Don't Make It Callable

There is a lot of code that overloads the __call__ method. This is the method that "calling" an object activates: something(x, y, z) will call something.__call__(x, y, z) if something is a member of a Python-defined class.

At first, like every operator overload, this seems like a nifty idea. And then, like most operator overload cases, we need to ask: why? Why is this better than a named method?

The first use-case is easily done better with a named method, and more readably: accepting callbacks. Let's say that the function interesting_files will call the passed-in callback with names of interesting files.

We can, of course, use __call__:

class PrefixMatcher:

    def __init__(self, prefix):
        self.prefix = prefix
        self.matches = []

    def __call__(self, name):
        if name.startswith(self.prefix):
            self.matches.append(name)

    def random_match(self):
        return random.choice(self.matches)

matcher = PrefixMatcher("prefix")
interesting_files(matcher)
print(matcher.random_match())

But it is more readable, and obvious, if we...don't:

class PrefixMatcher:

    def __init__(self, prefix):
        self.prefix = prefix
        self.matches = []

    def get_name(self, name):
        if name.startswith(self.prefix):
            self.matches.append(name)

    def random_match(self):
        return random.choice(self.matches)

matcher = PrefixMatcher("prefix")
interesting_files(matcher.get_name)
print(matcher.random_match())

We can pass the matcher.get_name method, which is already callable directly to interesting_files: there is no need to make PrefixMatcher callable by overloading __call__.

If something really is nothing more than a function call with some extra arguments, then either a closure or a partial would be appropriate.

In the example above, the random_match method was added to make sure that the class PrefixMatcher is justified. If this was not there, either of these implementations would be more appropriate:

def prefix_matcher(prefix):
    matches = []
    def callback(name):
        if name.startswith(prefix):
            matches.append(name)
    return callback, matches

matcher, matches = prefix_matcher("prefix")
interesting_files(matcher)
print(random.choice(matches))

This uses the function closure to capture some variables and return a function.

def prefix_matcher(prefix, matches, name):
    if name.startswith(prefix):
        matches.append(name)

matches = []
matcher = functools.partial(prefix_matcher, "prefix", matches)
interesting_files(matcher)
print(random.choice(matches))

This uses the funcotools.partial functions to construct a function that has some of the arguments "prepared".

There is one important use case for __call__, but it is specialized: it is a powerful tool when constructing a Python-based DSL. Indeed, this is exactly the time when we want to trade away "doing exactly when the operator always does" in favor of "succint syntax dedicated to the task at hand."

A good example of such a DSL is stan, where the __call__ function is used to construct XML tags with attributes: div(style="color: blue").

In almost every other case, avoid the temptation to make your objects callable. They are not functions, and should not be pretending.