Upgrade to Pro
— share decks privately, control downloads, hide ads and more …
Speaker Deck
Features
Speaker Deck
PRO
Sign in
Sign up for free
Search
Search
Discovering-Descriptors_ep.pdf
Search
Mariano Anaya
July 11, 2017
Programming
1
370
Discovering-Descriptors_ep.pdf
EuroPython 2017 - Tuesday 11th, July - 15:45
Room PythonAnywhere
Mariano Anaya
July 11, 2017
Tweet
Share
More Decks by Mariano Anaya
See All by Mariano Anaya
Demystifying Coroutines and Asynchronous Programming in Python
rmariano
1
150
Demystifying coroutines and asynchronous programming in Pyhon
rmariano
1
340
Exploring Generators & Coroutines
rmariano
1
790
Beyond Coverage
rmariano
0
190
Discovering Descriptors
rmariano
0
170
Clean Code in Python
rmariano
2
2.2k
Other Decks in Programming
See All in Programming
Is Xcode slowly dying out in 2025?
uetyo
1
280
NPOでのDevinの活用
codeforeveryone
0
840
Rails Frontend Evolution: It Was a Setup All Along
skryukov
0
160
AIエージェントはこう育てる - GitHub Copilot Agentとチームの共進化サイクル
koboriakira
0
600
PHPで始める振る舞い駆動開発(Behaviour-Driven Development)
ohmori_yusuke
2
390
なんとなくわかった気になるブロックテーマ入門/contents.nagoya 2025 6.28
chiilog
1
270
AIプログラマーDevinは PHPerの夢を見るか?
shinyasaita
1
230
“いい感じ“な定量評価を求めて - Four Keysとアウトカムの間の探求 -
nealle
2
10k
A2A プロトコルを試してみる
azukiazusa1
2
1.4k
GitHub Copilot and GitHub Codespaces Hands-on
ymd65536
2
150
ペアプロ × 生成AI 現場での実践と課題について / generative-ai-in-pair-programming
codmoninc
2
18k
地方に住むエンジニアの残酷な現実とキャリア論
ichimichi
5
1.6k
Featured
See All Featured
The Power of CSS Pseudo Elements
geoffreycrofte
77
5.9k
A better future with KSS
kneath
238
17k
Optimizing for Happiness
mojombo
379
70k
Build your cross-platform service in a week with App Engine
jlugia
231
18k
How to train your dragon (web standard)
notwaldorf
95
6.1k
The Language of Interfaces
destraynor
158
25k
Building Better People: How to give real-time feedback that sticks.
wjessup
367
19k
Facilitating Awesome Meetings
lara
54
6.4k
jQuery: Nuts, Bolts and Bling
dougneiner
63
7.8k
The Straight Up "How To Draw Better" Workshop
denniskardys
234
140k
Save Time (by Creating Custom Rails Generators)
garrettdimon
PRO
31
1.3k
Unsuck your backbone
ammeep
671
58k
Transcript
Discovering Descriptors Mariano Anaya EuroPython - July 2017 rmariano rmarianoa
def “Learning about descriptors not only provides access to a
larger toolset, it creates a deeper understanding of how Python works and an appreciation for the elegance of its design”. - Raymond Hettinger
Introduction In general: >>> obj = DomainModel() >>> obj.x =
'value' >>> obj.x 'value'
Control Access to Data But what if… When doing “obj.x”
we could run arbitrary code?
Control Access to Data But what if… When doing “obj.x”
we could run arbitrary code? By another object.
Control Access to Data But what if… When doing “obj.x”
we could run arbitrary code? By another object (of a different class).
A First Look at Descriptors
Introduction Descriptors enable control over core operations (get, set, delete),
of an attribute in an object.
Descriptor Methods __get__(self, instance, owner) __set__(self, instance, value) __delete__(self, instance)
__set_name__(self, owner, name) * * Python 3.6
None
Types of Descriptors • Non-data descriptors (a.k.a “non-overriding”) ◦ Don’t
implement __set__ ◦ Instance attributes take precedence • Data descriptors (a.k.a. “overriding”) ◦ Implement __get__, __set__ ◦ Override instance’s __dict__
__get__ Problem: automatically format date values of other attributes. Two
classes: Descriptor + Managed class
Descriptor
__get__: Default Value class DateFormatter: FORMAT = "%Y-%m-%d %H:%M" def
__init__(self, name=None): self.name = name def __get__(self, instance, owner): if instance is None: return self date_value = getattr(instance, self.name) if date_value is None: return '' return date_value.strftime(self.FORMAT)
Managed Class
__get__: Managed Class class FileStat: """Stats of a file in
a virtual file system""" str_created_at = DateFormatter('created_at') str_updated_at = DateFormatter('updated_at') str_removed_at = DateFormatter() def __init__(self, fname, created, updated=None, removed=None): self.filename = fname self.created_at = created self.updated_at = updated self.removed_at = removed
>>> created = updated = datetime(2017, 6, 9, 11, 15,
19) >>> f1 = FileStat('/home/mariano/file1', created, updated) >>> f1.str_created_at '2017-06-09 11:15' >>> f1.str_updated_at '2017-06-09 11:15' >>> f1.str_removed_at ''
Resolution Order
>>> f1 = FileStat(...) >>> f1.str_created_at Statement f1.__dict__ { 'created_at':
... 'filename': '/home/...', 'removed_at': ..., 'updated_at': ... }
>>> f1 = FileStat(...) >>> f1.str_created_at Statement FileStat.__dict__ mappingproxy({'__dict__': ...,
'__doc__': "...", '__init__': ..., 'str_created_at': <DateFormatter at 0x..>, 'str_removed_at': <DateFormatter at 0x..>, 'str_updated_at': <DateFormatter at 0x..>})
>>> f1 = FileStat(...) >>> f1.str_created_at Statement >>> hasattr(FileStat.__dict__['str_created_at'], '__get__')
True
__get__: Syntax Sugar >>> f1 = FileStat(...) >>> f1.str_created_at Translates
into: FileStat.str_created_at.__get__(f1, FileStat)
__get__(self, instance, owner) When called like <class>.<descriptor> instance is None
>>> FileStat.str_created_at <__main__.DateFormatter object at 0x...> Access Through the Class
Name of the Descriptor
class FileStat: """Stats of a file in a virtual file
system""" str_created_at = DateFormatter('created_at') str_updated_at = DateFormatter('updated_at') str_removed_at = DateFormatter()
Before __set_name__ Some techniques to have an “automatic configuration”: Class
decorator or metaclass
__set_name__(self, owner, name) Called automatically with the name of the
attribute, on the LHS. class owner: name = Descriptor()
__set_name__ class DateFormatter: def __init__(self, name=None): self.name = name ...
def __set_name__(self, owner, name): if self.name is None: _, _, self.name = name.partition('_')
__set__ Problem: Given an attribute of an object, keep count
of how many times its value was changed.
Data Descriptor: __set__ Some strategies: 1. Properties (with setter) 2.
Override __setattr__() 3. Descriptors!
class TracedProperty: """Count how many times an attribute changed its
value""" def __set_name__(self, owner, name): self.name = name self.count_name = f'count_{name}' def __set__(self, instance, value): ...
