奇怪的 if 语句

我发现这很奇怪if-别人代码中的语句：

if variable & 1 == 0:

我不明白。它应该有两个==, right?

有人可以解释一下吗？

条件式是一个按位运算符 https://wiki.python.org/moin/BitwiseOperators比较：

>>> 1 & 1
1
>>> 0 & 1
0
>>> a = 1
>>> a & 1 == 0
False
>>> b = 0
>>> b & 1 == 0
True

正如许多评论所说，对于整数，此条件对于偶数为 True，对于奇数为 False。普遍的写法是if variable % 2 == 0: or if not variable % 2:

Using timeit我们可以看到，性能上没有太大差异。

n & 1("== 0" and "not")

>>> timeit.Timer("bitwiseIsEven(1)", "def bitwiseIsEven(n): return n & 1 == 0").repeat(4, 10**6)
[0.2037370204925537, 0.20333600044250488, 0.2028651237487793, 0.20192503929138184]

>>> timeit.Timer("bitwiseIsEven(1)", "def bitwiseIsEven(n): return not n & 1").repeat(4, 10**6)
[0.18392395973205566, 0.18273091316223145, 0.1830739974975586, 0.18445897102355957]

n % 2("== 0" and "not")

>>> timeit.Timer("modIsEven(1)", "def modIsEven(n): return n % 2 == 0").repeat(4, 10**6)
[0.22193098068237305, 0.22170782089233398, 0.21924591064453125, 0.21947598457336426]

>>> timeit.Timer("modIsEven(1)", "def modIsEven(n): return not n % 2").repeat(4, 10**6)
[0.20426011085510254, 0.2046220302581787, 0.2040550708770752, 0.2044820785522461]

重载运算符：

这俩% and &运算符超载。

按位与运算符重载为set https://docs.python.org/2/library/sets.html. s.intersection(t)相当于s & t并返回一个“具有 s 和 t 共有元素的新集合”。

>>> {1} & {1}
set([1])

这不会影响我们的条件：

>>> def bitwiseIsEven(n):
...   return n & 1 == 0

>>> bitwiseIsEven('1')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in bitwiseIsEven
TypeError: unsupported operand type(s) for &: 'str' and 'int'
>>> bitwiseIsEven({1})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in bitwiseIsEven
TypeError: unsupported operand type(s) for &: 'set' and 'int'

模运算符也会抛出TypeError: unsupported operand type(s)对于大多数非整数。

>>> def modIsEven(n):
...   return n % 2 == 0

>>> modIsEven({1})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in modIsEven
TypeError: unsupported operand type(s) for %: 'set' and 'int'

它被重载为旧版本的字符串插值运算符%-formatting https://docs.python.org/2/library/stdtypes.html#string-formatting。它抛出TypeError: not all arguments converted during string formatting如果使用字符串进行比较。

>>> modIsEven('1')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 2, in modIsEven
TypeError: not all arguments converted during string formatting

如果字符串包含有效的转换说明符，则不会抛出此错误。

>>> modIsEven('%d')
False