In Python, when should I use a function instead of a method?

The Zen of Python states that there should only be one way to do things- yet frequently I run into the problem of deciding when to use a function versus when to use a method.

Let's take a trivial example- a ChessBoard object. Let's say we need some way to get all the legal King moves available on the board. Do we write ChessBoard.get_king_moves() or get_king_moves(chess_board)?

Here are some related questions I looked at:

The answers I got were largely inconclusive:

Why does Python use methods for some functionality (e.g. list.index()) but functions for other (e.g. len(list))?

The major reason is history. Functions were used for those operations that were generic for a group of types and which were intended to work even for objects that didn’t have methods at all (e.g. tuples). It is also convenient to have a function that can readily be applied to an amorphous collection of objects when you use the functional features of Python (map(), apply() et al).

In fact, implementing len(), max(), min() as a built-in function is actually less code than implementing them as methods for each type. One can quibble about individual cases but it’s a part of Python, and it’s too late to make such fundamental changes now. The functions have to remain to avoid massive code breakage.

While interesting, the above doesn't really say much as to what strategy to adopt.

This is one of the reasons - with custom methods, developers would be free to choose a different method name, like getLength(), length(), getlength() or whatsoever. Python enforces strict naming so that the common function len() can be used.

Slightly more interesting. My take is that functions are in a sense, the Pythonic version of interfaces.

Lastly, from Guido himself:

Talking about the Abilities/Interfaces made me think about some of our "rogue" special method names. In the Language Reference, it says, "A class can implement certain operations that are invoked by special syntax (such as arithmetic operations or subscripting and slicing) by defining methods with special names." But there are all these methods with special names like __len__ or __unicode__ which seem to be provided for the benefit of built-in functions, rather than for support of syntax. Presumably in an interface-based Python, these methods would turn into regularly-named methods on an ABC, so that __len__ would become
class container:
...
def len(self):
raise NotImplemented
Though, thinking about it some more, I don't see why all syntactic operations wouldn't just invoke the appropriate normally-named method on a specific ABC. "<", for instance, would presumably invoke "object.lessthan" (or perhaps "comparable.lessthan"). So another benefit would be the ability to wean Python away from this mangled-name oddness, which seems to me an HCI improvement.
Hm. I'm not sure I agree (figure that :-).

There are two bits of "Python rationale" that I'd like to explain first.

First of all, I chose len(x) over x.len() for HCI reasons (def __len__() came much later). There are two intertwined reasons actually, both HCI:

(a) For some operations, prefix notation just reads better than postfix -- prefix (and infix!) operations have a long tradition in mathematics which likes notations where the visuals help the mathematician thinking about a problem. Compare the easy with which we rewrite a formula like x*(a+b) into x*a + x*b to the clumsiness of doing the same thing using a raw OO notation.

(b) When I read code that says len(x) I know that it is asking for the length of something. This tells me two things: the result is an integer, and the argument is some kind of container. To the contrary, when I read x.len(), I have to already know that x is some kind of container implementing an interface or inheriting from a class that has a standard len(). Witness the confusion we occasionally have when a class that is not implementing a mapping has a get() or keys() method, or something that isn't a file has a write() method.

Saying the same thing in another way, I see 'len' as a built-in operation. I'd hate to lose that. I can't say for sure whether you meant that or not, but 'def len(self): ...' certainly sounds like you want to demote it to an ordinary method. I'm strongly -1 on that.

The second bit of Python rationale I promised to explain is the reason why I chose special methods to look __special__ and not merely special. I was anticipating lots of operations that classes might want to override, some standard (e.g. __add__ or __getitem__), some not so standard (e.g. pickle's __reduce__ for a long time had no support in C code at all). I didn't want these special operations to use ordinary method names, because then pre-existing classes, or classes written by users without an encyclopedic memory for all the special methods, would be liable to accidentally define operations they didn't mean to implement, with possibly disastrous consequences. Ivan Krstić explained this more concise in his message, which arrived after I'd written all this up.

-- --Guido van Rossum (home page: http://www.python.org/~guido/)

My understanding of this is that in certain cases, prefix notation just makes more sense (ie, Duck.quack makes more sense than quack(Duck) from a linguistic standpoint.) and again, the functions allow for "interfaces".

