5.3. OOP Identity

  • 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

  • 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.

  • is checks for object identity

  • is compares id() output for both objects

  • CPython: compares the memory address a object resides in

../../_images/oop-identity-str-1.png
../../_images/oop-identity-str-2.png
../../_images/oop-identity-str-3.png

5.3.1. Builtin id()

  • id(obj) -> int - Function id() returns an integer

  • Guaranteed to be unique and constant for object during its lifetime

  • Will change every time you execute script

  • In CPython id() shows the memory address of an object

  • Returned value of id() will change every time you execute script

>>> class User:
...     pass
>>>
>>>
>>> mark = User()
>>>
>>> mark  
<__main__.User at 0x12068b8f0>

In CPython id() shows the memory address of an object:

>>> id(mark)  
4838701296

Usually memory address is shown in hexadecimal format:

>>> hex(4838701296)  
'0x12068b8f0'

5.3.2. None Type Identity

  • NoneType object is a singleton

  • It always has the same identity (during one run)

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

Rationale:

>>> id(x)  
4351779640
>>>
>>> id(None)  
4351779640

It always has the same identity (during one run):

>>> id(None)  
4351779640
>>>
>>> id(None)  
4351779640
>>>
>>> id(None)  
4351779640

>>> x = None
>>> id(x)  
4351779640
>>>
>>> x = [None, None, None]
>>> id(x[0])  
4351779640

Performance:

>>> x = None
>>>
>>> # doctest: +SKIP
... %%timeit -r1000 -n1000
... x is None
...
10.9 ns ± 4.15 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
10.8 ns ± 6.06 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
9.98 ns ± 4.66 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
9.57 ns ± 3.63 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
10.4 ns ± 4.7 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)

>>> x = None
>>>
>>> # doctest: +SKIP
... %%timeit -r1000 -n1000
... x == None
...
15.8 ns ± 3.76 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
15.4 ns ± 3.59 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
15.3 ns ± 4.04 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
15.7 ns ± 4.4 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
15.4 ns ± 4.12 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)

  • Date: 2024-12-12

  • Python: 3.13.1

  • IPython: 8.30.0

  • System: macOS 15.1.1

  • Computer: MacBook M3 Max

  • CPU: 16 cores (12 performance and 4 efficiency) / 3nm

  • RAM: 128 GB RAM LPDDR5

Why:

>>> x.__eq__(None)
True
>>>
>>> None.__eq__(x)
True
>>>
>>> object.__eq__(x, None)
True
>>> x is None  # and finally, this will be executed
True

5.3.3. Bool Type Identity

  • Bool object is a singleton

  • It always has the same identity (during one run)

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

Rationale:

>>> id(x)  
4379755984
>>>
>>> id(True)  
4379755984

Performance:

>>> x = True
>>>
>>> # doctest: +SKIP
... %%timeit -r1000 -n1000
... x is True
...
9.26 ns ± 7.09 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
9.9 ns ± 3.49 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
10.3 ns ± 5.66 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
10.7 ns ± 4.19 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
10.6 ns ± 4.76 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)

>>> x = True
>>>
>>> # doctest: +SKIP
... %%timeit -r1000 -n1000
... x == True
...
15.3 ns ± 5.38 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
15.6 ns ± 3.88 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
15.3 ns ± 8.11 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
13.9 ns ± 1.99 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)
13.7 ns ± 4.74 ns per loop (mean ± std. dev. of 1000 runs, 1,000 loops each)

  • Date: 2024-12-12

  • Python: 3.13.1

  • IPython: 8.30.0

  • System: macOS 15.1.1

  • Computer: MacBook M3 Max

  • CPU: 16 cores (12 performance and 4 efficiency) / 3nm

  • RAM: 128 GB RAM LPDDR5

5.3.4. Int Type Identity

  • SyntaxWarning: "is" with 'int' literal. Did you mean "=="?

