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Type Checking Matrix Multiplication

I have historically struggled with NumPy matrices being the right size while working on ML-oriented code. After reading some of Learn You a Haskell and working on a couple Elm projects (1, 2), I felt like it was a problem I could solve using mypy’s more advanced features.

The basic idea is a wrapper class for NumPy matrices that overloads *, + and whatever else you want to type check the matrix dimensions using mypy before running the code. This only seemed possible because most of the matrices in a my typical ML problems are fixed sizes, and not changing at runtime.

The end result is:

a = Matrix[_100, _500](np.zeros((100, 500))) # 100 x 500 matrix
b = Matrix[_100, _500](np.zeros((100, 500))) # 100 x 500 matrix
c = Matrix[_500, _600](np.zeros((500, 600))) # 500 x 600 matrix
a + b
a * b # throws a mypy error
a * c
a + c # throws a mypy error
a.matrix # access the underlying numpy ndarray

I’m going to spend the rest of this explaining how it works.

Literal Types in MyPy

Literal types in mypy let you define a literal as its own type. This is useful for overloaded functions that return a different type based on a flag value. For example, in a made up function that returns a float or an int based on a flag:

def parse_float_or_int(s: str, is_float: bool) -> Union[float, int]:

You’d have to check if the return value was a float or an int every time you wanted to use the value later on. But if we know that the return type depends on the is_float flag, we can better model this function with literal types.

from typing import overload, Literal

def parse_float_or_int(s: str, is_float: Literal[True]) -> float:

def parse_float_or_int(s: str, is_float: Literal[False]) -> int:

This is basically ripped straight from the mypy docs on literal types

Now if we call parse_float_or_int with a literal True or False, mypy will know the return type. Note that if we pass a variable that is not of type Literal then mypy will need you to narrow the type down from a Union[float, int].

Can we use literals as the dimensions of our matrix to type check matrix multiplication? Ideally we could use Matrix[100, 200] or some variant as our type annotation, knowing that it will only work for matrices where we know the dimensions at “compile”-time.

To the best of my knowledge, we can’t define a generic Matrix class that will take integer literals to create concrete types for the dimensions that can be used later on. The Literal type isn’t a free type if you don’t give it a value. Here’s what I mean:

from typing import TypeVar, Literal, Generic

A = TypeVar("A")

class GenericOverTypeVar(Generic[A]):
    stuff: A # has type of A, whatever it is

# code below causes errors
class GenericOverLiteral(Generic[Literal[A]]):
    stuff: Literal[A] # value is just A

Ideally, I would be able to use GenericOverLiteral with any literal value and then get type checking, like this:

a = GenericOverLiteral[100]
a.stuff # always 100 later on in my code
b = GenericOverLiteral["stay"]
b.stuff # "stay"

I couldn’t bend mypy into this shape. If you know how, I’d love to hear. To get around this, we can define our TypeVar to be bound by int, like so:

# Represent the different dimensions in our matrices
A = TypeVar("A", bound=int)
B = TypeVar("B", bound=int)
C = TypeVar("C", bound=int)

Now A, B and C always have to be int-like. Then, we can use these bounded type variables in a Matrix class:

class Matrix(Generic[A, B]):
    rows: A
    cols: B

    def __init__(self) -> None: ...

Now Matrix is generic over two types, each of which have to be integer-like. The rows and cols will have those two types. But this alone doesn’t do anything useful. Basically, all we’ve said is that Matrix has two attributes, rows and cols, that are both integers. The real fun comes when we overload *:

def __mul__(self, other: Matrix[B, C]) -> Matrix[A, C]: ...

Now we see that we take another Matrix that has the same type of rows as we have columns. Not the same number, but the same type. How do we define these types?

Using Literal:

_100 = Literal[100]
_200 = Literal[200]
_300 = Literal[300]

Now we have three types, all of which are integer like, with values known at “compile”-time. We can use it like this:

a = Matrix[_100, _200]()
b = Matrix[_100, _200]()
c = Matrix[_200, _300]()
a * b # throws a mypy error
a * c # safe

Because __mul__ requires that the generic types for the columns and rows for the first and second matrices, respectively, a * b throws a error in mypy:

Unsupported operand types for * ("Matrix[Literal[100], Literal[200]]" and "Matrix[Literal[100], Literal[200]]").

Not the best error in the world, but clear enough and it happens before running.

The rest of it is just implementation details:

Obviously, the downside of this is having to define types for all the dimensions. If there was a way to do this with just integer literals, that would be a massive improvement. I’d also like someway to verify that the shape of the matrix matches the literals, but it would have to be at runtime anyways.

Please email me if you have any comments or want to discuss further.

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Sam Stevens, 2020