我发现默认的Python合成val = a if cond other b cumbersome,所以有时我这样做:

iif = lambda (cond, a, b): a if cond else b
# So I can then use it like:
val = iif(cond, a, b)

当然,它有一个缺点,总是评估双方(a和b),但合成对我来说更清楚。

做事的神秘方式:

"true" if var else "false"

但是,总是有不同的方式来做一个温和的状态:

"true" and var or "false"

你可能经常发现

cond and on_true or on_false

但这会导致一个问题,当 on_true == 0

>>> x = 0
>>> print x == 0 and 0 or 1
1
>>> x = 1
>>> print x == 0 and 0 or 1
1

在哪里你会期望这个结果为一个正常的特纳利运营商:

>>> x = 0
>>> print 0 if x == 0 else 1
0
>>> x = 1
>>> print 0 if x == 0 else 1
1

a if condition else b

这使得短循环,因为当状态是真实的,只有A被评估,而B根本不被评估,但当状态是虚假的,只有B被评估,而A根本不被评估。

例如:

>>> 'true' if True else 'false'
'true'
>>> 'true' if False else 'false'
'false'

請注意,條件是表達,而不是表達,這意味著你無法在條件表達中使用表達,如通過,或與 = (或「增加」的表達,如 +=) 的表達:

>>> pass if False else pass
  File "<stdin>", line 1
    pass if False else pass
         ^
SyntaxError: invalid syntax

>>> # Python parses this as `x = (1 if False else y) = 2`
>>> # The `(1 if False else x)` part is actually valid, but
>>> # it can't be on the left-hand side of `=`.
>>> x = 1 if False else y = 2
  File "<stdin>", line 1
SyntaxError: cannot assign to conditional expression

>>> # If we parenthesize it instead...
>>> (x = 1) if False else (y = 2)
  File "<stdin>", line 1
    (x = 1) if False else (y = 2)
       ^
SyntaxError: invalid syntax

(在 3.8 及以上, := “walrus” 操作员允许简单地将值定义为表达式,这与此同步,但请不要这样写代码,很快就会变得非常难以理解。

# Invalid syntax: we didn't specify what the value should be if the 
# condition isn't met. It doesn't matter if we can verify that
# ahead of time.
a if True

但是,您可以使用条件表达式来分配变量如下:

x = a if True else b

例如,返回值:

# Of course we should just use the standard library `max`;
# this is just for demonstration purposes.
def my_max(a, b):
    return a if a > b else b

请记住,它是由一些Pythonists为几种原因:

其他答案是正确的谈论Python操作员. 我想补充提到一个场景,操作员经常使用,但有一个更好的字符。

假设我们希望使用选项_值与默认值,如果它没有设置:

run_algorithm(option_value if option_value is not None else 10)

或者,如果选项_值从未设置为虚假值(0,“”,等),

run_algorithm(option_value if option_value else 10)

但是,在这种情况下,一个越来越好的解决方案只是写作。

run_algorithm(option_value or 10)

正如我已经回答过的那样,是的,在Python中有一个特纳里操作员:

<expression 1> if <condition> else <expression 2>

在许多情况下, < 表达 1> 也被用作 Boolean 评估的 < 条件>. 然后您可以使用短循环评估。

a = 0
b = 1

# Instead of this:
x = a if a else b
# Evaluates as 'a if bool(a) else b'

# You could use short-circuit evaluation:
x = a or b

短循环评估的一个大专业是链接超过两个表达式的可能性:

x = a or b or c or d or e

当与功能工作时,它在细节上更不同:

# Evaluating functions:
def foo(x):
    print('foo executed')
    return x


def bar(y):
    print('bar executed')
    return y


def blubb(z):
    print('blubb executed')
    return z


# Ternary Operator expression 1 equals to False
print(foo(0) if foo(0) else bar(1))
''' foo and bar are executed once
foo executed
bar executed
1
'''

# Ternary Operator expression 1 equals to True
print(foo(2) if foo(2) else bar(3))
''' foo is executed twice!
foo executed
foo executed
2
'''

# Short-circuit evaluation second equals to True
print(foo(0) or bar(1) or blubb(2))
''' blubb is not executed
foo executed
bar executed
1
'''

# Short-circuit evaluation third equals to True
print(foo(0) or bar(0) or blubb(2))
'''
foo executed
bar executed
blubb executed
2
'''

# Short-circuit evaluation all equal to False
print(foo(0) or bar(0) or blubb(0))
''' Result is 0 (from blubb(0)) because no value equals to True
foo executed
bar executed
blubb executed
0
'''

PS:当然,一个短循环评估不是一个电路运营商,但经常在短循环足够的情况下使用电路,它具有更好的可读性,可以连锁。