In such a case, my guess would be to implement get_king_moves based solely on Guido's first point. But that still leaves a lot of open questions regarding say, implementing a stack and queue class with similar push and pop methods- should they be functions or methods? (here I would guess functions, because I really want to signal a push-pop interface)

TLDR: Can someone explain what the strategy for deciding when to use functions vs. methods should be?

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Here's a simple rule of thumb: if the code acts upon a single instance of an object, use a method. Even better: use a method unless there is a compelling reason to write it as a function.

In your specific example, you want it to look like this:

chessboard = Chessboard()
...
chessboard.get_king_moves()

Don't over think it. Always use methods until the point comes where you say to yourself "it doesn't make sense to make this a method", in which case you can make a function.

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My general rule is this - is the operation performed on the object or by the object?

if it is done by the object, it should be a member operation. If it could apply to other things too, or is done by something else to the object then it should be a function (or perhaps a member of something else).

When introducing programming, it is traditional (albeit implementation incorrect) to describe objects in terms of real-world objects such as cars. You mention a duck, so let's go with that.

class duck:
def __init__(self):pass
def eat(self, o): pass
def crap(self) : pass
def die(self)
....

In the context of the "objects are real things" analogy, it is "correct" to add a class method for anything which the object can do. So say I want to kill off a duck, do I add a .kill() to the duck? No... as far as I know animals do not commit suicide. Therefore if I want to kill a duck I should do this:

def kill(o):
if isinstance(o, duck):
o.die()
elif isinstance(o, dog):
print "WHY????"
o.die()
elif isinstance(o, nyancat):
raise Exception("NYAN "*9001)
else:
print "can't kill it."

Moving away from this analogy, why do we use methods and classes? Because we want to contain data and hopefully structure our code in a manner such that it will be reusable and extensible in the future. This brings us to the notion of encapsulation which is so dear to OO design.

The encapsulation principal is really what this comes down to: as a designer you should hide everything about the implementation and class internals which it is not absolutely necessarily for any user or other developer to access. Because we deal with instances of classes, this reduces to "what operations are crucial on this instance". If an operation is not instance specific, then it should not be a member function.

TL;DR: what @Bryan said. If it operates on an instance and needs to access data which is internal to the class instance, it should be a member function.

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Use a class when you want to:

1) Isolate calling code from implementation details -- taking advantage of abstraction and encapsulation.

2) When you want to be substitutable for other objects -- taking advantage of polymorphism.

3) When you want to reuse code for similar objects -- taking advantage of inheritance.

Use a function for calls that make sense across many different object types -- for example, the builtin len and repr functions apply to many kinds of objects.

That being said, the choice sometimes comes down to a matter of taste. Think in terms of what is most convenient and readable for typical calls. For example, which would be better (x.sin()**2 + y.cos()**2).sqrt() or sqrt(sin(x)**2 + cos(y)**2)?

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Generally I use classes to implement a logical set of capabilities for some thing, so that in the rest of my program I can reason about the thing, not having to worry about all the little concerns that make up its implementation.

Anything that's part of that core abstraction of "what you can do with a thing" should usually be a method. This generally includes everything that can alter a thing, as the internal data state is usually considered private and not part of the logical idea of "what you can do with a thing".

When you come to higher level operations, especially if they involve multiple things, I find they are usually most naturally expressed as functions, if they can be built out of the public abstraction of a thing without needing special access to the internals (unless they're methods of some other object). This has the big advantage that when I decide to completely rewrite the internals of how my thing works (without changing the interface), I just have a small core set of methods to rewrite, and then all the external functions written in terms of those methods will Just Work. I find that insisting that all operations to do with class X are methods on class X leads to over-complicated classes.

It depends on the code I'm writing though. For some programs I model them as a collection of objects whose interactions give rise to the behavior of the program; here most important functionality is closely coupled to a single object, and so is implemented in methods, with a scattering of utility functions. For other programs the most important stuff is a set of functions that manipulate data, and classes are in use only to implement the natural "duck types" that are manipulated by the functions.

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I usually think of an object like a person.

Attributes are the person's name, height, shoe size, etc.

Methods and functions are operations that the person can perform.

If the operation could be done by just any ol' person, without requiring anything unique to this one specific person (and without changing anything on this one specific person), then it's a function and should be written as such.

If an operation is acting upon the person (e.g. eating, walking, ...) or requires something unique to this person to get involved (like dancing, writing a book, ...), then it should be a method.

Of course, it is not always trivial to translate this into the specific object you're working with, but I find it is a good way to think of it.