How to implement C-like unsigned int 32 in Python with no external dependencies?

Question

I need a class that would keep all the Python's functionality of int class, but make sure its results are within 32-bit integers, just like in C programming language. The type has to be "poisonous" - performing an operation on int and this type should result in returning this type. As suggested in one of the answers to my other question, I used to use numpy.uint32 for this purpose, but it feels too silly to add such a big dependency just for a single, relatively simple, type. How do I achieve that? Here are my attempts so far:

MODULO = 7  # will be 2**32 later on

class u32:
    def __init__(self, num = 0, base = None):
        print(num)
        if base is None:
            self.int = int(num) % MODULO
        else:
            self.int = int(num, base) % MODULO
    def __coerce__(self, x):
        return None
    def __str__(self):
        return "<u32 instance at 0x%x, int=%d>" % (id(self), self.int)
    def __getattr__(self, x):
        r = getattr(self.int, x)
        if callable(r):
            def f(*args, **kwargs):
                ret = r(*args, **kwargs) % MODULO
                print("x=%s, args=%s, kwargs=%s, ret=%s" % (x, args, kwargs, ret))
                if x not in ['__str__', '__repr__']:
                    return u32(ret)
                return r(*args, **kwargs)
            return f
        return r

u = u32(4)
print("u/2")
a = u * 2
assert(isinstance(a, u32))

print("\n2/u")
a = 2 * u
assert(isinstance(a, u32))

print("\nu+u")
"""
Traceback (most recent call last):
  File "u32.py", line 44, in <module>
    a = u + u
  File "u32.py", line 18, in f
    ret = r(*args, **kwargs) % MODULO
TypeError: unsupported operand type(s) for %: 'NotImplementedType' and 'int'
"""
a = u + u
assert(isinstance(a, u32))

Compatibility. I need to be able to copy some formulas from C to Python and they rely on the overflow behavior. — d33tah, Oct 26 '13 at 19:48
Other than "silliness", why not using `numpy`? Fine work has already been done, for you to enjoy. — shx2, Oct 26 '13 at 19:49
I'd rather keep C-compiled code to minimum, so I'd prefer to keep this type in pure Python. Also, I'm interested in how would that look like. — d33tah, Oct 26 '13 at 19:50
After my experiences with Cygwin, I decided to avoid ctypes. Also, not sure how it's supported in other Python implementations, like Jython. — d33tah, Oct 27 '13 at 13:15

score 1 · Accepted Answer · edited May 23 '17 at 11:57

1

The obviously correct method is going to be to write all the special methods (which can be a pain).

Otherwise, try this for your __getattr__

def __getattr__(self, x):
    r = getattr(self.int, x)
    if callable(r):
        def f(*args, **kwargs):
            if args and isinstance(args[0], u32):
                args = (args[0].int, ) + args[1:]
            ret = r(*args, **kwargs)
            if ret is NotImplemented:
                return ret
            if x in ['__str__', '__repr__', '__cmp__', '__index__']:
                return ret
            ret %= MODULO
            return u32(ret)
        return f
    return r

As Martijn notes, this only works for old-style classes. See his answer for a method that works with new-style classes.

edited May 23 '17 at 11:57

Community

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answered Oct 26 '13 at 19:49

Ethan Furman

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I already tried the version without % MODULO. Has the same problem. – d33tah Oct 26 '13 at 19:51
@d33tah: Okay, try this one. – Ethan Furman Oct 26 '13 at 20:05
self.int = int(num) % MODULO TypeError: int() argument must be a string or a number, not 'tuple' – d33tah Oct 26 '13 at 20:17
@d33tah: Do you still have the `__coerce__` method in place? Which operations gives that error? – Ethan Furman Oct 26 '13 at 20:23
1

`struct.unpack()` returns a tuple. ;) – Ethan Furman Oct 26 '13 at 20:47
@d33tah: and you'll want to add `__index__` to the exclusions. – Ethan Furman Oct 26 '13 at 20:48
What a silly mistake! Looks like the code works now that I applied your changes. Thank you! :) – d33tah Oct 26 '13 at 20:59
`u32(2) == 2`: `TypeError: comparison did not return an int` – shx2 Oct 31 '13 at 22:34
@shx2: Added `__cmp__` to list of non-wrapped items. – Ethan Furman Nov 01 '13 at 00:37
Note, this works **only** on old-style classes. On new-style classes, special methods are looked up on the **type** (so for instances, on the class), bypassing any `__getattr__` or `__getattribute__` hooks, on the instance, the class *and* the meta type! – Martijn Pieters Dec 06 '13 at 19:03
See the [Special Method lookup](http://docs.python.org/2/reference/datamodel.html#special-method-lookup) as to why this is so. I present a workaround in http://stackoverflow.com/a/20430720. – Martijn Pieters Dec 06 '13 at 19:06

How to implement C-like unsigned int 32 in Python with no external dependencies?

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