（x不是None）和（y不是None）和（x和y）之间的python区别不是None

Question

The following conditions give the same results. Are there any performance (or any other) differences between them ?

1.

if (x != None) and (y != None):
    # Proceed

2.

if (x and y) is not None:
    # Proceed

Answer 1

In [44]: x = False

In [45]: y = None

In [46]: (x != None) and (y != None)
Out[46]: False

In [47]: (x and y) is not None
Out[47]: True

---

By the way, when testing if something is None, it is better to test

if x is None

rather than

if x == None

The result can be different:

class Egalitarian(object):
    def __eq__(self, other):
        return True

x = Egalitarian()

print(x == None)
# True

print(x is None)
# False

Answer 2

A programmer's wife asks him: do you want sausage or bacon for breakfast? - Yes, he responds. (a true story).

When using boolean constructs in everyday speaking we tend to skip common parts so that X(Y) @ X(Z) becomes X @ (Y,Z) :

Her bunny is happy and her hamster is happy (1)

is the same as

Her bunny and her hamster are happy

In programming, however, we cannot contract things like that. The first statement

bunny == happy and hamster == happy

will be true if both pets are fine. The second phrase translates literally to this:

(bunny and hamster) == happy 

here, (bunny and hamster) evaluates to

- a falsy value, if she's got no bunny
- the hamster otherwise

So (bunny and hamster) == happy actually reads as:

She's got a bunny and her hamster is happy

which is quite different from the statement (1)

In python:

>>> happy = 1
>>> sad = 2
>>> bunny = sad
>>> hamster = happy
>>> bunny == happy and hamster == happy
False
>>> (bunny and hamster) == happy
True
>>> 

Answer 3

You need to understand what it is you are doing to understand the real differences, you might think the following is a simple statement:

>>> if (x and y) is not None:

It is not a simple conditional statement. Here, you are first evaluating (x and y) which will return the last Truthy value it finds, or the first Falsey value it finds, and then compare that to is not None . Which will yield sometimes unexpected results. (Consult The Peculiar Nature of and and or )

If you want to check to make sure that both x and y are not None you should do something like the following:

>>> if all(var is not None for var in [x, y]):

Which is almost the same as:

>>> if x is not None and y is not None:

Only a bit easier to read, and more robust (if you add more variables to check),

---

Either way, the difference between your two statements are that the first one will actually do what you expect, but you should be using is not instead of != when it comes to None . And the second statement might work sometimes, but is doing something that you probably were not expecting.

Answer 4

Usually

if x and y:
    #Proceed

is good enough in the Python world.

Here I says good enough because in most cases a function should not return False and None while they have different meanings in the function logic.

For those downvoters, seriously will you write the following codes in real world?

if x is None:
    #xxx
elif x == False:
    #xxxx
else:
    #xxx

I just want to point out that the both example are not pythonic and should be avoided.

Answer 5

"is" and "is not" are fast as they check identity (ie comparing pointers/references)

The other comparisons and logical operators will inspect the object's values (or truthiness).

Generally to check for "None" values, you therefore want to use "is"/"is not".

Personally I find that relying on the behaviour of None values in an expression is bad form (things that would often trigger NullPointerException or similar errors in many other languages), so while I'm not completely sure what is you are trying to achieve, generally similar code I write looks like:

if x is not None and y is not None: