maartenb/metautils

Utilities for writing metaclasses.

metautils

Utilities for writing and composing metaclasses.

Template Model

Why do we need or want to write class templates.

Consider the two metaclasses.

class AllLower(type):
    def __new__(mcls, name, bases, dict_):
        dict_ = {k.lower(): v for k, v in dict_.items()}
        return super().__new__(mcls, name, bases, dict_)


class MethodCatcher(type):
    def __new__(mcls, name, bases, dict_):
        dict_['methods'] = [v for v in dict_.values() if callable(v)]
        return super().__new__(mcls, name, bases, dict_)

What would we do if we wanted to make a class that used BOTH of these metaclasses? Using a class that subclasses both AllLower and MethodCatcher does not work, what we want is a way to chain them.

With the class template model, we could have written our metaclasses as:

from metautils import T, templated

class AllLower(T):
    @templated
    def __new__(mcls, name, bases, dict_, T_):
        dict_ = {k.lower(): v for k, v in dict_.items()}
        return T_.__new__(mcls, name, bases, dict_)


class MethodCatcher(T):
    @templated
    def __new__(mcls, name, bases, dict_, T_):
        dict_['methods'] = [v for v in dict_.values() if callable(v)];
        return T_.__new__(mcls, name, bases, dict_)

Now we can define classes that use BOTH of these metaclasses like so:

class C(object, metaclass=MethodCatcher(AllLower())):
    def F():
        pass

    def g():
        pass

    a = 'a'
    B = 'b'

We can see that this applied the composition of the metaclasses.

>>> C.f
<function __main__.C.F>
>>> C.g
<function __main__.C.g>
>>> C.b
'b'
>>> C.a
'a'
>>> C.methods
[<function __main__.C.g>, <function __main__.C.F>]

The order that the metaclasses are composed is explicit as they act as transformers over each other.

Template

While the previous example only showed metaclasses, you can use this for any class; however, it is most useful for metaclasses where having a compatible metaclass hierarchy is important.

A Template is a callable that takes a type object and returns a new type object. It takes the following arguments:

• base: A type object. default: type.
• adjust_name: Should we prepend the name of the base to the new type object. default: True.

These can be chained together with any concrete metaclass at the end, e.g.:

new_class = m(n,p(q(...z(type)...)))

You can also use the compose function to do this:

from metautils import compose

new_class_template = compose(m, n, p, q, ..., z)