diff --git a/Doc/tutorial/datastructures.rst b/Doc/tutorial/datastructures.rst --- a/Doc/tutorial/datastructures.rst +++ b/Doc/tutorial/datastructures.rst @@ -235,89 +235,139 @@ List Comprehensions ------------------- -List comprehensions provide a concise way to create lists without resorting to -use of :func:`map`, :func:`filter` and/or :keyword:`lambda`. The resulting list -definition tends often to be clearer than lists built using those constructs. -Each list comprehension consists of 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. :: +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 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:: + + >>> [(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)] + +and it's equivalent to: + + >>> 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)] + +Note how the order of the :keyword:`for` and :keyword:`if` statements is the +same in both these snippets. + +If the expression is a tuple (e.g. the ``(x, y)`` in this 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 elements >>> freshfruit = [' banana', ' loganberry ', 'passion fruit '] >>> [weapon.strip() for weapon in freshfruit] ['banana', 'loganberry', 'passion fruit'] - >>> vec = [2, 4, 6] - >>> [3*x for x in vec] - [6, 12, 18] - >>> [3*x for x in vec if x > 3] - [12, 18] - >>> [3*x for x in vec if x < 2] - [] - >>> [[x,x**2] for x in vec] - [[2, 4], [4, 16], [6, 36]] - >>> [x, x**2 for x in vec] # error - parens required for tuples - File "", line 1, in ? - [x, x**2 for x in vec] + >>> # 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 "", line 1 + [x, x**2 for x in range(6)] ^ SyntaxError: invalid syntax - >>> [(x, x**2) for x in vec] - [(2, 4), (4, 16), (6, 36)] - >>> 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] + >>> # 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] -List comprehensions are much more flexible than :func:`map` and can be applied -to complex expressions and nested functions:: +List comprehensions can contain complex expressions and nested functions:: - >>> [str(round(355/113.0, 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], - 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:: - >>> 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.