copy.deepcopy raises TypeError on objects with self-defined __new__() method

Question

I want to implement a symbol type, which keeps track of the symbols we already have(saved in _sym_table), and return them if they exist, or create new ones othe

Answer 1

one problem is that deepcopy and copy have no way of knowing which arguments to pass to __new__, therefore they only work with classes that don't require constructor arguments.

the reason why you can have __init__ arguments is that __init__ isn't called when copying an object, but __new__ must be called to create the new object.

so if you want to control copying, you'll have to define the special __copy__ and __deepcopy__ methods:

def __copy__(self):
    return self

def __deepcopy__(self, memo):
    return self

by the way, singletons are evil and not really needed in python.