Skip to main content
This is a pre-production deployment of Warehouse. Changes made here affect the production instance of PyPI (
Help us improve Python packaging - Donate today!

Utilities for working with metaclasses.

Project Description

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):
    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):
    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():

    def g():

    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
>>> C.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.


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)
Release History

Release History

This version
History Node


History Node


History Node


Download Files

Download Files

Download the file for your platform. If you're not sure which to choose, learn more about installing packages.

File Name & Checksum SHA256 Checksum Help Version File Type Upload Date
metautils-0.1.2.tar.gz (8.6 kB) Copy SHA256 Checksum SHA256 Source Apr 13, 2016

Supported By

WebFaction WebFaction Technical Writing Elastic Elastic Search Pingdom Pingdom Monitoring Dyn Dyn DNS Sentry Sentry Error Logging CloudAMQP CloudAMQP RabbitMQ Heroku Heroku PaaS Kabu Creative Kabu Creative UX & Design Fastly Fastly CDN DigiCert DigiCert EV Certificate Rackspace Rackspace Cloud Servers DreamHost DreamHost Log Hosting