python中使用if not x 語句用法
阿新 • • 發佈：2018-11-04
在Python中，None、空列表[]、空字典{}、空元組()、0等一系列代表空和無的物件會被轉換成False。除此之外的其它物件都會被轉化成True。
#!/usr/bin/python
# -*- coding: UTF-8 -*-
######測試if not########
x=0
#x='aa'
#x=[]
if x is None:
print("x in None!")
if not x:
print('not x!')
if not x is None:
print('not x is None!')
if x is not None:
print('x is not None!')
y=1
if y is not None:
print('y is not None!')
if not y:
print('not y')上面會輸出：
not x! not x is None! x is not None! y is not None!
看下面程式碼
>>> x=0 >>> not x True >>> x is not None True >>> not x is None True >>> >>> >>> x=156 >>> not x False >>> x is not None True >>> not x is None True >>> >>>
if not 有三種表達方式
第一種是`if x is None`；
第二種是 `if not x：`；
第三種是`if not x is None`（這句這樣理解更清晰`if not (x is None)`）
注意：[]不等於None型別，也就是x==[]和x==None
重點看下面例子：
>>> x=[] >>> y='' >>> z=0 >>> w=None >>> x is None False >>> y is None False >>> z is None False >>> w is None True >>> not x True >>> not y True >>> not z True >>> not w True >>> not x is None True >>> not y is None True >>> not z is None True >>> not z is None True >>> not w is None False