问题描述：
判断是否是字符串 然后将大写改为小写：
自己代码：
#-*-coding:utf-8-*-
... L1 = {'Hello','World',18,'Apple',None}
>>> [x for x in L1 if isinstance(x,str)]
['Hello', 'Apple', 'World']
>>> L2 = [x for x in L1 if isinstance(x,str)]
>>> [x.lower() for x in L2]
['hello', 'apple', 'world']
别人参考代码：
L2 = [s.lower() for s in L1 if isinstance(s, str)]
>>> print(L2)
['hello', 'apple', 'world']
#!/usr/bin/env python3
# -*- coding: utf-8 -*-
from collections import Iterable, Iterator
def g():
yield 1
yield 2
yield 3
print('Iterable? [1, 2, 3]:', isinstance([1, 2, 3], Iterable))
print('Iterable? \'abc\':', isinstance('abc', Iterable))
print('Iterable? 123:', isinstance(123, Iterable))
print('Iterable? g():', isinstance(g(), Iterable))
print('Iterator? [1, 2, 3]:', isinstance([1, 2, 3], Iterator))
print('Iterator? iter([1, 2, 3]):', isinstance(iter([1, 2, 3]), Iterator))
print('Iterator? \'abc\':', isinstance('abc', Iterator))
print('Iterator? 123:', isinstance(123, Iterator))
print('Iterator? g():', isinstance(g(), Iterator))
# iter list:
print('for x in [1, 2, 3, 4, 5]:')
for x in [1, 2, 3, 4, 5]:
print(x)
print('for x in iter([1, 2, 3, 4, 5]):')
for x in iter([1, 2, 3, 4, 5]):
print(x)
print('next():')
it = iter([1, 2, 3, 4, 5])
print(next(it))
print(next(it))
print(next(it))
print(next(it))
print(next(it))
d = {'a': 1, 'b': 2, 'c': 3}
# iter each key:
print('iter key:', d)
for k in d.keys():
print('key:', k)
# iter each value:
print('iter value:', d)
for v in d.values():
print('value:', v)
# iter both key and value:
print('iter item:', d)
for k, v in d.items():
print('item:', k, v)
# iter list with index:
print('iter enumerate([\'A\', \'B\', \'C\']')
for i, value in enumerate(['A', 'B', 'C']):
print(i, value)
# iter complex list:
print('iter [(1, 1), (2, 4), (3, 9)]:')
for x, y in [(1, 1), (2, 4), (3, 9)]:
print(x, y)
