5.12. OOP Object Identity

5.12.1. Rationale

  • = assignment

  • == checks for object equality

  • is checks for object identity

found = True
found == True
found is True

5.12.2. 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'

5.12.3. Value Comparison

  • == checks for object equality

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

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

a == b
# True

5.12.4. 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.

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

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

a is b

name = None

name is None
name is True
name is False

name = None

type(name) is int
type(name) is float
type(name) is complex
type(name) is bool
type(name) is None
type(name) is str
type(name) is bytes
type(name) is list
type(name) is tuple
type(name) is set
type(name) is frozenset
type(name) is dict

5.12.5. Problem

'Mark Watney' is 'Mark Watney'
# True

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

>>> a == b
True

>>> a is b
True

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

>>> a == b
True

>>> a is b
False

5.12.6. Compare Value and Identity

name = 'Mark Watney'
expected = 'Mark Watney'

name == expected
# True

name is expected
# False

name == 'Mark Watney'
# True

name is 'Mark Watney'
# False

5.12.7. Assignments