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(python2.1-tut.info)List Comprehensions

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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
following 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 ', '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]
     [[2, 4], [4, 16], [6, 36]]
     >>> [x, x**2 for x in vec]	# error - parens required for tuples
       File "<stdin>", line 1
         [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]