python map和reduce的用法

map(function, sequence[, sequence, ...]) -> list 

Return a list of the results of applying the function to the items of 
the argument sequence(s). If more than one sequence is given, the 
function is called with an argument list consisting of the corresponding 
item of each sequence, substituting None for missing values when not all 
sequences have the same length. If the function is None, return a list of 
the items of the sequence (or a list of tuples if more than one sequence). 

reduce(...) 
reduce(function, sequence[, initial]) -> value 

Apply a function of two arguments cumulatively to the items of a sequence, 
from left to right, so as to reduce the sequence to a single value. 
For example, reduce(lambda x, y: x+y, [1, 2, 3, 4, 5]) calculates 
((((1+2)+3)+4)+5). If initial is present, it is placed before the items 
of the sequence in the calculation, and serves as a default when the 
sequence is empty.

 

 

>>> L1 = ['a', 'b', 'c']
>>> L2 = [1, 2, 3]
>>> zip(L1, L2)
[('a', 1), ('b', 2), ('c', 3)]
>>> map(L1, L2)
Traceback (most recent call last):
  File "", line 1, in ?
TypeError: 'list' object is not callable
>>> map(None, L1, L2)
[('a', 1), ('b', 2), ('c', 3)]
>>> dict(zip(L1, L2))
{'a': 1, 'c': 3, 'b': 2}
>>> dict(map(None, L1, L2))
{'a': 1, 'c': 3, 'b': 2}

>>> L3 = []
>>> zip(L1, L3)
[]
>>> map(L1, L3)
[]
>>> map(None, L1, L3)
[('a', None), ('b', None), ('c', None)]
>>> dict(zip(L1, L3))
{}
>>> dict(map(None, L1, L3))
{'a': None, 'c': None, 'b': None}

>>> def fun(A, B):
...     if B == None: B = ''
...     return A, B
...
map(fun, L1, L2)
[('a', 1), ('b', 2), ('c', 3)
>>> map(fun, L1, L3)
[('a', ''), ('b', ''), ('c', '')]
>>> dict(map(fun, L1, L3))
{'a': '', 'c': '', 'b': ''}