class TracedProperty: ... def __set__(self, instance, value): try: current_value =
instance.__dict__[self.name] except KeyError: instance.__dict__[self.count_name] = 0 else: if current_value != value: instance.__dict__[self.count_name] += 1 instance.__dict__[self.name] = value
class Traveller: city = TracedProperty() country = TracedProperty() def __init__(self,
name): self.name = name
>>> tourist = Traveller('John Smith') >>> tourist.city = 'Barcelona' >>>
tourist.country = 'Spain' >>> tourist.count_city 0 >>> tourist.count_country 0 >>> tourist.city = 'Stockholm' >>> tourist.country = 'Sweden' >>> tourist.count_city 1 >>> tourist.count_country 1 >>> tourist.city = 'Gothenburg' >>> tourist.count_city 2 >>> tourist.count_country 1 >>> tourist.country = 'Sweden' >>> tourist.count_country 1
tourist = Traveller() tourist.city = 'Stockholm' Traveller.city.__set__(tourist, 'Stockholm') __set__: Syntax
sugar Translates to:
__delete__ Called when deleting an attribute by using the descriptor,
like: del <instance>.<descriptor>
__delete__ class ProtectedAttribute: """Attribute that is protected against deletion""" def
__set_name__(self, owner, name): self.name = name def __delete__(self, instance): raise AttributeError(f"Can't delete {self.name} for {instance!s}") def __set__(self, instance, value): ...
class ProtectedUser: username = ProtectedAttribute() def __init__(self, username, location): self.username
= username self.location = location def __str__(self): return f"{self.__class__.__name__}[{self.username}]"
>>> usr = ProtectedUser('jsmith', '127.0.0.1') >>> usr.username 'jsmith' >>> del
usr.username Traceback (most recent call last): ... AttributeError: Can't delete username for ProtectedUser[jsmith] >>> usr.location '127.0.0.1' >>> del usr.location >>> usr.location Traceback (most recent call last): ... AttributeError: 'ProtectedUser' object has no attribute 'location'
What makes a good descriptor?
What makes a good descriptor? The same thing that makes
any good Python object: consistency with Python itself (to be Pythonic).
Descriptors are deployed in the language infrastructure. • @property, @classmethod,
@staticmethod • Methods (functions) Descriptors in CPython
Functions are Descriptors They have a __get__ method. That’s why
they can work as instance methods! <function>.__get__ returns the function bound to an object.
class Class: def method(self, *args): return f'{self!s} got {args}' >>>
Class.__dict__ mappingproxy({'__dict__': ... 'method': <function Class.method>}) >>> isinstance(Class.__dict__['method'], types.FunctionType) True
>>> instance = Class() >>> instance.method('arg1', 'arg2') "instance got ('arg1',
'arg2')" Method Call >>> Class.method.__get__(instance, Class)('arg1', 'arg2') "instance got ('arg1', 'arg2')" It’s actually...
Extended Uses
Improve decorators that change the signature.
Apply to Functions & Methods as well Problem: A decorator
that changes the signature, has to work both for functions and methods. E.g. abstract away repeated code.
def resolver_function(root, args, context, info): helper = DomainObject(root, args, context,
info) ... helper.process() helper.task1() helper.task2() return helper.task1()
class DomainArgs: def __init__(self, func): self.func = func wraps(func)(self) def
__call__(self, root, args, context, info): helper = DomainObject(root, args, context, info) return self.func(helper) @DomainArgs def resolver_function(helper): helper.task1() ...
class ViewResolver: @DomainArgs def resolve_method(self, helper): response = helper.process() return
f"Method: {response}" Try to Decorate a Method
>>> vr1.resolve_method('root', 'args', 'context', 'info') ------------------------------------ TypeError Traceback (most recent
call last) 39 def __call__(self, root, args, context, info): 40 helper = DomainObject(root, args, context, info) ---> 41 return self.func(helper) 42 TypeError: resolve_method() missing 1 required positional argument: 'helper' Doesn’t handle self!
class DomainArgs: ... def __get__(self, instance, owner): mapped = self.func.__get__(instance,
owner) return self.__class__(mapped) >>> vr = ViewResolver() >>> vr.method_resolver('root', 'args', 'context', 'info') 'Method resolver: root, args, context, info' Fix: __get__
Closing Remarks
Implement the minimum required interface.
Use for general-purpose solutions.
Thanks! @rmarianoa