Data Model

Attribute Access

Attribute hooks customize lookup, missing attributes, and assignment.

The starting point is ordinary: __init__ stores one real attribute, the _values backing dictionary. The hooks in the next cells customize lookup and assignment around it.

Source

class Settings:
    def __init__(self, values):
        self._values = dict(values)

settings = Settings({"theme": "dark"})
print(settings._values)

Output

{'theme': 'dark'}

__getattr__ runs only for missing attributes, so it can provide fallback lookup.

Source

class Settings:
    def __init__(self, values):
        self._values = dict(values)

    def __getattr__(self, name):
        try:
            return self._values[name]
        except KeyError as error:
            raise AttributeError(name) from error

settings = Settings({"theme": "dark"})
print(settings.theme)

Output

dark

__setattr__ intercepts every assignment, including the ones in __init__. Underscore names are stored as real attributes through object.__setattr__, which avoids recursing through your own hook; public names go to the backing dictionary.

Source

class Settings:
    def __init__(self, values):
        self._values = dict(values)

    def __setattr__(self, name, value):
        if name.startswith("_"):
            object.__setattr__(self, name, value)
        else:
            self._values[name] = value

settings = Settings({"theme": "dark"})
settings.volume = 7
print(settings._values["volume"])

Output

Notes

__getattr__ is narrower than __getattribute__ because it handles only missing attributes.
__setattr__ affects every assignment on the instance.
Use property or descriptors when the behavior is attached to a known attribute name.

Run the complete example

Expected output

dark
7

Execution time appears here after you run the example.

Attribute Access

See also

Run the complete example

Example code

Expected output