It is standard convention to use if foo is None
rather than if foo == None
to test if a value is specifically None
.
If you want to determine whether a value is exactly True
(not just a true-like value), is there any reason to use if foo == True
rather than if foo is True
? Does this vary between implementations such as CPython (2.x and 3.x), Jython, PyPy, etc.?
Example: say True
is used as a singleton value that you want to differentiate from the value 'bar'
, or any other true-like value:
if foo is True: # vs foo == True
...
elif foo == 'bar':
...
Is there a case where using if foo is True
would yield different results from if foo == True
?
NOTE: I am aware of Python booleans - if x:, vs if x == True, vs if x is True. However, it only addresses whether if foo
, if foo == True
, or if foo is True
should generally be used to determine whether foo
has a true-like value.
UPDATE: According to PEP 285 § Specification:
The values False and True will be singletons, like None.
If you want to determine whether a value is exactly True (not just a true-like value), is there any reason to use if foo == True rather than if foo is True?
If you want to make sure that foo
really is a boolean and of value True
, use the is
operator.
Otherwise, if the type of foo
implements its own __eq__()
that returns a true-ish value when comparing to True
, you might end up with an unexpected result.
As a rule of thumb, you should always use is
with the built-in constants True
, False
and None
.
Does this vary between implementations such as CPython (2.x and 3.x), Jython, PyPy, etc.?
In theory, is
will be faster than ==
since the latter must honor types' custom __eq__
implementations, while is
can directly compare object identities (e.g., memory addresses).
I don't know the source code of the various Python implementations by heart, but I assume that most of them can optimize that by using some internal flags for the existence of magic methods, so I suspect that you won't notice the speed difference in practice.