#13549: improve tutorial section about listcomps.
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Ezio Melotti
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@ -163,107 +163,137 @@ have fast appends and pops from both ends. For example::
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List Comprehensions
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-------------------
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List comprehensions provide a concise way to create lists from sequences.
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Common applications are to make lists where each element is the result of
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some operations applied to each member of the sequence, or to create a
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subsequence of those elements that satisfy a certain condition.
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List comprehensions provide a concise way to create lists.
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Common applications are to make new lists where each element is the result of
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some operations applied to each member of another sequence or iterable, or to
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create a subsequence of those elements that satisfy a certain condition.
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For example, assume we want to create a list of squares, like::
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>>> squares = []
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>>> for x in range(10):
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... squares.append(x**2)
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...
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>>> squares
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[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
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We can obtain the same result with::
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squares = [x**2 for x in range(10)]
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This is also equivalent to ``squares = map(lambda x: x**2, range(10))``,
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but it's more concise and readable.
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A list comprehension consists of brackets containing an expression followed
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by a :keyword:`for` clause, then zero or more :keyword:`for` or :keyword:`if`
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clauses. The result will be a list resulting from evaluating the expression in
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the context of the :keyword:`for` and :keyword:`if` clauses which follow it. If
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the expression would evaluate to a tuple, it must be parenthesized.
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clauses. The result will be a new list resulting from evaluating the expression
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in the context of the :keyword:`for` and :keyword:`if` clauses which follow it.
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For example, this listcomp combines the elements of two lists if they are not
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equal::
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Here we take a list of numbers and return a list of three times each number::
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>>> [(x, y) for x in [1,2,3] for y in [3,1,4] if x != y]
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[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
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>>> vec = [2, 4, 6]
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>>> [3*x for x in vec]
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[6, 12, 18]
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and it's equivalent to::
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Now we get a little fancier::
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>>> combs = []
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>>> for x in [1,2,3]:
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... for y in [3,1,4]:
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... if x != y:
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... combs.append((x, y))
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...
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>>> combs
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[(1, 3), (1, 4), (2, 3), (2, 1), (2, 4), (3, 1), (3, 4)]
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>>> [[x, x**2] for x in vec]
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[[2, 4], [4, 16], [6, 36]]
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Note how the order of the :keyword:`for` and :keyword:`if` statements is the
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same in both these snippets.
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Here we apply a method call to each item in a sequence::
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If the expression is a tuple (e.g. the ``(x, y)`` in the previous example),
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it must be parenthesized. ::
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>>> vec = [-4, -2, 0, 2, 4]
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>>> # create a new list with the values doubled
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>>> [x*2 for x in vec]
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[-8, -4, 0, 4, 8]
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>>> # filter the list to exclude negative numbers
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>>> [x for x in vec if x >= 0]
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[0, 2, 4]
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>>> # apply a function to all the elements
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>>> [abs(x) for x in vec]
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[4, 2, 0, 2, 4]
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>>> # call a method on each element
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>>> freshfruit = [' banana', ' loganberry ', 'passion fruit ']
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>>> [weapon.strip() for weapon in freshfruit]
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['banana', 'loganberry', 'passion fruit']
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Using the :keyword:`if` clause we can filter the stream::
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>>> [3*x for x in vec if x > 3]
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[12, 18]
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>>> [3*x for x in vec if x < 2]
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[]
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Tuples can often be created without their parentheses, but not here::
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>>> [x, x**2 for x in vec] # error - parens required for tuples
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>>> # create a list of 2-tuples like (number, square)
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>>> [(x, x**2) for x in range(6)]
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[(0, 0), (1, 1), (2, 4), (3, 9), (4, 16), (5, 25)]
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>>> # the tuple must be parenthesized, otherwise an error is raised
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>>> [x, x**2 for x in range(6)]
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File "<stdin>", line 1, in ?
