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')