Exhaustiveness Checking with Mypy

Fail at compile time, not at run time


Mypy is an optional static type checker for Python. It's been around since 2012 and is gaining traction even since. One of the main benefits of using a type checker is getting errors at "compile time" rather than at run time.

Exhaustiveness checking is a common feature of type checkers, and a very useful one! In this article I'm going to show you how you can get mypy to perform exhaustiveness checking!

Playing cards are also useful for explaining enumeration types...<br><small>Photo by <a href="https://unsplash.com/photos/G6wlppP4EN8">Daniel Rykhev</a></small>
Playing cards are also useful for explaining enumeration types...
Photo by Daniel Rykhev

Table of Contents


Exhaustiveness Checking

Say you have a system to manage orders. To represent the status of an order, you have the following enum:

import enum

class OrderStatus(enum.Enum):
    Ready = 'ready'
    Shipped = 'shipped'

You also have the following code to process an Order:

def handle_order(status: OrderStatus) -> None:
    if status is OrderStatus.Ready:
        print('ship order')

    elif status is OrderStatus.Shipped:
        print('charge order')

When the order is ready, you ship it; and when it's shipped, you charge it.

A few months go by and your system becomes big. So big in fact, that you can no longer ship orders immediately, and you add a new status:

import enum

class OrderStatus(enum.Enum):
    Ready = 'ready'
    Scheduled = 'scheduled'
    Shipped = 'shipped'

Before you push this change to production, you run a quick check with mypy to make sure everything is OK:

$ mypy main.py
Success: no issues found in 1 source file

Mypy does not see anything wrong in this code, Do you? The problem is that you forgot to handle the new status in your function.

One way to make sure you always handle all possible order statuses is to add an assert, or throw an exception:

def handle_order(status: OrderStatus) -> None:
    if status is OrderStatus.Ready:
        print('ship order')

    elif status is OrderStatus.Shipped:
        print('charge order')

    assert False, f'Unhandled status "{status}"'

Now, when you execute the function with the new status OrderStatus.Scheduled, you will get a runtime error:

>>> handle_order(OrderStatus.Scheduled)
AssertionError: Unhandled status "OrderStatus.Scheduled"

Another way to with deal with cases like this is to go over your test suite and add scenarios in all the places that use order status. But... if you forgot to change the function when you added the status, what are the chances you'll remember to update the tests? That's not a good solution...

Exhaustiveness Checking in Mypy

What if mypy could warn you at "compile time" about such cases? Well... it can, using this little magic function:

from typing import NoReturn
import enum

def assert_never(value: NoReturn) -> NoReturn:
    assert False, f'Unhandled value: {value} ({type(value).__name__})'

Before you dig into the implementation, try to use it to see how it works. In the function above, place assert_never after you handled all the possible order statuses, where you previously used assert or raises an exception:

def handle_order(status: OrderStatus) -> None:
    if status is OrderStatus.Ready:
        print('ship order')

    elif status is OrderStatus.Shipped:
        print('charge order')

    else:
        assert_never(status)

Now, check the code with Mypy:

$ mypy main.py
error: Argument 1 to "assert_never" has incompatible type "Literal[OrderStatus.Scheduled]";
expected "NoReturn"
Found 1 error in 1 file (checked 1 source file)

Amazing! Mypy warns you about a status you forgot to handle! The message also includes the value, OrderStatus.Scheduled. If you use a modern editor such as VSCode you can get these warnings immediately as you type:

mypy Error in VSCode
mypy Error in VSCode

You can now go ahead and fix your function to handle the missing status:

def handle_order(status: OrderStatus) -> None:
    if status is OrderStatus.Pending:
        print('schedule order')

    elif status is OrderStatus.Scheduled:
        print('ship order')

    elif status is OrderStatus.Shipped:
        print('charge order')

    else:
        assert_never(status)

Check with mypy again:

$ mypy main.py
Success: no issues found in 1 source file

Great! You can now rest assure you handled all order statuses. The best part is that you did that with no unit tests, and there were no runtime errors. If you include mypy in your CI, the bad code will never make it into production.


Enumeration types

In the previous section you used mypy to perform exhaustiveness check on an Enum. You can use mypy, and assert_never to perform exhaustiveness check on other enumeration types as well.

Exhaustiveness Checking of a Union

A Union type represents several possible types. For example, a function that casts an argument to float can look like this:

from typing import Union

def get_float(num: Union[str, float]) -> float:
    if isinstance(num, str):
        return float(num)

    else:
        assert_never(num)

Check the function with mypy:

$ mypy main.py
error: Argument 1 to "assert_never" has incompatible type "float"; expected "NoReturn"

Whoops... you forgot to handle the float type in the code:

from typing import Union

def get_float(num: Union[str, float]) -> float:
    if isinstance(num, str):
        return float(num)

    elif isinstance(num, float):
        return num

    else:
        assert_never(num)

Check again:

$ mypy main.py
Success: no issues found in 1 source file

Great! mypy is happy...

Exhaustiveness Checking of a Literal

Another useful type is Literal. It is included in the built-in typing module since Python3.8, and prior to that it is part of the complementary typing_extensions package.

