3.6. OOP Static and Dynamic Attrs¶
3.6.1. Static Fields¶
Fields created on class
Must have default values
Share state
Static Fields:
>>> class Astronaut:
... agency = 'NASA'
>>>
>>>
>>> watney = Astronaut()
>>> jimenez = Astronaut()
>>>
>>> print(watney.agency)
NASA
>>>
>>> print(jimenez.agency)
NASA
>>>
>>> print(Astronaut.agency)
NASA
3.6.2. Dynamic Fields¶
Fields created on instance
Do not share state
By convention initialized in
__init__()You can also initialize on living object directly
Dynamic fields:
>>> class Astronaut:
... def __init__(self, agency='NASA'):
... self.agency = agency
>>>
>>>
>>> watney = Astronaut()
>>> twardowski = Astronaut()
>>>
>>> print(watney.agency)
NASA
>>>
>>> print(twardowski.agency)
NASA
>>>
>>> print(Astronaut.agency)
Traceback (most recent call last):
AttributeError: type object 'Astronaut' has no attribute 'agency'
3.6.3. Static vs. Dynamic Fields¶
Static vs. Dynamic fields:
>>> class Astronaut:
... agency = 'NASA'
>>>
>>>
>>> watney = Astronaut()
>>> twardowski = Astronaut()
>>> ivanovic = Astronaut()
>>>
>>> # Print field
>>> print(watney.agency)
NASA
>>> print(twardowski.agency)
NASA
>>> print(ivanovic.agency)
NASA
>>> print(Astronaut.agency)
NASA
>>>
>>> # Change field on a class
>>> Astronaut.agency = 'ESA'
>>>
>>> # Print field
>>> print(watney.agency)
ESA
>>> print(twardowski.agency)
ESA
>>> print(ivanovic.agency)
ESA
>>> print(Astronaut.agency)
ESA
>>>
>>> # Change field on the instance
>>> ivanovic.agency = 'Roscosmos'
>>>
>>> # Print field
>>> print(watney.agency)
ESA
>>> print(twardowski.agency)
ESA
>>> print(ivanovic.agency)
Roscosmos
>>> print(Astronaut.agency)
ESA
>>>
>>> # Change field on a class
>>> Astronaut.agency = 'POLSA'
>>>
>>> # Print field
>>> print(watney.agency)
POLSA
>>> print(twardowski.agency)
POLSA
>>> print(ivanovic.agency)
Roscosmos
>>> print(Astronaut.agency)
POLSA
3.6.4. Static or Dynamic?¶
Static Fields:
>>> class Astronaut:
... firstname = ...
... lastname = ...
Dynamic Fields:
>>> class Cosmonaut:
... def __init__(self):
... self.firstname = ...
... self.lastname = ...
Dynamic Fields:
>>> from dataclasses import dataclass
>>>
>>>
>>> @dataclass
... class GaganYatri:
... firstname: str = ...
... lastname: list = ...
Dynamic Fields:
>>> class Taikonaut:
... pass
>>>
>>> t = Taikonaut()
>>> t.firstname = ...
>>> t.lastname = ...
Static Fields:
>>> class Taikonaut:
... pass
>>>
>>> Taikonaut.firstname = ...
>>> Taikonaut.lastname = ...