Python支持布尔表达式中的短路吗?
是的,and
和or
运营商都发生短路 - 请参阅the docs。
and
, or
:让我们首先定义一个有用的函数来确定是否执行某些操作。一个接受参数的简单函数,打印一条消息并返回输入,不变。
>>> def fun(i):
... print "executed"
... return i
...
在下面的例子中,可以观察到Python's short-circuiting behavior,and
算子的or
:
>>> fun(1)
executed
1
>>> 1 or fun(1) # due to short-circuiting "executed" not printed
1
>>> 1 and fun(1) # fun(1) called and "executed" printed
executed
1
>>> 0 and fun(1) # due to short-circuiting "executed" not printed
0
注意:解释器认为以下值表示错误:
False None 0 "" () [] {}
any()
, all()
:Python的any()
和all()
函数也支持短路。如文档中所示;他们按顺序评估序列的每个元素,直到找到允许在评估中提前退出的结果。请考虑以下示例来理解两者。
函数any()
检查是否有任何元素为True。一旦遇到True,它就会停止执行并返回True。
>>> any(fun(i) for i in [1, 2, 3, 4]) # bool(1) = True
executed
True
>>> any(fun(i) for i in [0, 2, 3, 4])
executed # bool(0) = False
executed # bool(2) = True
True
>>> any(fun(i) for i in [0, 0, 3, 4])
executed
executed
executed
True
函数all()
检查所有元素是否为True并在遇到False时立即停止执行:
>>> all(fun(i) for i in [0, 0, 3, 4])
executed
False
>>> all(fun(i) for i in [1, 0, 3, 4])
executed
executed
False
另外,在Python中
Comparisons can be chained arbitrarily;例如,
x < y <= z
相当于x < y and y <= z
,除了y
仅被评估一次(但在两种情况下,当发现z
为假时,x < y
根本没有被评估)。
>>> 5 > 6 > fun(3) # same as: 5 > 6 and 6 > fun(3)
False # 5 > 6 is False so fun() not called and "executed" NOT printed
>>> 5 < 6 > fun(3) # 5 < 6 is True
executed # fun(3) called and "executed" printed
True
>>> 4 <= 6 > fun(7) # 4 <= 6 is True
executed # fun(3) called and "executed" printed
False
>>> 5 < fun(6) < 3 # only prints "executed" once
executed
False
>>> 5 < fun(6) and fun(6) < 3 # prints "executed" twice, because the second part executes it again
executed
executed
False
编辑:
还有一点需要注意: - Python中的逻辑and
, or
运算符返回操作数的值而不是布尔值(True
或False
)。例如:
操作
x and y
给出结果if x is false, then x, else y
与其他语言不同, &&
,C中的||
运算符返回0或1。
例子:
>>> 3 and 5 # Second operand evaluated and returned
5
>>> 3 and ()
()
>>> () and 5 # Second operand NOT evaluated as first operand () is false
() # so first operand returned
类似地,or
运算符返回最左边的值bool(value)
== True
其他最正确的值(根据短路行为),示例:
>>> 2 or 5 # left most operand bool(2) == True
2
>>> 0 or 5 # bool(0) == False and bool(5) == True
5
>>> 0 or ()
()
那么,这有用吗? Practical Python提供的一个示例用法由Magnus Lie Hetland提供:
假设用户应该输入他或她的名字,但可以选择不输入任何内容,在这种情况下你想使用默认值'<unknown>'
。您可以使用if语句,但您也可以非常简洁地声明:
In [171]: name = raw_input('Enter Name: ') or '<Unkown>'
Enter Name:
In [172]: name
Out[172]: '<Unkown>'
换句话说,如果raw_input的返回值为true(不是空字符串),则将其分配给name(没有任何更改);否则,默认的'<unknown>'
被分配给name
。
是。在python解释器中尝试以下操作:
和
>>>False and 3/0
False
>>>True and 3/0
ZeroDivisionError: integer division or modulo by zero
要么
>>>True or 3/0
True
>>>False or 3/0
ZeroDivisionError: integer division or modulo by zero