python3 内置函数 filter()

filter(function or None, iterable) --> filter object

Return an iterator yielding those items of iterable for which function(item)
is true. If function is None, return the items that are true.

函数有两个参数，第一个可以为一个函数，或者None,第二个参数为一个可迭代对象

保返回真值(非零，非空)返回值为一个filter object

 ls = [1, 0 -1, 3]

 l =(filter(None,ls))
 print(l)

 <filter object at 0x0000023036E7A390>

filter1

这个filter　object 是个可迭代对象

 ls = [1, 0 -1, 3]

 def t(x ):
     if x > 0:
         return True
     else:
         return False

 l =(filter(t,ls))
 for i in l:
     print(i)

 1
 3

filter2