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
リバースエンジニアリング新時代へ! GhidraとClaude DesktopをMCPで繋ぐ/findy202507
tkmru
3
960
NPOでのDevinの活用
codeforeveryone
0
900
ソフトウェア設計とAI技術の活用
masuda220
PRO
17
3.7k
なぜ「共通化」を考え、失敗を繰り返すのか
rinchoku
1
680
PHP 8.4の新機能「プロパティフック」から学ぶオブジェクト指向設計とリスコフの置換原則
kentaroutakeda
2
1k
可変変数との向き合い方 $$変数名が踊り出す$$ / php conference Variable variables
gunji
0
180
20250704_教育事業におけるアジャイルなデータ基盤構築
hanon52_
5
1.1k
はじめてのWeb API体験 ー 飲食店検索アプリを作ろうー
akinko_0915
0
140
React は次の10年を生き残れるか:3つのトレンドから考える
oukayuka
9
3k
Agentic Coding: The Future of Software Development with Agents
mitsuhiko
0
130
おやつのお供はお決まりですか?@WWDC25 Recap -Japan-\(region).swift
shingangan
0
140
20250708_JAWS_opscdk
takuyay0ne
2
130
Featured
See All Featured
Measuring & Analyzing Core Web Vitals
bluesmoon
7
520
How to Ace a Technical Interview
jacobian
278
23k
Templates, Plugins, & Blocks: Oh My! Creating the theme that thinks of everything
marktimemedia
31
2.4k
The Illustrated Children's Guide to Kubernetes
chrisshort
48
50k
Done Done
chrislema
184
16k
No one is an island. Learnings from fostering a developers community.
thoeni
21
3.4k
Java REST API Framework Comparison - PWX 2021
mraible
31
8.7k
Building a Scalable Design System with Sketch
lauravandoore
462
33k
Building Better People: How to give real-time feedback that sticks.
wjessup
367
19k
Raft: Consensus for Rubyists
vanstee
140
7k
XXLCSS - How to scale CSS and keep your sanity
sugarenia
248
1.3M
It's Worth the Effort
3n
185
28k
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