Question

I’m a Python newbie with a very particular itch to experiment with Python’s dot-name-lookup process. How do I code either a class or function in “make.py” so that these assignment statements work succesfully?

import make

make.a.dot.separated.name = 666
make.something.else.up = 123
make.anything.i.want = 777

Answer 1

#!/usr/bin/env python

class Make:
    def __getattr__(self, name):
        self.__dict__[name] = Make()
        return self.__dict__[name]

make = Make()

make.a.dot.separated.name = 666
make.anything.i.want = 777

print make.a.dot.separated.name
print make.anything.i.want

The special __getattr__ method is called when a named value isn’t found. The line make.anything.i.want ends up doing the equivalent of:

m1 = make.anything    # calls make.__getattr__("anything")
m2 = m1.i             # calls m1.__getattr__("i")
m2.want = 777

The above implementation uses these calls to __getattr__ to create a chain of Make objects each time an unknown property is accessed. This allows the dot accesses to be nested arbitrarily deep until the final assignment at which point a real value is assigned.

Python documentation – customizing attribute access:

object.__getattr__(self, name)

Called when an attribute lookup has not found the attribute in the usual places (i.e. it is not an instance attribute nor is it found in the class tree for self). name is the attribute name. This method should return the (computed) attribute value or raise an AttributeError exception.

Note that if the attribute is found through the normal mechanism, __getattr__() is not called. (This is an intentional asymmetry between __getattr__() and __setattr__().) This is done both for efficiency reasons and because otherwise __getattr__() would have no way to access other attributes of the instance. Note that at least for instance variables, you can fake total control by not inserting any values in the instance attribute dictionary (but instead inserting them in another object). See the __getattribute__() method below for a way to actually get total control in new-style classes.

object.__setattr__(self, name, value)

Called when an attribute assignment is attempted. This is called instead of the normal mechanism (i.e. store the value in the instance dictionary). name is the attribute name, value is the value to be assigned to it.

If __setattr__() wants to assign to an instance attribute, it should not simply execute self.name = value — this would cause a recursive call to itself. Instead, it should insert the value in the dictionary of instance attributes, e.g., self.__dict__[name] = value. For new-style classes, rather than accessing the instance dictionary, it should call the base class method with the same name, for example, object.__setattr__(self, name, value).