英文:
python "and" evaluation sequence
问题
问题：为什么像(True and a)这样的表达式不会在括号内完全求值？
在Python中，逻辑运算符（如and）是短路运算符，它们只在需要时才会计算表达式的值。对于and运算符，如果第一个表达式是False，则整个表达式将返回False，并且不会计算第二个表达式的值。因此，如果第一个表达式是True，则会继续计算第二个表达式。
在你的示例中，(True and a)，由于第一个表达式是True，Python会继续计算第二个表达式a的值，而不会短路。这与你的预期不同，因为你可能期望由于第一个表达式已经为True，所以整个表达式会直接返回True，而不会计算第二个表达式。
这是Python的设计决定，以便在逻辑表达式中能够正确处理各种情况。如果你想要在第一个表达式为True时直接返回True，可以使用条件表达式或者其他方法来实现你的预期行为。
英文:
I am trying to understand how python keywords and operators interact with object dunder methods and have encounter a situation I don't quite understand.
Setup:
I created two simple classes with the __bool__ dunder method that I believe should be used when checking for truthy conditions. I have them always return true which should be the same behavior as if the method was not defined on the class, but I have added a print statement so I can see when the method is called.
class ClassA(object):
def __bool__(self):
print('__bool__ check classA')
return True
class ClassB(object):
def __bool__(self):
print('__bool__ check classB')
return True
Test 1:
if a and b: print(True)
The output is what I expected to see:
__bool__ check classA
__bool__ check classB
True
Test 2:
c = (a and b)
print(c)
The output is NOT what I expected to see:
__bool__ check classA
<__main__.ClassB object at 0x0000020ECC6E7F10>
My best guess is that Python is evaluating the logic from left to right, but doesn't call the __bool__ method on the final object until needed. (I don't understand why, but I think that is what is happening)
Test 3:
c = (a and b and a)
print(c)
This output agrees with my assumption.
__bool__ check classA
__bool__ check classB
<__main__.ClassA object at 0x0000026A81FA7FD0>
Test 4:
c = (a and b and a)
if c: print(True)
Further calling if c then evaluates the check on the object.
__bool__ check classA
__bool__ check classB
__bool__ check classA
True
Question
Why doesn't an expression such as (True and a) fully evaluate inside the parenthesis?
答案1
得分: 3
表达式 a and b 如果 a 为假，则返回左操作数，如果 a 为真，则返回右操作数。不需要评估 b 的布尔值。
然而，在 if 语句中使用时，有必要检查整个表达式是否产生一个真值。
英文:
The expression a and b returns the left hand operand if a is falsy and returns the right hand operand if a is truthy. There is no need to evaluate the boolean value of b.
However, when used in an if statement, it is necessary to check if the entire expression gives a truthy value.
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