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An Introduction to Python
by Guido van Rossum and Fred L. Drake, Jr.
Paperback (6"x9"), 124 pages
ISBN 0954161769
RRP £12.95 ($19.95)

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5.1.4 List Comprehensions

List comprehensions provide a concise way to create lists without resorting to use of map(), filter() and/or 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 for clause, then zero or more for or if clauses. The result will be a list resulting from evaluating the expression in the context of the for and if clauses which follow it. If the expression would evaluate to a tuple, it must be parenthesized.

    >>> freshfruit = ['  banana', '  loganberry ', 'plum  ']
    >>> [weapon.strip() for weapon in freshfruit]
    ['banana', 'loganberry', 'plum']
    >>> 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 - need () for tuples
      File "<stdin>", line 1, in ?
        [x, x**2 for x in vec]
                   ^
    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]

List comprehensions are much more flexible than map() and can be applied to complex expressions and nested functions:

    >>> [str(round(355/113.0, i)) for i in range(1,6)]
    ['3.1', '3.14', '3.142', '3.1416', '3.14159']
ISBN 0954161769An Introduction to PythonSee the print edition