Automatically-generated Python Constructor

I have countless Python classes from various projects from SQLAlchemy (and a couple from Pygame as well), and I recently noticed a pattern in many of them: their constructors alway

Solution 1:

For python >= 3.7, the proper way of handling this is through dataclasses:

This module provides a decorator and functions for automatically adding generated special methods such as __init__() and __repr__() to user-defined classes. It was originally described in PEP 557.

Solution 2:

You can probably do this with Metaclasses. Here's an example of a metaclass which overrides __init__(): Python Class Decorator

You will need to somehow specify the field/argument names, of course - or used named arguments, if you prefer. Here's one way to do that:

# This is the mataclass-defined __init__
def auto_init(self, *args, **kwargs):
    for arg_val, arg_name in zip(args, self.init_args):
        setattr(self, arg_name, arg_val)

    # This would allow the user to explicitly specify field values with named arguments
    self.__dict__.update(kwargs)

class MetaBase(type):
    def __new__(cls, name, bases, attrs):
        attrs['__init__'] = auto_init
        return super(MetaBase, cls).__new__(cls, name, bases, attrs)

class Base(object):
    __metaclass__ = MetaBase

# No need to define __init__
class Foo(Base):
    init_args = ['first', 'last', 'email', 'mi']

Solution 3:

You can do something like this:

class Foo(Base):
    def __init__(self, **kwargs):
        self.__dict__.update(kwargs)

One problem with doing this is there is no guarantee that all instances will have the same members. Another problem is now all constructors must be called with keyword arguments.

Solution 4:

Check out namedtuple:

>>> from collections import namedtuple
>>> Foo = namedtuple("Foo", "first last email mi")
>>> f = Foo("Alfred", "Neumann", "aen@madmagazine.com", "E")
>>> f
Foo(first='Alfred', last='Neumann', email='aen@madmagazinecom', mi='E')