Python Descriptors Demystified
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The document discusses Python descriptors and how they can be used to disguise methods as attributes, create reusable properties, and store instance-specific data. Descriptors allow other code to interact with data stored in a class by providing getter and setter methods that make attributes appear to directly access data, when they are actually calling methods behind the scenes. This provides a cleaner interface for users than requiring explicit method calls.
![Instance-specific data
class Foo(object):
x = Descriptor()
f1 = Foo()
f2 = Foo()
f1.x = 5
f2.x = 4
class Descriptor(object):
def __init__(self):
self._data = {}
def __get__(self, instance):
return self._data[instance]](https://image.slidesharecdn.com/descriptortalk-130424174820-phpapp02/85/Python-Descriptors-Demystified-24-320.jpg)
Python Descriptors Demystified
- 1. Descriptors Python's most obscure language feature demystified. In 5 minutes. Chris Beaumont @BeaumontChris graduate student, astrophysics Harvard University / University of Hawaii
- 2. Learning about descriptors creates an appreciation for the elegance of Python's design. -Raymond Hettinger
- 3. Learning about descriptors creates an appreciation for the elegance of Python's design. -Raymond Hettinger I DON'T GET IT
- 4. class Email(object): def __init__(self, sender, subject, message): self.sender_widget = QLineEdit(sender) self.subject_widget = QLineEdit(subject) self.message_widget = QLineEdit(message) How does other code interact with email data?
- 5. class Email(object): ... def get_subject(self): return self.subject_widget.text() def set_subject(self, subject): self.subject_widget.setText(subject) ... e = Email() e.set_subject('Hi there') sub = e.get_subject()
- 6. class Email(object): ... def get_subject(self): return self.subject_widget.text() def set_subject(self, subject): self.subject_widget.setText(subject) ... e = Email() e.set_subject('Hi there') sub = e.get_subject() Gross
- 8. class Email(object): ... @property def subject(self): return self.subject_widget.text() @subject.setter def subject(self, subject): self.subject_widget.setText(subject) ...
- 9. e = Email() sub = e.subject e.subject = 'hi there' class Email(object): ... @property def subject(self): return self.subject_widget.text() @subject.setter def subject(self, subject): self.subject_widget.setText(subject) ...
- 10. e = Email() sub = e.subject e.subject = 'hi there' class Email(object): ... @property def subject(self): return self.subject_widget.text() @subject.setter def subject(self, subject): self.subject_widget.setText(subject) ... Nice.
- 11. class Email(object): def __init__(self, sender, subject, message): self._sender_widget = QLineEdit(sender) self._subject_widget = QLineEdit(subject) self._message_widget = QLineEdit(message) @property def sender(self): return self._sender_widget.text() @sender.setter def sender(self, sender): self._sender_widget.setText(sender) @property def subject(self): return self._subject_widget.text() @subject.setter def subject(self, subject): self._subject_widget.setText(subject) @property def message(self): return self._message_widget.text() @message.setter def message(self, message): self._message_widget.setText(message)
- 12. class Email(object): def __init__(self, sender, subject, message): self._sender_widget = QLineEdit(sender) self._subject_widget = QLineEdit(subject) self._message_widget = QLineEdit(message) @property def sender(self): return self._sender_widget.text() @sender.setter def sender(self, sender): self._sender_widget.setText(sender) @property def subject(self): return self._subject_widget.text() @subject.setter def subject(self, subject): self._subject_widget.setText(subject) @property def message(self): return self._message_widget.text() @message.setter def message(self, message): self._message_widget.setText(message) Gross
- 14. class TextWrapper(object): def __init__(self, widget_name): self.widget_name = widget_name def __get__(self, object): widget = getattr(object, self.widget_name) return widget.text() def __set__(self, object, value): widget = getattr(object, self.widget_name) widget.setText(value) class Email(object): sender = TextWrapper('sender_widget') subject = TextWrapper('subject_widget') message = TextWrapper('message_widget') def __init__(self, sender, subject, message): self.sender_widget = QLineEdit(sender) self.subject_widget = QLineEdit(subject) self.message_widget = QLineEdit(message) ... ...