A Literal is used to type primitive values such as strings and numbers. Literal is also an enumeration type, so you can use exhaustiveness checking on it as well:

from typing_extensions import Literal

Color = Literal['R', 'G', 'B']

def get_color_name(color: Color) -> str:
    if color == 'R':
        return 'Red'
    elif color == 'G':
        return 'Green'
    # elif color == 'B':
    #     return 'Blue'
    else:
        assert_never(color)

Checking the code without the commented part will produce the following error:

$ mypy main.py
error: Argument 1 to "assert_never" has incompatible type "Literal['B']"; expected "NoReturn"

Very handy indeed!


Type Narrowing in Mypy

Now that you've seen what assert_never can do, you can try and understand how it works. assert_never works alongside "type narrowing", which is a mypy feature where the type of a variable is narrowed based on the control flow of the program. In other words, mypy is gradually eliminating possible types for a variable.

First, it's important to understand how various things translate to a Union type in mypy:

Optional[int]
# Equivalent to Union[int, None]

Literal['string', 42, True]
# Equivalent to Union[Literal['string'], Literal[42], Literal[True]]

class Suit(Enum):
    Clubs = "♣"
    Diamonds = "♦"
    Hearts = "♥"
    Spades = "♠"

Suit
# ~Equivalent to Union[
#   Literal[Suit.Clubs],
#   Literal[Suit.Diamonds],
#   Literal[Suit.Hearts],
#   Literal[Suit.Spades]
# ]

To display the type of an expression, mypy provides a useful utility called reveal_type. Using reveal_type you can ask mypy to show you the inferred type for a variable at the point it's called:

def describe_suit(suit: Optional[Suit]) -> str:
    # Revealed type is Union[Suit, None]
    reveal_type(suit)

In the function above, the reveled type of suit is Union[Suit, None], which is the type of the argument suit.

At this point you haven't done anything in the function, so mypy is unable to narrow down the type. Next, add some logic and see how mypy narrows down the type of the variable suit:

def describe_suit(suit: Optional[Suit]) -> str:
    assert suit is not None
    # Revealed type is Suit
    reveal_type(suit)

After eliminating the option of suit being None, the revealed type is Suit. Mypy used your program's logic to narrow the type of the variable.

Keep in mind, the type Suit is equivalent to the type Union[Literal[Suit.Clubs], Literal[Suit.Diamonds], Literal[Suit.Hearts], Literal[Suit.Spades]], so next, try to narrow down the type even more:

def describe_suit(suit: Optional[Suit]) -> str:
    assert suit is not None

    if suit is Suit.Clubs:
        # Revealed type is Literal[Suit.Clubs]
        reveal_type(suit)
        return "Clubs"

    # Revealed type is Literal[Suit.Diamonds, Suit.Hearts, Suit.Spades]
    reveal_type(suit)

After checking if suit is Suit.Clubs, mypy is able to narrow down the type to Suit.Clubs. Mypy is also smart enough to understand that if the condition does not hold, the variable is definitely not Clubs, and narrows down the type to Diamonds, Hearts or Spades.

Mypy can also use other conditional statements to further narrow the type, for example:

def describe_suit(suit: Optional[Suit]) -> str:
    assert suit is not None

    if suit is Suit.Clubs:
        # Revealed type is Literal[Suit.Clubs]
        reveal_type(suit)
        return "Clubs"

    # Revealed type is Literal[Suit.Diamonds, Suit.Hearts, Suit.Spades]
    reveal_type(suit)

    # `and`, `or` and `not` also work.
    if suit is Suit.Diamonds or suit is Suit.Spades:
        # Revealed type is Literal[Suit.Diamonds, Suit.Spades]
        reveal_type(suit)
        return "Diamonds or Spades"

    # Revealed type is Literal[Suit.Hearts]
    reveal_type(suit)

By the end of the function, mypy narrowed down the type of suit to Suit.Hearts. If, for example, you add a condition that imply a different type for suit, mypy will issue an error:

def describe_suit(suit: Optional[Suit]) -> str:
    assert suit is not None

    if suit is Suit.Clubs:
        # Revealed type is Literal[Suit.Clubs]
        reveal_type(suit)
        return "Clubs"

    # Revealed type is Literal[Suit.Diamonds, Suit.Hearts, Suit.Spades]
    reveal_type(suit)

    # `and`, `or` and `not` also work.
    if suit is Suit.Diamonds or suit is Suit.Spades:
        # Revealed type is Literal[Suit.Diamonds, Suit.Spades]
        reveal_type(suit)
        return "Diamonds or Spades"

    # Revealed type is Literal[Suit.Hearts]
    reveal_type(suit)

    # mypy error [comparison-overlap]: Non-overlapping identity check
    # left operand type: "Literal[Suit.Hearts]"
    # right operand type: "Literal[Suit.Diamonds]"
    if suit is Suit.Diamonds:
        # mypy error [unreachable]: Statement is unreachable
        return "Diamonds"

After mypy narrowed down the type of suit to Literal[Suit.Hearts], it knows the next condition suit is Suit.Diamonds will always evaluate to False, and issues an error.

