Generate classes based on a list of names in Python

Question

The approach might be just wrong to begin with, but I'm trying to do the following:

class Material:
    pass


class Vacuum(Material):
    def __str__(self):
        return 'vacuum'


class Aluminum(Material):
    def __str__(self):
        return 'aluminum'


class Graphite(Material):
    def __str__(self):
        return 'graphite'


class Beryllium(Material):
    def __str__(self):
        return 'beryllium'

I have different pieces of code that deals with different materials. Instead of passing a string as argument to that other pieces I would prefer to give it objects. This allows to have tab-completion with ipython and it is also a way to enforce the type.

To avoid changing the already written pieces, those will just do str(argument): if it is a string it recovers the old behavior, if it is one of the objects it will work.

The question is now: I want to support a given list of materials:

allowed_materials = ['vacuum', 'aluminum', 'graphite',]

and that list might be growing. Instead of manually writing the classes, how could I generate them based on the list?

do you need different classes for all materials ? or the name could be an attribute of a class Material ? the class can return this attribute in __str__ — PRMoureu
– PRMoureu, Commented Jul 21, 2018 at 13:25
@PRMoureu Agreed, I also thought about it but in the end the classes will do more than just that. — Cedric H.
– Cedric H., Commented Jul 21, 2018 at 13:49

Patrick Haugh · Accepted Answer · 2018-07-21 14:05:08Z

1

You can define a metaclass that can generate your classes for you.

class mattype(type):
    def __new__(mcls, name, bases=(), d=None):
        def __str__(self):
            return name.lower()
        if not d:
            d = {}
        d['__str__'] = __str__
        bases = (*bases, Material)
        return super().__new__(mcls, name.title(), bases, d)

allowed_materials = ['vacuum', 'aluminum', 'graphite',]
classes = {name: mattype(name) for name in allowed_materials}
str(classes['vacuum']())
# 'vacuum'

answered Jul 21, 2018 at 14:05

Patrick Haugh

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1 Comment

Cedric H. Over a year ago

Thanks. I have to see how that compares to what I'm doing.

Marcus.Aurelianus · Accepted Answer · 2018-07-21 13:27:26Z

1

If you do not need different class name for different material you can simply initialise it inside the material class. If not I will delete my answer.

class Material:
    def __init__(self,name):
        self.name=name
    def __str__(self):
        return self.name

allowed_materials = ['vacuum', 'aluminum', 'graphite',]

obj_lst=[Material(material) for material in allowed_materials]

for obj in obj_lst:
    print(str(obj))

output:

vacuum
aluminum 
graphite

answered Jul 21, 2018 at 13:27

Marcus.Aurelianus

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Comments

Cedric H. · Accepted Answer · 2018-07-21 14:11:08Z

1

I ended up doing the following, also adding objects to the module.

import sys


class Material:
    def __str__(self):
        return self.__class__.__name__

    pass


print(sys.modules[__name__])

_materials = ['Copper', 'Vacuum']

for m in _materials:
    setattr(sys.modules[__name__], m, type(m, (Material,), {})())

answered Jul 21, 2018 at 14:11

Cedric H.

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