Why should you avoid using "is" and "is not"?

None

is singleton, so you can compare the identity ( is

/ is not

) object instead of equality ( ==

/ !=

) - whenever you access None

, you get the same object.

For almost all other cases, you should use equality (see, for example, Why does string comparison in Python using "==" or "is" sometimes lead to different results? ). In terms of "avoid forced type conversion" - Python is strongly typed and will never convert implicitly. Also, you cannot "distinguish" a type in Python - conversion, for example. a int

to str

creates a new separate object.

This is especially important as it is None

often used to avoid flagging a return — if False

either 0

or []

or anything else that evaluates False

-y, it is a valid return evaluating "truth" that will give false results.

def yes_or_no(s):
    """Convert 'yes' or 'no' to boolean, or implicitly return None."""
    if s.lower() == "yes":
        return True
    elif s.lower() == "no":
        return False

result = some_func("foo")

if result: # wrong, covers None and False
    ...

if result is not None: # right, covers None only
    ...

      

        
        
        
      

    

Per style guide :

Comparison with singlons of a type None

should always be done using is

or is not

, never-executed equality operators.

Also, beware of writing if x

if you really mean if x is not None

- eg. when checking if a variable or an argument was, the default for is None

set to some other value. Another value could be of a type (eg container), which could be false in a boolean context!

Note that you do n't usually compare True

or False

. Again, from the style guide:

Do not compare boolean values ​​with True

or False

with ==

.

Yes:   if greeting:
No:    if greeting == True:
Worse: if greeting is True:

      

        
        
        
      

    

See also: Using True, False and None as return values ​​in python functions