[PYTHON] __Getattr__ and __getattribute__ to customize the acquisition of object attributes by dots

Getting attributes with .

I think that the attributes of objects such as variables and methods defined in the class statement are usually acquired by .. For example, suppose you define the following class.

python

class Foo:
    def __init__(self):
        self.foo = "foo"
        self.bar = "bar"

The attributes foo and bar of the instance foo of Foo can be obtained with ..

python

>>> foo = Foo()
>>> foo.foo
'foo'
>>> foo.bar
'bar'

This . behavior can be customized using the special attributes __getattr__ and __getattribute__ methods.

Customization with __getattr__

If you define __getattr__, when you try to get an attribute in ., the usual name search is done first, and if the specified attribute is still not found, __getattr__ is called and the returned value is used. Returns as that attribute.

The following class Foo defines __getattr__ to always return the member variable foo.

python

class Foo:

    def __init__(self):
        self.foo = "foo"
        self.bar = "bar"
    
    def __getattr__(self, name):
        return self.foo

The instance foo of Foo has a member variable called self.bar, so foo.bar returns self.bar, but there is no member variable called self.anything. So self.__ getattr__ ("anything") is executed and self.foo (ie the string " foo ") is returned.

python

>>> foo = Foo()
>>> foo.foo
'foo'
>>> foo.bar
'bar'
>>> foo.anything
'foo'

Customization with __getattribute__

If __getattribute__ is defined, this method will be called whenever you try to get an attribute in .. In the definition of __getattribute__, if you try to get the attribute of that object using ., your own __getattribute__ will be called recursively, so to avoid that, the superclass ʻobject Use __getattribute__ of`.

python

class Bar:

    def __init__(self):
        self.foo = "foo"
        self.bar = "bar"
    
    def __getattribute__(self, name):
        return object.__getattribute__(self, "foo")

As you can see from the following execution result, the instance bar of Bar has a member variable called self.bar, but even if it is bar.bar, it is __getattribute__. self.foo(ie the string" foo "`) is returned.

python

>>> bar = Bar()
>>> bar.foo
'foo'
>>> bar.bar
'foo'
>>> bar.anything
'foo'

Built-in functions getattr and hasattr

The built-in functions getattr and hasattr are based on attribute acquisition customized with __getattr__ and __hasattr__. So getting the attributes of an object with getattr is the same as getting it with .. If you check the instances of Foo and Bar in the above example, the output will be like this.

python

>>> getattr(foo, "bar")
'bar'
>>> getattr(bar, "bar")
'foo'

If __getattr__ or __getattribute__ returns a value no matter what attribute you specify (that is, ʻAttributeError does not occur), hasattralways returnsTrue`.

python

>>> hasattr(foo, "anything")
True
>>> hasattr(bar, "anything")
True