python map/filter/reduce

python有几个内置的函数很有意思：map/filter/reduce，都是对一个集合进行处理，filter很容易理解用于过滤，map用于映射，reduce用于归并，这几个词还是太抽象了，下面的代码精确的说明了这几个函数的用途：

def map_imp(function, sequence) :
    if function is None: return list(sequence)
    retvals = []
    for element in sequence:
        retvals.append(function(element))
    return retvals

def reduce_imp(function, sequence) :
    arg1 = function(sequence[0])
    for arg2 in sequence[1:] :
        arg1 = function(arg1, arg2)
    return arg1

def filter_imp(function, sequence) :
    retvals = []
    for element in sequence:
        if (function is None and element) or function(element) :
            retvals.append(element)
    return retvals

对于这种逻辑性强的东西，用语言来描述是无助的，代码则要清晰的多。