Once all the possibilities have been narrowed-out, the rest of the function becomes unreachable:

def describe_suit(suit: Optional[Suit]) -> str:
    assert suit is not None

    if suit is Suit.Clubs:
        return "Clubs"

    if suit is Suit.Diamonds or suit is Suit.Spades:
        return "Diamonds or Spades"

    if suit == Suit.Hearts:
        return 'Hearts'

    # This is currently unreachable
    assert_never(suit)

assert_never works by taking an argument of type NoReturn, which is only possible when the argument type is "empty". That is, when all possibilities have been narrowed-out and the statement is unreachable. If the statement does become reachable, then the NoReturn is not allowed and mypy issues an error. To illustrate, remove the last condition and check the code with mypy:

def describe_suit(suit: Optional[Suit]) -> str:
    assert suit is not None

    if suit is Suit.Clubs:
        return "Clubs"

    if suit is Suit.Diamonds or suit is Suit.Spades:
        return "Diamonds or Spades"

    # if suit == Suit.Hearts:
    #     return 'Hearts'

    # mypy error: Argument 1 to "assert_never" has
    # incompatible type "Literal[Suit.Hearts]"; expected "NoReturn"
    assert_never(suit)

Mypy narrowed down the type of suit to Suit.Hearts, but assert_never expects NoReturn. This mismatch triggers the error, which effectively performs exhaustiveness checking for suit.


The Future

In 2018 Guido though assert_never is a pretty clever trick, but it never made it into mypy. Instead, exhaustiveness checking will become officially available as part of mypy if/when PEP 622 - Structural Pattern Matching is implemented. Until then, you can use assert_never instead.


Bonus: Exhaustiveness Checking in Django

Django provides a very useful attribute to most model field types called choices:

from django.db import models
from django.utils.translation import gettext_lazy as _

class Order(models.Model):

    status: str = models.CharField(
        max_length = 20,
        choices = (
            ('ready', _('Ready')),
            ('scheduled', _('Scheduled')),
            ('shipped', _('Shipped')),
        ),
    )

When you provide choices to a field, Django adds all sorts of nice things to it:

  • Add a validation check to ModelForm (which are used by Django admin, among others)
  • Render the field as a <select> html element in forms
  • Add a get_{field}_display_name method to get the description

However, mypy can't know that a Django field with choices has a limited set of values, so it cannot perform exhaustiveness checking on it. To adapt our example from before:

# Will not perform exhaustiveness checking!
def handle_order(order: Order) -> None:
    if order.status == 'ready':
        print('ship order')

    elif order.status == 'shipped':
        print('charge order')

    else:
        assert_never(status)

The function is not handling the status "scheduled", but mypy can't know that.

One way to overcome this is to use an enum to generate the choices:

import enum
from django.db import models

class OrderStatus(enum.Enum):
    Ready = 'ready'
    Scheduled = 'scheduled'
    Shipped = 'shipped'

class Order(models.Model):
    status: str = models.CharField(
        max_length = 20,
        choices = ((e.value, e.name) for e in OrderStatus),
    )

Now, you can achieve exhaustiveness checking with a slight change to the code:

def handle_order(order: Order) -> None:
    status = OrderStatus(order.status)

    if status is OrderStatus.Pending:
        print('ship order')

    elif status is OrderStatus.Shipped:
        print('charge order')

    else:
        assert_never(status)

The tricky part here is that the model field status is actually a string, so to achieve exhaustiveness checking you have to turn the value into an instance of the OrderStatus enum. There are two downsides to this approach:

  1. You have to cast the value every time: This is not very convenient. This can possibly be solved by implementing a custom "Enum field" in Django.

  2. The status descriptions are not translated: Previously you used gettext (_) to translate the enum values, but now you just used the description of the enum.

While the first is still a pain, the second issue was addressed in Django 3.1 with the addition of Django enumeration types:

from django.db import models

class OrderStatus(models.TextChoices):
    Ready = 'ready', _('Ready')
    Scheduled = 'scheduled', _('Scheduled')
    Shipped = 'shipped', _('Shipped')

class Order(models.Model):
    status: str = models.CharField(
        max_length = 20,
        choices = OrderStatus.choices,
    )

Notice how you replaced the enum with a TextChoices. The new enumeration type looks a lot like an Enum (it actually extends Enum under the hood), but it let's you provide a tuple with a value and a description instead of just the value.


Updates

After publishing this article a few readers suggested ways to improve the implementation, so I made the following edits:

  1. 2020-12-09: The initial version of the article had assert_never take a value of type NoReturn. A commenter on Lobsters made an excellent suggestion to use the more intuitive Union[()] type instead. This also results in a better error message.

  2. 2020-12-09: The initial version of the article used assert False, ... in assert_never instead of raise AssertionError(...). A commenter on Lobsters mentioned that assert statements are removed when python is run with the -O flag. Since the assert in assert_never should not be removed, I changed it to raise AssertionError instead.

  3. 2020-12-10: After looking some more, tmcb found that Union[()] is not currently accepted by Python at runtime, so I reverted the argument to NoReturn again.