A metaclass in Python

The class of a class: it runs at definition time and can rewrite the class.

class Uppercase(type):
    def __new__(mcls, name, bases, namespace):
        constants = {k: v for k, v in namespace.items()
                     if k.isupper() and isinstance(v, str)}
        for key, value in constants.items():
            namespace[key] = value.upper()
        namespace["CONSTANTS"] = tuple(sorted(constants))
        return super().__new__(mcls, name, bases, namespace)


class Config(metaclass=Uppercase):
    THEME = "monokai"
    MODE = "editor"
    other = "left alone"


print(Config.THEME, Config.MODE, Config.other)
print(Config.CONSTANTS, type(Config).__name__)

How it works

1. __new__ on the metaclass receives the namespace being built.
2. Reach for it only when __init_subclass__ cannot do the job.
3. type(cls) tells you which metaclass made a class.

Keywords and builtins used here

• Config
• Uppercase
• class
• def
• for
• if
• isinstance
• print
• return
• sorted
• str
• super
• tuple
• type

The run, in numbers

Lines

18

Characters to type

511

Tokens

148

Three-star pace

110 tpm

At the three-star pace of 110 tokens a minute, this run takes about 81 seconds.

Type this snippet

Step 3 of 3 in Class machinery, step 37 of 53 in Pythonic Python.

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