Create a class dynamically without instantiating it - no metaclasses?

Question

Using WTForms form definition classes as an example:

class RegistrationForm(Form):
    username     = StringField('Username', [validators.Length(min=4, max=25)])
    email        = StringField('Email Address', [validators.Length(min=6, max=35)])
    accept_rules = BooleanField('I accept the site rules', [validators.InputRequired()])

and looking at the source of the library it seems WTForms allows a user to define a very simple class of a customised form structure (as above), which in turn then gets used to construct a new field class which is not instantiated when the class is generated.

I've read a number of tutorials about class factories and metaclasses and the general consensus is to avoid metaclasses and use things like class decorators instead. The problem is the tutorials either start importing extra libraries eg: import six, mix explanations of different Python versions together, use overly complex examples or advise not to use metaclasses at all.

Please can somebody provide a very simple explanation (for Python 3) of how to use a simple class definition (like the WTForms example above) along with metaclasses to customise a brand new construction of a class without actually instantiating the class when it's constructed.

Edit: Apologies for finding it difficult to explain what my end goal is but as I have gone through tutorials it has been unclear as to whether class decorators, metaclasses, magic methods (call, new, init) or a combination of such are what I needed to achieve what I visualised, or if what I was visualising was the wrong way of doing things. Unfortunately it seems impossible to judge if my goal was wrong without being able to understand the mechanisms needed to achieve it. I've realised metaclasses are the way to go and just need pointing in the right direction for a very simple metaclass example done the Python 3.x way.

Answer 1

You can create classes dynamically - with no custom metaclasses and no decorators with what looks to the programmer as simple function call.

Just make a call to Python's builtin type with three parameters: the name of the class, a tuple with its bases, and a mapping object with its namespace (i.e. a dictionary containing the attributes and methods you would ordinarily define on the class body).

def __init__(self):
    ...

namespace = {
   '__init__': init,
   'name': 'default name'
}

MyClass = type("MyClass", (object,), namespace)

You loose some features that are only possible due to the compiler doing a couple special things during building functions declared within a class body - mostly the ability to use paramterless super and name mangling of attributes starting with __, but that is it.

That said it should be noted this is not with "no metaclasses". "type" is itself a metaclass - the default Python metaclass for all objects - and is calling a metaclass that create a class. There is no other way to create a class. A "class decorator" is just a method that can makes changes to a class object after it is created.

Any function or method that yields a new, dynamic class, will have inside it to, at some point, call type or other metaclass. In the same mood, a "metaclass" does not create dynamic classes by itself - it needs do be either used in a class body declaration, or called with (at least) the same parameters used for calling type.

As for the recommendations for "class decorators" instead of metaclasses, I am not sure is that true (beyond the fact there is no way a "class decorator" can create classes dynamically by itself): their main drawback is that there is no ordinary way for subclasses of decorated classes to have the parent's class decorators applied to themselves automatically, while metaclasses are inherited.

In Python 3.6 you have the __init_subclass__ protocol which, yes, can avoid a lot of the traditional uses for a metaclass (but still, it won't "create classes dynamically" - calling type does that).