dsimcha dsimcha - 4 years ago 108
Python Question

Metaclass to parametrize Inheritance

I've read some tutorials on Python metaclasses. I've never used one before, but I need one for something relatively simple and all the tutorials seem geared towards much more complex use cases. I basically want to create a template class that has some pre-specified body, but takes its base class as a parameter. Since I got the idea from C++/D templates, here's an example of what the code I want to write would look like in C++:

template<class T>
class Foo : T {
void fun() {}
}

Answer Source

Although it certainly can be done with metaclasses, you can do what you want without them because in Python classes are themselves objects. The means that—surprisingly—essentially nothing more than an almost one-to-one translation of the C++ code is required. Besides being relatively uncomplicated because of this, it'll also work without modification in both Python 2 & 3.

def template(class_T):
    """Factory function to create subclasses of class_T."""

    class Foo(class_T):
        def fun(self):
            print('%s.fun()' % self.__class__.__name__)

    Foo.__name__ += '_' + class_T.__name__  # rename the subclass to reflect its heritage
    return Foo

class Base1:
    def bar(self):
        print('Base1.bar()')

class Base2:
    def bar(self):
        print('Base2.bar()')

Foo_Base1 = template(Base1)
print('Foo_Base1 base classes: {}'.format(Foo_Base1.__bases__))

Foo_Base2 = template(Base2)
print('Foo_Base2 base classes: {}'.format(Foo_Base2.__bases__))

subclass1 = Foo_Base1()
subclass1.fun()
subclass1.bar()
subclass2 = Foo_Base2()
subclass2.fun()
subclass2.bar()

Output:

Foo_Base1 base classes: (<class __main__.Base1 at 0x00A79C38>,)
Foo_Base2 base classes: (<class __main__.Base2 at 0x00A79DC0>,)
Foo_Base1.fun()
Base1.bar()
Foo_Base2.fun()
Base2.bar()

The code the (unimaginatively-named) template() function is an example of what is commonly called a class factory or an implementation of the Factory pattern. So, incidentally, you might find my answer to the question What exactly is a Class Factory? informative.

Edit: Added code to create different class names for each subclass returned—which was inspired by @aaronasterling's insight (in a now deleted comment) about potential confusion when debugging if the class manufactured always has the same name.

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