This is a followup to my last blog post Function Composition in Python. In that post, I talked about how to implement function composition with various operators in Python. Today, I’m going to make use of one of the features that is new in Python 3.5, and show you how to take it one step further to typed function composition.

Function composition has an interesting type signature – it is a function that takes two functions, and returns a new function with the input type of the second function and the output type of the first. In Haskell, the type signature would look something like this:

compose :: (b -> c) -> (a -> b) -> (a -> c)

compose is a function that takes two arguments, the first, a function that takes a value of type b and returns a value of type c, the second, a function that takes a value of type a and returns a value of type b. compose returns a new function, which takes a value of type a, and returns a value of type c (it’s basically cutting out the middleman).

For those who took a logic class, this might seem a little familiar:

a -> b
b -> c
a -> c

Now in Haskell, this is easy to specify and type check. What I am interested in exploring is whether or not Python’s new “type-hints” system is powerful enough to express this same meaning, and capture errors when the output type of the second function doesn’t match the input type of the first. Let’s start with a simple definition of function composition, stolen from the last article.

def compose(f1, f2):
    def inner(argument):
        return f1(f2(argument))
    return inner

Now, we’ll annotate that with types, from Python 3.5’s new type hints system. This might get a little messy.

from typing import Callable, TypeVar

T1 = TypeVar('T1')
T2 = TypeVar('T2')
T3 = TypeVar('T3')

def compose(f1: Callable[[T2], T3],
            f2: Callable[[T1], T2]) -> Callable[[T1], T3]:
    def inner(arg: T1) -> T3:
        return f1(f2(arg))
return inner

Python 3.5 introduced the typing module, which has a variety of functions classes for defining types. One of the more powerful features is TypeVar. TypeVar allows you to define a variable that can be many types, but within a declaration, all instances of a specific TypeVar must be the same type. For something like function composition, this is a necessity, because the compose function does not know or care what the input/output types of the input functions are, so long as the input of one matches the output of the other.

For function composition, we need three TypeVars - the first, to represent the input of second function (in this case, T1), the second, to represent the output of the second function (T2), which is also the input of the first function, and the third, to represent the output of the first function (T3). Now, let’s dig into the actual signature of compose.

Let’s start with our two arguments, f1 and f2. They both have the structure Callable[[InType], OutType], where InType is the input type of the function, and OutType is the return type of the function. InType is encapsulated by a list because functions can have multiple arguments in Python. The Callable after the -> indicates the return type, in this case, our composed function, with an input type of T1 and an output type of T3. This declaration of compose is a little wordy, but I think it is actually pretty elegant, once you get past the syntax. Now, the real question is, does it work? Let’s say we have the following toy functions:

def plus2(x):
    return x + 2

def minus2(x):
    return float(x - 2)

Uh-oh - minus2 silently converts everything to a float. Let’s see what these would look like with types:

def plus2(x: int) -> int:
    return x + 2

def minus2(x: int) -> float:
    return float(x - 2)

So let’s say we wanted to make a completely useless function, add0. We can do this with compose in two ways

add0_plus_first = compose(minus2, plus2)
add0_minus_first = compose(plus2, minus2)

Now, let’s figure out what the types of these functions are. plus2 takes an int and returns an int, and minus2 takes an int and returns a float. This means that add0_plus_first would have the type:

plus2: (T1: int) -> (T2: int)
minus2: (T2: int) -> (T3: float)
add0_plus_first: (T1: int) -> (T3: float)

This is perfectly fine, type-wise. The input type T2 of minus2 matches the output type T2 of plus2. T2 is easily resolvable to int. The problem is with add0_minus_first. minus2 returns a float, but plus2 expects an int.

minus2: (T1: int) -> (T2: float)
plus2: (T2: int) -> (T3: int)

This means that in the compose function, T2 would have to be of type int and of type float. Obviously, it cannot be both, so any good type system would reject it.

Python 3.5 technically only included the ability to add type-hints to your code, but does not actually provide any type-checking functionality. For that, I used a static type-checker for python called mypy. The result of running mypy (you can download here) is: error: Cannot infer type argument 1 of "compose"

Success! This failed on line 24, the line on which we create add0_minus_first. Unfortunately, the error is kind of vague (what is type argument 1?), but I think it’s fantastic that mypy was able to catch this error at all – improving the error messages is the easier part. So there we have it, a type-checkable compose function in Python. Thanks for sticking with me through that, and I hope it wasn’t too hard to follow.