be any arbitrary expression,
+including another list comprehension.
-Consider the following example of a 3x3 matrix held as a list containing three
-lists, one list per row::
+Consider the following example of a 3x4 matrix implemented as a list of
+3 lists of length 4::
- >>> mat = [
- ... [1, 2, 3],
- ... [4, 5, 6],
- ... [7, 8, 9],
- ... ]
+ >>> matrix = [
+ ... [1, 2, 3, 4],
+ ... [5, 6, 7, 8],
+ ... [9, 10, 11, 12],
+ ... ]
-Now, if you wanted to swap rows and columns, you could use a list
-comprehension::
+The following list comprehension will transpose rows and columns::
- >>> print([[row[i] for row in mat] for i in [0, 1, 2]])
- [[1, 4, 7], [2, 5, 8], [3, 6, 9]]
+ >>> [[row[i] for row in matrix] for i in range(4)]
+ [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
-Special care has to be taken for the *nested* list comprehension:
+As we saw in the previous section, the nested listcomp is evaluated in
+the context of the :keyword:`for` that follows it, so this example is
+equivalent to::
- To avoid apprehension when nesting list comprehensions, read from right to
- left.
+ >>> transposed = []
+ >>> for i in range(4):
+ ... transposed.append([row[i] for row in matrix])
+ ...
+ >>> transposed
+ [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
-A more verbose version of this snippet shows the flow explicitly::
+which, in turn, is the same as::
- for i in [0, 1, 2]:
- for row in mat:
- print(row[i], end="")
- print()
+ >>> transposed = []
+ >>> for i in range(4):
+ ... # the following 3 lines implement the nested listcomp
+ ... transposed_row = []
+ ... for row in matrix:
+ ... transposed_row.append(row[i])
+ ... transposed.append(transposed_row)
+ ...
+ >>> transposed
+ [[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
-In real world, you should prefer built-in functions to complex flow statements.
+In the real world, you should prefer built-in functions to complex flow statements.
The :func:`zip` function would do a great job for this use case::
- >>> list(zip(*mat))
- [(1, 4, 7), (2, 5, 8), (3, 6, 9)]
+ >>> zip(*matrix)
+ [(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)]
See :ref:`tut-unpacking-arguments` for details on the asterisk in this line.
--
Repository URL: http://hg.python.org/cpython
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