6.12. Boolean Identity

  • = assignment

  • == checks for object equality

  • is checks for object identity

6.12.1. Identity Check

  • 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.

>>> data = None
>>>
>>> data is None
True
>>> data is not None
False

Is empty:

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

>>> data = None
>>>
>>> if data is None:
...     print('Empty')
Empty

Has value:

>>> data = 1
>>>
>>> if data:
...     print('Has value')
Has value

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

6.12.2. Bool Identity

  • True and False are singletons

  • Comparing identity is faster

  • Comparing values will yield the same result

>>> name = None
>>>
>>> name is None
True
>>> name is False
False

>>> found = True
>>>
>>> found == True
True
>>> found is True
True

Example:

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

6.12.3. String Identity

  • 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

>>> a = 'Mark Watney'
>>> b = 'Mark Watney'
>>>
>>> a == b
True
>>> a is b
False

>>> 'Mark Watney' is 'Mark Watney'  
<...>:1: SyntaxWarning: "is" with a literal. Did you mean "=="?
True

6.12.4. Type Checking

>>> data = 1337
>>>
>>> if type(data) is int:
...     print('Integer')
Integer
>>>
>>> if type(data) in (int, float):
...     print('Numeric')
Numeric

>>> data = 'Mark'
>>>
>>> if type(data) is str:
...     print('String')
String

>>> data = []
>>>
>>> if type(data) is list:
...     print('List')
List
>>>
>>> if type(data) in (list, tuple, set):
...     print('Sequence')
Sequence

>>> data = {}
>>>
>>> if type(data) is dict:
...     print('Dict')
Dict