Python descriptors are a way to create managed attributes. Among their many advantages, managed attributes are used to protect an attribute from changes or to automatically update the values of a dependant attribute.
Questions tagged [python-descriptors]
169 questions
395
votes
8 answers
Understanding __get__ and __set__ and Python descriptors
I am trying to understand what Python's descriptors are and what they are useful for. I understand how they work, but here are my doubts. Consider the following code:
class Celsius(object):
def __init__(self, value=0.0):
self.value =…
Matt Bronson
- 3,969
- 3
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- 4
29
votes
2 answers
How to use super() with one argument?
While reading the Python documentation on super(), I stumbled on the following statement:
If the second argument is omitted, the super object returned is unbound.
What does ‘unbound’ mean and how to use super() with one argument?
user3282758
- 1,379
- 11
- 29
25
votes
2 answers
Why is @staticmethod not preserved across classes, when @classmethod is?
Take the following example script:
class A(object):
@classmethod
def one(cls):
print("I am class")
@staticmethod
def two():
print("I am static")
class B(object):
one = A.one
two =…
Alex Forbes
- 3,039
- 2
- 19
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17
votes
2 answers
How can I get the attribute name when working with descriptor protocol in Python?
The descriptor protocol works fine but I still have one issue I would like to resolve.
I have a descriptor:
class Field(object):
def __init__(self, type_, name, value=None, required=False):
self.type = type_
self.name = "_" +…
Johan Vergeer
- 5,208
- 10
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- 105
16
votes
3 answers
Why do people default owner parameter to None in __get__?
I've seen this quite often:
def __get__(self, instance, owner=None):
Why do some people use the default value of None for the the owner parameter?
This is even done in the Python docs:
descr.__get__(self, obj, type=None) --> value
Markus Meskanen
- 19,939
- 18
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- 119
14
votes
3 answers
Why does setting a descriptor on a class overwrite the descriptor?
Simple repro:
class VocalDescriptor(object):
def __get__(self, obj, objtype):
print('__get__, obj={}, objtype={}'.format(obj, objtype))
def __set__(self, obj, val):
print('__set__')
class B(object):
v =…
Michael Carilli
- 371
- 2
- 12
13
votes
3 answers
Type hinting with descriptors
In this pull request it looks like type hinting support for descriptors was added.
However it looks like no finalized "correct" usage example was ever posted, nor does it looks like any documentation was ever added to the typing module or to…
shadowtalker
- 12,529
- 3
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- 96
13
votes
1 answer
Setting a class __name__ declaratively
Why can't you override a class name declaratively, e.g. to use a class name which is not a valid identifier?
>>> class Potato:
... __name__ = 'not Potato'
...
>>> Potato.__name__ # doesn't stick
'Potato'
>>> Potato().__name__ # .. but…
wim
- 338,267
- 99
- 616
- 750
13
votes
1 answer
python data and non-data descriptors
According to Python's documentation,
Data descriptors with __set__() and __get__() defined always override a redefinition in an instance dictionary.
I have no problem understanding this sentence, but can someone clarify for me why such a rule is…
antony
- 2,877
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12
votes
5 answers
How to assign member variables temporarily?
I often find that I need to assign some member variables temporarily, e.g.
old_x = c.x
old_y = c.y
# keep c.z unchanged
c.x = new_x
c.y = new_y
do_something(c)
c.x = old_x
c.y = old_y
but I wish I could simply write
with c.x = new_x; c.y =…
MWB
- 11,740
- 6
- 46
- 91
11
votes
2 answers
Neat way to get descriptor object
In Python 3
class A(object):
attr = SomeDescriptor()
...
def somewhere(self):
# need to check is type of self.attr is SomeDescriptor()
desc = self.__class__.__dict__[attr_name]
return isinstance(desc,…
Antigluk
- 1,176
- 2
- 11
- 30
10
votes
1 answer
mypy error: Callable has no attribute "__get__"
I have something like the following:
from typing import TypeVar, Callable, Generic, Type, Union, Optional
T = TypeVar("T")
V = TypeVar("V")
class DescClass(Generic[T, V]):
"""A descriptor."""
def __init__(self, func: Callable[[T], V]) ->…
Rick
- 43,029
- 15
- 76
- 119
9
votes
1 answer
Using Typing and Mypy with Descriptors
I have looked at a few SO posts and github issues related to using Typing with descriptors but I have not been able to get my issue resolved.
I have wrapper classes and I want to define properties as descriptos that can get and "cast" properties of…
gtalarico
- 4,409
- 1
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- 42
7
votes
1 answer
typing: How to bind owner class to generic descriptor?
Can I implement a generic descriptor in Python in a way it will support/respect/understand inheritance hierarchy of his owners?
It should be more clear in the code:
from typing import (
Generic, Optional, TYPE_CHECKING,
Type, TypeVar, Union,…
NobbyNobbs
- 1,341
- 1
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6
votes
1 answer
Class properties in Python 3.11+
In Python 3.9, we gained the ability to chain @classmethod and @property to sensibly create class properties.
class Foo:
@property
def instance_property(self):
return "A regular property"
@classmethod
@property
def…
kg583
- 408
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