这个Python表达式有意义吗？

Question

I found this kind of expression several times in a python program:

if variable is not None:
    dothings(variable)

It seems strange to me, and I think that it has no more sense than:

if variable:
    dothings(variable)

Maybe I don't know Python enough, and the expression is explained somewhere?

Answer 1

variable could be 0 , or False , or [] , or () ; be 'falsy' in other words, and then the if statement would be skipped.

See Truth testing for more detail on what is considered false in a boolean context.

In short, testing if variable is not None allows variable to be anything else , including values that would otherwise be considered False in a boolean context.

Answer 2

Some values are falsy , but are not None . You may still want to dothings() to those values.

Here are the things that are falsy in Python 2. (You may want to change the '2' in the URL to a '3' to get the values for Python 3, but it didn't change much.)

Answer 3

Eg

i = 0

if i is not None:
    print i    # prints i

if i:
    print i    # prints nothing

Answer 4

In the google voice python implementation I came across this as well, their use was during a function call. The parameters are defaulted to "None" and then when the function is called they can check if the values are changed or the default. IE

def login (user=None, password=None)
  if user is None:
     user = input('Please enter your username');
  ...
  return <something>;

called by

login()

OR

login(user='cool_dude')

OR

any combination of user/password you wish.

Additionally your "update" of the logic implies that variable == true or false. That is not correct for all cases (it may work in some cases but I'm throwing them out, since it's not a general case). What you are testing by using the "NONE" logic is whether the variable contains anything besides NONE. Similar to what I was saying above, all the "login function" was doing was determining if the user passed anything, not whether the value was valid, true, etc.