5.1. OOP Mutability

5.1.1. Rationale

  • Function and method arguments should not be mutable

5.1.2. Immutable Types

  • int

  • float

  • complex

  • bool

  • None

  • str

  • bytes

  • tuple

  • frozenset

  • mappingproxy

5.1.3. Mutable Types

  • list

  • set

  • dict

5.1.4. Argument Mutability

Note, You should not set mutable objects as a default function argument. More information in Argument Mutability. This is how all dynamically typed languages work (including PHP, Ruby, Perl etc).

>>> class Astronaut:
...     def __init__(self, name, missions=[]):
...         self.name = name
...         self.missions = missions
>>>
>>>
>>> watney = Astronaut('Mark Watney')
>>> lewis = Astronaut('Melissa Lewis')
>>>
>>> watney.missions.append('Ares 1')
>>> watney.missions.append('Ares 2')
>>> watney.missions.append('Ares 3')
>>>
>>> print(f'Name: {watney.name}, Missions: {watney.missions}')
Name: Mark Watney, Missions: ['Ares 1', 'Ares 2', 'Ares 3']
>>>
>>> print(f'Name: {lewis.name}, Missions: {lewis.missions}')
Name: Melissa Lewis, Missions: ['Ares 1', 'Ares 2', 'Ares 3']

>>> class Astronaut:
...     def __init__(self, name, missions=None):
...         self.name = name
...         self.missions = missions if missions else []
>>>
>>>
>>> watney = Astronaut('Mark Watney')
>>> lewis = Astronaut('Melissa Lewis')
>>>
>>> watney.missions.append('Ares 1')
>>> watney.missions.append('Ares 2')
>>> watney.missions.append('Ares 3')
>>>
>>> print(f'Name: {watney.name}, Missions: {watney.missions}')
Name: Mark Watney, Missions: ['Ares 1', 'Ares 2', 'Ares 3']
>>>
>>> print(f'Name: {lewis.name}, Missions: {lewis.missions}')
Name: Melissa Lewis, Missions: []

5.1.5. Assignments