In [79]: _cache = {}

In [80]: def cached(cls):
    ...:     from functools import wraps
    ...:     import inspect
    ...:     sig = inspect.signature(cls)
    ...:     @wraps(cls)
    ...:     def wrapped(*args, **kwargs):
    ...:         bound_values = sig.bind(*args, **kwargs)
    ...:         bound_values.apply_defaults()
    ...:         s = frozenset(bound_values.arguments.items())
    ...:         if s in _cache:
    ...:             return _cache[s]
    ...:         _cache[s] = cls(*args, **kwargs)
    ...:         return _cache[s]
    ...:     return wrapped
    ...:

In [81]: @cached
    ...: class Spam:
    ...:     def __init__(self, x, y, z=None):
    ...:         self.x = x
    ...:         self.y = y
    ...:         self.z = z
    ...:

In [82]: s1 = Spam(10, 20)

In [83]: s2 = Spam(x=10, y=20)

In [84]: s1 is s2
Out[84]: True

In [85]: s3 = Spam(x=10, y=20, z=None)

In [86]: s2 is s3
Out[86]: True