以下是filter函数的官方文档，注意最后一段，当function不为None时，函数相似于[item for item in iterable if function(item)]，function为None时，函数相似于[item for item in iterable if item]。

filter(function, iterable)

Construct a list from those elements of iterable for which function returns true. iterable may be either a sequence, a container which supports iteration, or an iterator. If iterable is a string or a tuple, the result also has that type; otherwise it is always a list. If function is None, the identity function is assumed, that is, all elements of iterable that are false are removed.

Note that filter(function, iterable) is equivalent to [item for item in iterable if function(item)] if function is not None and [item for item in iterable if item] if function is None.

根据官方文档，为了方便理解，自己试着去实现了下，没有带异常处理。分析filter(function, iterable)，满足以下条件

• function, 为None, 或者函数。
• iterable, 为list, tuple, 或者basestring。

根据function生成列表，再根据iterable对列表进行相应处理。

def myFilter(func, seq):

    if func is None:

        results = [i for i in seq if i]

    else:

        results = [i for i in seq if func(i)]

    if isinstance(seq, tuple):

        results = tuple(results)

    elif isinstance(seq, basestring):

        results = ''.join(results)

    return results

运行结果与filter自建函数输出一致。

print '-------------------filter------------------------------'

print filter(lambda x: x > 4, range(10))

print filter(lambda x: x > 4, tuple(range(10)))

print filter(None, range(10))

print filter(lambda x: x > '3', '12345')

print filter(None, '01234')

print '-------------------myFilter----------------------------'

print myFilter(lambda x: x > 4, range(10))

print myFilter(lambda x: x > 4, tuple(range(10)))

print myFilter(None, range(10))

print myFilter(lambda x: x > '3', '12345')

print myFilter(None, '01234')