Though correct the current answers are potentially not complete.
e.g.
In [1]: a = TestClass()
In [2]: b = a - 5
In [3]: type(b)
Out[3]: <class 'int'>
Shows b as an integer, where you might want it to be a TestClass.
Here is an improved answer, where the functions of the base class are overloaded to return the correct type.
class positive(int):
def __new__(cls, value, *args, **kwargs):
if value < 0:
raise ValueError("positive types must not be less than zero")
return super(cls, cls).__new__(cls, value)
def __add__(self, other):
res = super(positive, self).__add__(other)
return self.__class__(max(res, 0))
def __sub__(self, other):
res = super(positive, self).__sub__(other)
return self.__class__(max(res, 0))
def __mul__(self, other):
res = super(positive, self).__mul__(other)
return self.__class__(max(res, 0))
def __div__(self, other):
res = super(positive, self).__div__(other)
return self.__class__(max(res, 0))
def __str__(self):
return "%d" % int(self)
def __repr__(self):
return "positive(%d)" % int(self)
Now the same sort of test
In [1]: a = positive(10)
In [2]: b = a - 9
In [3]: type(b)
Out[3]: <class '__main__.positive'>
UPDATE:
Added __repr__ and __str__ examples so that the new class prints itself properly. Also changed to Python 3 syntax, even though OP used Python 2, to maintain relevancy.
UPDATE 04/22:
I found myself wanting to do something similar on two recent projects, one where I wanted an Unsigned type (i.e. x-y, where x is 0 and y is positive is still zero).
I also wanted a set-like type that was able to be updated and queried in a certain way.
The above method works but it's repetitive and tedious. What if there was a generic solution using metaclasses?
I could not find one so I wrote one. This will only work in recent Python (I would guess 3.8+, tested on 3.10)
First, the MetaClass
class ModifiedType(type):
"""
ModifedType takes an exising type and wraps all its members
in a new class, such that methods return objects of that new class.
The new class can leave or change the behaviour of each
method and add further customisation as required
"""
# We don't usually need to wrap these
_dont_wrap = {
"__str__", "__repr__", "__hash__", "__getattribute__", "__init_subclass__", "__subclasshook__",
"__reduce_ex__", "__getnewargs__", "__format__", "__sizeof__", "__doc__", "__class__"}
@classmethod
def __prepare__(typ, name, bases, base_type, do_wrap=None, verbose=False):
return super().__prepare__(name, bases, base_type, do_wrap=do_wrap, verbose=verbose)
def __new__(typ, name, bases, attrs, base_type, do_wrap=None, verbose=False):
bases += (base_type,)
# Provide a call to the base class __new__
attrs["__new__"] = typ.__class_new__
cls = type.__new__(typ, name, bases, attrs)
if "dont_wrap" not in attrs:
attrs["dont_wrap"] = {}
attrs["dont_wrap"].update(typ._dont_wrap)
if do_wrap is not None:
attrs["dont_wrap"] -= set(do_wrap)
base_members = set(dir(base_type))
typ.wrapped = base_members - set(attrs) - attrs["dont_wrap"]
for member in typ.wrapped:
obj = object.__getattribute__(base_type, member)
if callable(obj):
if verbose:
print(f"Wrapping {obj.__name__} with {cls.wrapper.__name__}")
wrapped = cls.wrapper(obj)
setattr(cls, member, wrapped)
return cls
def __class_new__(typ, *args, **kw):
"Save boilerplate in our implementation"
return typ.base_type.__new__(typ, *args, **kw)
An example usage to create a new Unsigned type
# Create the new Unsigned type and describe its behaviour
class Unsigned(metaclass=ModifiedType, base_type=int):
"""
The Unsigned type behaves like int, with all it's methods present but updated for unsigned behaviour
"""
# Here we list base class members that we won't wrap in our derived class as the
# original implementation is still useful. Other common methods are also excluded in the metaclass
# Note you can alter the metaclass exclusion list using 'do_wrap' in the metaclass parameters
dont_wrap = {"bit_length", "to_bytes", "__neg__", "__int__", "__bool__"}
import functools
def __init__(self, value=0, *args, **kw):
"""
Init ensures the supplied initial data is correct and passes the rest of the
implementation onto the base class
"""
if value < 0:
raise ValueError("Unsigned numbers cannot be negative")
@classmethod
def wrapper(cls, func):
"""
The wrapper handles the behaviour of the derived type
This can be generic or specific to a particular method
Unsigned behavior is:
If a function or operation would return an int of less than zero it is returned as zero
"""
@cls.functools.wraps(func)
def wrapper(*args, **kw):
ret = func(*args, **kw)
ret = cls(max(0, ret))
return ret
return wrapper
And some tests for the example
In [1]: from unsigned import Unsigned
In [2]: a = Unsigned(10)
...: print(f"a={type(a).__name__}({a})")
a=Unsigned(10)
In [3]: try:
...: b = Unsigned(-10)
...: except ValueError as er:
...: print(" !! Exception\n", er, "(This is expected)")
...: b = -10 # Ok, let's let that happen but use an int type instead
...: print(f" let b={b} anyway")
...:
!! Exception
Unsigned numbers cannot be negative (This is expected)
let b=-10 anyway
In [4]: c = a - b
...: print(f"c={type(c).__name__}({c})")
c=Unsigned(20)
In [5]: d = a + 10
...: print(f"d={type(d).__name__}({d})")
d=Unsigned(20)
In [6]: e = -Unsigned(10)
...: print(f"e={type(e).__name__}({e})")
e=int(-10)
In [7]: f = 10 - a
...: print(f"f={type(f).__name__}({f})")
f=Unsigned(0)
UPDATE for @Kazz:
To answer your question: "what do I need to do to make positive(10)*0.2 work?" Though it would be simpler to just int(u) * 0.2
Here is a small updated wrapper to handle the exception case e.g. (Unsigned * float) that serves as an example of how to modify behavior to match the desired subclass behaviour without having to individually overload each possible combination of argument types.
# NOTE: also add '__float__' to the list of non-wrapped methods
@classmethod
def wrapper(cls, func):
fn_name = func.__name__
@cls.functools.wraps(func)
def wrapper(*args, **kw):
compatible_types = [issubclass(type(a), cls.base_type) for a in args]
if not all(compatible_types):
# Try converting
type_list = set(type(a) for a in args) - set((cls.base_type, cls))
if type_list != set((float,)):
raise ValueError(f"I can't handle types {type_list}")
args = (float(x) for x in args)
ret = getattr(float, fn_name)(*args, **kw)
else:
ret = func(*args, **kw)
ret = cls(max(0, ret))
return ret
return wrapper
