dict.keys()
Python 中字典(Dictionary) ， keys() 函数返回一个字典所有的键。

Python2.x和Python3.x有所不同：

在python2.x中，dict.keys()返回一个列表

~ $  /usr/local/python2/bin/python     
Python 2.7.15 (default, Feb 21 2019, 10:58:06) 
[GCC 4.8.5 20150623 (Red Hat 4.8.5-36)] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> data={'project': '4', 'put_zone': '12', 'system': '28'}
>>> data.keys()
['project', 'put_zone', 'system']
>>> type(data.keys())
<type 'list'>

在python3.x中，dict.keys()返回一个dict_keys对象，比起列表，这个对象的行为更像是集合set，而不是列表list。

~ $  /usr/local/python3/bin/python3
Python 3.7.2 (default, Feb 21 2019, 10:55:58) 
[GCC 4.8.5 20150623 (Red Hat 4.8.5-36)] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> data={'project': '4', 'put_zone': '12', 'system': '28'}
>>> data.keys()
dict_keys(['project', 'put_zone', 'system'])
>>> type(data.keys())
<class 'dict_keys'>

这种情况会导致在for循环中对字典做增删时，导致运行时错误。

~ $  vi t.py
import os
data={'project': '4', 'put_zone': '12', 'system': '28'}
for x in data.keys():
    print(x)
    if x == 'system':
        data.pop('system')
print(data.keys())

~ $  /usr/local/python2/bin/python t.py
project
put_zone
system
['project', 'put_zone']

~ $  /usr/local/python3/bin/python t.py
project
put_zone
system
Traceback (most recent call last):
  File "t.py", line 4, in <module>
    for x in data.keys():
RuntimeError: dictionary changed size during iteration

解决方案：list(dict.keys())

~ $  vi t.py 
import os

data={'project': '4', 'put_zone': '12', 'system': '28'}
for x in list(data.keys()):
    print(x)
    if x == 'system':
        data.pop('system')
print(data.keys())

~ $  /usr/local/python3/bin/python3 t.py
project
put_zone
system
dict_keys(['project', 'put_zone'])