- 15. class TextWrapper(object): def __init__(self, widget_name): self.widget_name = widget_name def __get__(self, object): widget = getattr(object, self.widget_name) return widget.text() def __set__(self, object, value): widget = getattr(object, self.widget_name) widget.setText(value) class Email(object): sender = TextWrapper('sender_widget') subject = TextWrapper('subject_widget') message = TextWrapper('message_widget') def __init__(self, sender, subject, message): self.sender_widget = QLineEdit(sender) self.subject_widget = QLineEdit(subject) self.message_widget = QLineEdit(message) e = Email() sender = e.sender e.sender = 'foo' ... ...
- 16. e = Email() sender = e.sender e.sender = 'foo' ... class TextWrapper(object): def __init__(self, widget_name): self.widget_name = widget_name def __get__(self, object): widget = getattr(object, self.widget_name) return widget.text() def __set__(self, object, value): widget = getattr(object, self.widget_name) widget.setText(value) class Email(object): sender = TextWrapper('sender_widget') subject = TextWrapper('subject_widget') message = TextWrapper('message_widget') def __init__(self, sender, subject, message): self.sender_widget = QLineEdit(sender) self.subject_widget = QLineEdit(subject) self.message_widget = QLineEdit(message)
- 17. e = Email() sender = e.sender e.sender = 'foo' class TextWrapper(object): def __init__(self, widget_name): self.widget_name = widget_name def __get__(self, object): widget = getattr(object, self.widget_name) return widget.text() def __set__(self, object, value): widget = getattr(object, self.widget_name) widget.setText(value) class Email(object): sender = TextWrapper('sender_widget') subject = TextWrapper('subject_widget') message = TextWrapper('message_widget') def __init__(self, sender, subject, message): self.sender_widget = QLineEdit(sender) self.subject_widget = QLineEdit(subject) self.message_widget = QLineEdit(message)
- 18. e = Email() sender = e.sender e.sender = 'foo' Nice. class TextWrapper(object): def __init__(self, widget_name): self.widget_name = widget_name def __get__(self, object): widget = getattr(object, self.widget_name) return widget.text() def __set__(self, object, value): widget = getattr(object, self.widget_name) widget.setText(value) class Email(object): sender = TextWrapper('sender_widget') subject = TextWrapper('subject_widget') message = TextWrapper('message_widget') def __init__(self, sender, subject, message): self.sender_widget = QLineEdit(sender) self.subject_widget = QLineEdit(subject) self.message_widget = QLineEdit(message)
- 19. e = Email() sender = e.sender e.sender = 'foo' Nice. Nice. class TextWrapper(object): def __init__(self, widget_name): self.widget_name = widget_name def __get__(self, object): widget = getattr(object, self.widget_name) return widget.text() def __set__(self, object, value): widget = getattr(object, self.widget_name) widget.setText(value) class Email(object): sender = TextWrapper('sender_widget') subject = TextWrapper('subject_widget') message = TextWrapper('message_widget') def __init__(self, sender, subject, message): self.sender_widget = QLineEdit(sender) self.subject_widget = QLineEdit(subject) self.message_widget = QLineEdit(message)
- 20. e = Email() sender = e.sender e.sender = 'foo' Nice. Nice. Meh?class TextWrapper(object): def __init__(self, widget_name): self.widget_name = widget_name def __get__(self, object): widget = getattr(object, self.widget_name) return widget.text() def __set__(self, object, value): widget = getattr(object, self.widget_name) widget.setText(value) class Email(object): sender = TextWrapper('sender_widget') subject = TextWrapper('subject_widget') message = TextWrapper('message_widget') def __init__(self, sender, subject, message): self.sender_widget = QLineEdit(sender) self.subject_widget = QLineEdit(subject) self.message_widget = QLineEdit(message)
- 24. Instance-specific data class Foo(object): x = Descriptor() f1 = Foo() f2 = Foo() f1.x = 5 f2.x = 4 class Descriptor(object): def __init__(self): self._data = {} def __get__(self, instance): return self._data[instance]
- 25. Accessing Descriptor Methods class Descriptor(object) def __get__(self, instance): if instance == None: return self def cool_descriptor_method(self): pass desc = Foo.x # instance = None desc.cool_descriptor_method()