5.8. Object Identity¶

5.8.1. Rationale¶

== checks for object equality
is checks for object identity

found = True

found is True
found == True

5.8.2. Identity¶

id(obj) -> int
id() will change every time you execute script
id() returns an integer which is guaranteed to be unique and constant for object during its lifetime
Two objects with non-overlapping lifetimes may have the same id() value
In CPython it's also the memory address of the corresponding C object

id('Watney')
# 4499664048

hex(id('Watney'))
# '0x10c336cb0'

5.8.3. Compare Value and Identity¶

>>> a = 'Mark Watney'
>>> b = 'Mark Watney'

>>> a == b
True
>>> a is b
False

>>> a = 'Mark Watney'
... b = 'Mark Watney'

>>> a == b
True
>>> a is b
True

5.8.4. Value Comparison¶

== checks for object equality

'Mark Watney' == 'Mark Watney'
# True

5.8.5. Identity Check¶

is checks for object identity
Compares id() output for both objects
CPython: compares the memory address a object resides in
Is used for checking if None
Testing strings with is only works when the strings are interned

Changed in version 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.

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

name = None

if name is None:
    print('Your name is empty')
else:
    print(f'Hello {name}')

5.8.6. Assignments¶

Todo

Create assignments