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[x, x**2 for x in vec]
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[x, x**2 for x in range(6)]
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^
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SyntaxError: invalid syntax
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>>> [(x, x**2) for x in vec]
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[(2, 4), (4, 16), (6, 36)]
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>>> # flatten a list using a listcomp with two 'for'
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>>> vec = [[1,2,3], [4,5,6], [7,8,9]]
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>>> [num for elem in vec for num in elem]
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[1, 2, 3, 4, 5, 6, 7, 8, 9]
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Here are some nested for loops and other fancy behavior::
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List comprehensions can contain complex expressions and nested functions::
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>>> vec1 = [2, 4, 6]
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>>> vec2 = [4, 3, -9]
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>>> [x*y for x in vec1 for y in vec2]
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[8, 6, -18, 16, 12, -36, 24, 18, -54]
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>>> [x+y for x in vec1 for y in vec2]
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[6, 5, -7, 8, 7, -5, 10, 9, -3]
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>>> [vec1[i]*vec2[i] for i in range(len(vec1))]
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[8, 12, -54]
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List comprehensions can be applied to complex expressions and nested functions::
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>>> [str(round(355/113, i)) for i in range(1, 6)]
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>>> from math import pi
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>>> [str(round(pi, i)) for i in range(1, 6)]
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['3.1', '3.14', '3.142', '3.1416', '3.14159']
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Nested List Comprehensions
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--------------------------
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If you've got the stomach for it, list comprehensions can be nested. They are a
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powerful tool but -- like all powerful tools -- they need to be used carefully,
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if at all.
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The initial expression in a list comprehension can be any arbitrary expression,
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including another list comprehension.
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Consider the following example of a 3x3 matrix held as a list containing three
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lists, one list per row::
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Consider the following example of a 3x4 matrix implemented as a list of
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3 lists of length 4::
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>>> mat = [
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... [1, 2, 3],
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... [4, 5, 6],
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... [7, 8, 9],
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... ]
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>>> matrix = [
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... [1, 2, 3, 4],
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... [5, 6, 7, 8],
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... [9, 10, 11, 12],
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... ]
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Now, if you wanted to swap rows and columns, you could use a list
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comprehension::
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The following list comprehension will transpose rows and columns::
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>>> print([[row[i] for row in mat] for i in [0, 1, 2]])
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[[1, 4, 7], [2, 5, 8], [3, 6, 9]]
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>>> [[row[i] for row in matrix] for i in range(4)]
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[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
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Special care has to be taken for the *nested* list comprehension:
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As we saw in the previous section, the nested listcomp is evaluated in
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the context of the :keyword:`for` that follows it, so this example is
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equivalent to::
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To avoid apprehension when nesting list comprehensions, read from right to
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left.
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>>> transposed = []
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>>> for i in range(4):
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... transposed.append([row[i] for row in matrix])
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...
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>>> transposed
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[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
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A more verbose version of this snippet shows the flow explicitly::
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which, in turn, is the same as::
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for i in [0, 1, 2]:
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for row in mat:
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print(row[i], end="")
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print()
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>>> transposed = []
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>>> for i in range(4):
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... # the following 3 lines implement the nested listcomp
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... transposed_row = []
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... for row in matrix:
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... transposed_row.append(row[i])
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... transposed.append(transposed_row)
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...
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>>> transposed
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[[1, 5, 9], [2, 6, 10], [3, 7, 11], [4, 8, 12]]
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In real world, you should prefer built-in functions to complex flow statements.
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In the real world, you should prefer built-in functions to complex flow statements.
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The :func:`zip` function would do a great job for this use case::
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>>> list(zip(*mat))
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[(1, 4, 7), (2, 5, 8), (3, 6, 9)]
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>>> zip(*matrix)
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[(1, 5, 9), (2, 6, 10), (3, 7, 11), (4, 8, 12)]
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See :ref:`tut-unpacking-arguments` for details on the asterisk in this line.
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