问题
In [20]: print None or False
-------> print(None or False)
False
In [21]: print False or None
-------> print(False or None)
None
This behaviour confuses me. Could someone explain to me why is this happening like this? I expected them to both behave the same.
回答1:
The expression x or y evaluates to x if x is true, or y if x is false.
Note that "true" and "false" in the above sentence are talking about "truthiness", not the fixed values True and False. Something that is "true" makes an if statement succeed; something that's "false" makes it fail. "false" values include False, None, 0 and [] (an empty list).
回答2:
The "or" operator returns the value of its first operand, if that value is true in the Pythonic boolean sense (aka its "truthiness"), otherwise it returns the value of its second operand, whatever it happens to be. See the subsection titled Boolean operations in the section on Expressions in the current online documentation.
In both your examples, the first operand is considered false, so the value of the second one becomes the result of the evaluating the expression.
回答3:
You must realize that None, False and True are all singletons.
For example, if foo is not None means that foo has some value other than None. This works the same as just having if foo which is basically if foo == True.
So, not None and True work the same way. Also, None and False work the same way.
>>> foo = not None
>>> bool(foo)
True
>>> foo = 5 # Giving an arbitrary value here
>>> bool(foo)
True
>>> foo = None
>>> bool(foo)
False
>>> foo = 5 # Giving an arbitrary value here
>>> bool(foo)
True
The important thing to realize and to be aware of when coding is that when comparing two things, None needs is, but True and False need ==. Avoid if foo == None and do only if foo is None and avoid if foo != None and do only if foo is not None. In the case of if foo is not None, simply do if foo. In the case of if foo is None, simply do if not foo.
Note: True is basically 1 and False is basically 0. In the old days of Python, we had only 1 for a value of true and we had 0 for a value of false. It's more understandable and human-friendly to say True instead of 1 and more understandable and human-friendly to say False instead of 0.
回答4:
A closely related topic: Python's or and and short-circuit. In a logical or operation, if any argument is true, then the whole thing will be true and nothing else needs to be evaluated; Python promptly returns that "true" value. If it finishes and nothing was true, it returns the last argument it handled, which will be a "false" value.
and is the opposite, if it sees any false values, it will promptly exit with that "false" value, or if it gets through it all, returns the final "true" value.
>>> 1 or 2 # first value TRUE, second value doesn't matter
1
>>> 1 and 2 # first value TRUE, second value might matter
2
>>> 0 or 0.0 # first value FALSE, second value might matter
0.0
>>> 0 and 0.0 # first value FALSE, second value doesn't matter
0
回答5:
From a boolean point of view they both behave the same, both return a value that evaluates to false.
or just "reuses" the values that it is given, returning the left one if that was true and the right one otherwise.
回答6:
Condition1 or Condition2
if Condition1 is False then evalute and return Condition2. None evalutes to False.
来源：https://stackoverflow.com/questions/3914667/false-or-none-vs-none-or-false