Rules of thumb for when to use operator overloading in python

From what I remember from my C++ class, the professor said that operator overloading is cool, but since it takes relatively a lot of thought and code to cover all end-cases (e.g. when overloading + you probably also want to overload ++ and +=, and also make sure to handle end cases like adding an object to itself etc.), you should only consider it in those cases where this feature wil开发者_如何学JAVAl have a major impact on your code, like overloading the operators for the matrix class in a math application.

Does the same apply to python? Would you recommend overriding operator behavior in python? And what rules of thumb can you give me?

Operator overloading is mostly useful when you're making a new class that falls into an existing "Abstract Base Class" (ABC) -- indeed, many of the ABCs in standard library module collections rely on the presence of certain special methods (and special methods, one with names starting and ending with double underscores AKA "dunders", are exactly the way you perform operator overloading in Python). This provides good starting guidance.

For example, a Container class must override special method __contains__, i.e., the membership check operator item in container (as in, if item in container: -- don't confuse with the for statement, for item in container:, which relies on __iter__!-). Similarly, a Hashable must override __hash__, a Sized must override __len__, a Sequence or a Mapping must override __getitem__, and so forth. (Moreover, the ABCs can provide your class with mixin functionality -- e.g., both Sequence and Mapping can provide __contains__ on the basis of your supplied __getitem__ override, and thereby automatically make your class a Container).

Beyond the collections, you'll want to override special methods (i.e. provide for operator overloading) mostly if your new class "is a number". Other special cases exist, but resist the temptation of overloading operators "just for coolness", with no semantic connection to the "normal" meanings, as C++'s streams do for << and >> and Python strings (in Python 2.*, fortunately not in 3.* any more;-) do for % -- when such operators do not any more mean "bit-shifting" or "division remainder", you're just engendering confusion. A language's standard library can get away with it (though it shouldn't;-), but unless your library gets as widespread as the language's standard one, the confusion will hurt!-)

I've written software with significant amounts of overloading, and lately I regret that policy. I would say this:

Only overload operators if it's the natural, expected thing to do and doesn't have any side effects.

So if you make a new RomanNumeral class, it makes sense to overload addition and subtraction etc. But don't overload it unless it's natural: it makes no sense to define addition and subtraction for a Car or a Vehicle object.

Another rule of thumb: don't overload ==. It makes it very hard (though not impossible) to actually test if two objects are the same. I made this mistake and paid for it for a long time.

As for when to overload +=, ++ etc, I'd actually say: only overload additional operators if you have a lot of demand for that functionality. It's easier to have one way to do something than five. Sure, it means sometimes you'll have to write x = x + 1 instead of x += 1, but more code is ok if it's clearer.

In general, like with many 'fancy' features, it's easy to think that you want something when you don't really, implement a bunch of stuff, not notice the side effects, and then figure it out later. Err on the conservative side.

EDIT: I wanted to add an explanatory note about overloading ==, because it seems various commenters misunderstand this, and it's caught me out. Yes, is exists, but it's a different operation. Say I have an object x, which is either from my custom class, or is an integer. I want to see if x is the number 500. But if you set x = 500, then later test x is 500, you will get False, due to the way Python caches numbers. With 50, it would return True. But you can't use is, because you might want x == 500 to return True if x is an instance of your class. Confusing? Definitely. But this is the kind of detail you need to understand to successfully overload operators.

Here is an example that uses the bitwise or operation to simulate a unix pipeline. This is intended as a counter example to most of the rules of thumb.

I just found Lumberjack which uses this syntax in real code

class pipely(object):
    def __init__(self, *args, **kw):
        self._args = args
        self.__dict__.update(kw)

    def __ror__(self, other):
        return ( self.map(x) for x in other if self.filter(x) )

    def map(self, x):
        return x

    def filter(self, x):
        return True

class sieve(pipely):
    def filter(self, x):
        n = self._args[0]
        return x==n or x%n

class strify(pipely):
    def map(self, x):
        return str(x)

class startswith(pipely):
    def filter(self, x):
        n=str(self._args[0])
        if x.startswith(n):
            return x

print"*"*80
for i in xrange(2,100) | sieve(2) | sieve(3) | sieve(5) | sieve(7) | strify() | startswith(5):
    print i

print"*"*80
for i in xrange(2,100) | sieve(2) | sieve(3) | sieve(5) | sieve(7) | pipely(map=str) | startswith(5):
    print i

print"*"*80
for i in xrange(2,100) | sieve(2) | sieve(3) | sieve(5) | sieve(7) | pipely(map=str) | pipely(filter=lambda x: x.startswith('5')):
    print i

Python's overloading is "safer" in general than C++'s -- for example, the assignment operator can't be overloaded, and += has a sensible default implementation.

In some ways, though, overloading in Python is still as "broken" as in C++. Programmers should restrain the desire to "re-use" an operator for unrelated purposes, such as C++ re-using the bitshifts to perform string formatting and parsing. Don't overload an operator with different semantics from your implementation just to get prettier syntax.

Modern Python style strongly discourages "rogue" overloading, but many aspects of the language and standard library retain poorly-named operators for backwards compatibility. For example:

• %: modulus and string formatting
• +: addition and sequence concatenation
• *: multiplication and sequence repetition

So, rule of thumb? If your operator implementation will surprise people, don't do it.