staticmethod and recursion?

Question

I have following code:

class Foo(object):
    def __init__(self):
        baz=self.bar(10)

    @staticmethod
    def bar(n):
        if n==0:
            return 'bar'
        else:
            return bar(n-1)

bar() as a recursive function it needs reference to itself. However, bar() is inside a class, and calling return bar(n-1) will not work, invoking NameError: global name 'bar' is not defined. How can I deal with this kind of situation? Should I change bar() to a class or instance method, allowing access to self or cls?

Answer 1

You can refer to bar by prefixing it with the class name:

class Foo(object):
    def __init__(self):
        baz=self.bar(10)

    @staticmethod
    def bar(n):
        if n==0:
            return 'bar'
        else:
            return Foo.bar(n-1)

Static methods are nothing but regular functions contained within the namespace of a class after all.

Alternatively, define bar as a regular function instead:

def bar(n):
    if n==0:
        return 'bar'
    else:
        return bar(n-1)

class Foo(object):
    def __init__(self):
        baz=bar(10)

which avoids the whole issue altogether.

Answer 2

An alternative to using a class method or calling the class by name (as shown by others) is to use a closure to hold the reference to the function:

class Foo(object):
    def bar():
        def bar(n):
            if n == 0:
               return "bar"
            return bar(n-1)
        return bar
    bar = staticmethod(bar())

The (minor) advantage is that this is somewhat less susceptible to breaking when names change. For example, if you have a reference to Foo.bar inside bar, this relies on Foo continuing to be a global name for the class that bar is defined in. Usually this is the case but if it isn't, then the recursive call breaks.

The classmethod approach will provide the method with a reference to the class, but the class isn't otherwise needed in the method, which seems inelegant. Using the closure will also be marginally faster because it isn't doing an attribute lookup on each call.