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This means that when there are multiple :keyword:`for`...\ :keyword:`in` clauses, the resulting list will be equal to the product of the lengths of all the sequences. If you have two lists of length 3, the output list is 9 elements long::
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seq1 = 'abc' seq2 = (1,2,3) >>> [ (x,y) for x in seq1 for y in seq2] [('a', 1), ('a', 2), ('a', 3), ('b', 1), ('b', 2), ('b', 3), ('c', 1), ('c', 2), ('c', 3)]
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To avoid introducing an ambiguity into Python's grammar, if *expression* is creating a tuple, it must be surrounded with parentheses. The first list comprehension below is a syntax error, while the second one is correct::
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# Syntax error [ x,y for x in seq1 for y in seq2] # Correct [ (x,y) for x in seq1 for y in seq2]
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The idea of list comprehensions originally comes from the functional programming language Haskell (http://www.haskell.org). Greg Ewing argued most effectively for adding them to Python and wrote the initial list comprehension patch, which was then discussed for a seemingly endless time on the python-dev mailing list and kept up-to-date by Skip Montanaro.
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Augmented assignment operators, another long-requested feature, have been added to Python 2.0. Augmented assignment operators include ``+=``, ``-=``, ``*=``, and so forth. For example, the statement ``a += 2`` increments the value of the variable ``a`` by 2, equivalent to the slightly lengthier ``a = a + 2``.
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The full list of supported assignment operators is ``+=``, ``-=``, ``*=``, ``/=``, ``%=``, ``**=``, ``&=``, ``|=``, ``^=``, ``>>=``, and ``<<=``. Python classes can override the augmented assignment operators by defining methods named :meth:`__iadd__`, :meth:`__isub__`, etc. For example, the following :class:`Number` class stores a number and supports using += to create a new instance with an incremented value.
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class Number: def __init__(self, value): self.value = value def __iadd__(self, increment): return Number( self.value + increment) n = Number(5) n += 3 print n.value
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The :meth:`__iadd__` special method is called with the value of the increment, and should return a new instance with an appropriately modified value; this return value is bound as the new value of the variable on the left-hand side.
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