7.11. Boolean Identity

  • = assignment

  • == checks for object equality

  • is checks for object identity

  • is compares id() output for both objects

  • CPython: compares the memory address a object resides in

  • Testing strings with is only works when the strings are interned

Since Python 3.8 - Compiler produces a SyntaxWarning when identity checks (is and is not) are used with certain types of literals (e.g. str, int). These can often work by accident in CPython, but are not guaranteed by the language spec. The warning advises users to use equality tests (== and !=) instead.

7.11.1. Has Value

Has value:

>>> x = 1
>>>
>>> if x:
...     print('Has value')
Has value
>>>
>>> if x is not None:
...     print('Has value')
Has value

7.11.2. Is Empty

>>> x = None
>>>
>>> x is None
True
>>>
>>> x == None
True

Example:

>>> data = None
>>>
>>> if not data:
...     print('Empty')
Empty
>>>
>>> if data is None:
...     print('Empty')
Empty

7.11.3. Is True of False

  • True and False are singletons

  • Comparing identity is faster

  • Comparing values will yield the same result

>>> x = False
>>>
>>> x == False
True
>>>
>>> x is False
True

>>> x = True
>>>
>>> x == True
True
>>>
>>> x is True
True

Example:

>>> adult = True
>>>
>>> if adult:
...     print('Yes')
Yes
>>>
>>> if adult is True:
...     print('Yes')
Yes
>>>
>>> if adult == True:
...     print('Yes')
Yes

>>> x = True
>>>
>>> id(x)  
4385159736
>>>
>>> id(True)  
4385159736

7.11.4. Is Numeric

  • Type int caches value from -5 to 256

  • For those values identity check is True

  • For values lower than -5 or greater than 256 identity check is False

>>> x = 256
>>>
>>> x == 256
True
>>>
>>> x is 256  
<...>: SyntaxWarning: "is" with a literal. Did you mean "=="?
True

>>> x = 257
>>>
>>> x == 257
True
>>>
>>> x is 257  
<...>: SyntaxWarning: "is" with a literal. Did you mean "=="?
False

7.11.5. Is String

  • String instances differs

  • You cannot compare their identity

  • There is a caching mechanism in Python, which sometimes yield the same result

  • In order to compare strings, you should compare their values, not identities

>>> name = 'Mark Watney'
>>>
>>> name == 'Mark Watney'
True
>>> name is 'Mark Watney'  
<...>: SyntaxWarning: "is" with a literal. Did you mean "=="?
True

7.11.6. Is Type or Instance

  • type()

  • isinstance()

Int:

>>> x = 1
>>>
>>> type(x) is int
True
>>>
>>> isinstance(x, int)
True

Float:

>>> x = 1.0
>>>
>>> type(x) is float
True
>>>
>>> isinstance(x, float)
True

Numeric:

>>> x = 1.0
>>>
>>> type(x) in (int, float)
True
>>>
>>> isinstance(x, int | float)
True

Bool:

>>> x = True
>>>
>>> type(x) is bool
True
>>>
>>> isinstance(x, bool)
True

>>> x = True
>>>
>>> type(x) is int
False
>>>
>>> isinstance(x, int)
True
>>>
>>> bool.mro()  # bool inherits from int
[<class 'bool'>, <class 'int'>, <class 'object'>]

String:

>>> x = 'Mark Watney'
>>>
>>> type(x) is str
True
>>>
>>> isinstance(x, str)
True

List:

>>> x = [1, 2, 3]
>>>
>>> type(x) is list
True
>>>
>>> isinstance(x, list)
True

Iterable:

>>> x = [1, 2, 3]
>>>
>>> type(x) in (list, tuple, set)
True
>>>
>>> isinstance(x, list | tuple | set)
True

Dict:

>>> x = {'firstname': 'Mark', 'lastname': 'Watney'}
>>>
>>> type(x) is dict
True
>>>
>>> isinstance(x, dict)
True

7.11.7. Performance

  • Tested on Python 3.11.2

SetUp:

>>> x = True

Value comparison:

>>> %%timeit -r 1000 -n 1000  
... x == True
...
52 ns ± 23.8 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)

Identity check:

>>> %%timeit -r 1000 -n 1000  
... x is True
...
39.5 ns ± 18.7 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)