  • Integer interning (caching) for values from -5 to 256

  • -5 to 256 are interned (cached) and have the same id() value

  • -5 to 256 are singletons

  • -5 to 256 are preallocated in memory

  • they are never deleted from memory (during one run)

>>> x = 1
>>>
>>> x == 1
True
>>>
>>> x is 1  # SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
True

>>> x = 256
>>>
>>> x == 256
True
>>>
>>> x is 256  # SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
True

>>> x = 257
>>> x == 257
True
>>> x is 257  # SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
False

>>> x = -5
>>> x == -5
True
>>> x is -5  # SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
True

>>> x = -6
>>> x == -6
True
>>> x is -6  # SyntaxWarning: "is" with 'int' literal. Did you mean "=="?
False

Rationale:

>>> data = ['a', 'b', 'c', 'd']
>>>
>>> data[0]
'a'
>>>
>>> data[1:3]
['b', 'c']
>>>
>>> data[slice(1,3)]
['b', 'c']

5.3.5. Float Type Identity

  • SyntaxWarning: "is" with 'float' literal. Did you mean "=="?

  • Floats are never cached

>>> x = 1.23
>>>
>>> x == 1.23
True
>>>
>>> x is 1.23  # SyntaxWarning: "is" with 'float' literal. Did you mean "=="?
False

5.3.6. String Types Identity

  • Strings are interned only if they do not contains special characters

  • SyntaxWarning: "is" with 'float' literal. Did you mean "=="?

>>> x = 'Czesc'
>>>
>>> x == 'Czesc'
True
>>>
>>> x is 'Czesc'
True

>>> x = 'Cześć'
>>>
>>> x == 'Cześć'  # SyntaxWarning: "is" with 'float' literal. Did you mean "=="?
True
>>>
>>> x is 'Cześć'
False

5.3.7. Sequences Identity

>>> x = (1, 2, 3)
>>>
>>> x == (1, 2, 3)
True
>>>
>>> x is (1, 2, 3)  # SyntaxWarning: "is" with 'tuple' literal. Did you mean "=="?
False

>>> x = [1, 2, 3]
>>>
>>> x == [1, 2, 3]
True
>>>
>>> x is [1, 2, 3]
False

>>> x = {1, 2, 3}
>>>
>>> x == {1, 2, 3}
True
>>>
>>> x is {1, 2, 3}
False

5.3.8. Mappings Identity

>>> x = {'a':1, 'b':2, 'c':3}
>>>
>>> x == {'a':1, 'b':2, 'c':3}
True
>>>
>>> x is {'a':1, 'b':2, 'c':3}
False

5.3.9. Collections Identity

Increment add with lists:

>>> data = [1, 2, 3]
>>> hex(id(data))  
'0x106b4ab40'
>>>
>>> data += [4, 5, 6]
>>> hex(id(data))  
'0x106b4ab40'
>>>
>>> data
[1, 2, 3, 4, 5, 6]

Increment add with tuples:

>>> data = (1, 2, 3)
>>> hex(id(data))  
'0x10685e0c0'
>>>
>>> data += (4, 5, 6)
>>> hex(id(data))  
'0x106b4cf40'
>>>
>>> data
(1, 2, 3, 4, 5, 6)

Tuple identities:

>>> (1,2,3) is (1,2,3)  # SyntaxWarning: "is" with 'tuple' literal. Did you mean "=="?
True

>>> a = (1, 2, 3)
>>> b = (1, 2, 3)
>>>
>>> a is b  # SyntaxWarning: "is" with 'tuple' literal. Did you mean "=="?
False

>>> a=(1,2,3); b=(1,2,3)
>>>
>>> a is b  # SyntaxWarning: "is" with 'tuple' literal. Did you mean "=="?
True

Why:

>>> id(a)  
4411490432
>>>
>>> id(b)  
4421110208

5.3.10. Object Identity

>>> class User:
...     pass
>>>
>>>
>>> x = User()
>>>
>>> x == User()
False
>>>
>>> x is User()
False

5.3.11. Class Identity

  • Class object is a singleton

  • It always has the same identity (during one run)

>>> class User:
...     pass
>>>
>>>
>>> x = User()
>>>
>>> type(x) == User
True
>>>
>>> type(x) is User
True

5.3.12. Type Identity

  • Type object is a singleton

>>> class User:
...     pass
>>>
>>>
>>> cls = User
>>>
>>> cls == User
True
>>>
>>> cls is User
True