Python static type checker (mypy)

mypy is a compile-time static type checker for Python, allowing optional, gradual typing of Python code. Zulip was fully annotated with mypy’s Python 2 syntax in 2016, before our migration to Python 3 in late 2017. In 2018 and 2020, we migrated essentially the entire codebase to the nice PEP 484 (Python 3 only) and PEP 526 (Python 3.6) syntax for static types:

user_dict: Dict[str, UserProfile] = {}

def get_user(email: str, realm: Realm) -> UserProfile:
    ... # Actual code of the function here

You can learn more about it at:

The mypy type checker is run automatically as part of Zulip’s Travis CI testing process in the backend build.

Installing mypy

mypy is installed by default in the Zulip development environment. If you’d like to install just the version of mypy that we’re using (useful if e.g. you want mypy installed on your laptop outside the Vagrant guest), you can do that with pip install -r requirements/mypy.txt.

Running mypy on Zulip’s code locally

To run mypy on Zulip’s python code, you can run the command:


Mypy outputs errors in the same style as a compiler would. For example, if your code has a type error like this:

foo = 1
foo = '1'

you’ll get an error like this: note: In function "test": error: Incompatible types in assignment (expression has type "str", variable has type "int")

Mypy is there to find bugs in Zulip before they impact users

For the purposes of Zulip development, you can treat mypy like a much more powerful linter that can catch a wide range of bugs. If, after running tools/run-mypy on your Zulip branch, you get mypy errors, it’s important to get to the bottom of the issue, not just do something quick to silence the warnings, before we merge the changes. Possible explanations include:

  • A bug in any new type annotations you added.

  • A bug in the existing type annotations.

  • A bug in Zulip!

  • Some Zulip code is correct but confusingly reuses variables with different types.

  • A bug in mypy (though this is increasingly rare as mypy is now fairly mature as a project).

Each explanation has its own solution, but in every case the result should be solving the mypy warning in a way that makes the Zulip codebase better. If you’re having trouble, silence the warning with an Any or # type: ignore[code] so you’re not blocked waiting for help, add a # TODO: comment so it doesn’t get forgotten in code review, and ask for help in

mypy stubs for third-party modules.

For the Python standard library and some popular third-party modules, the typeshed project has stubs, basically the equivalent of C header files defining the types used in these Python APIs.

For other third-party modules that we call from Zulip, one either needs to add an ignore_missing_imports entry in mypy.ini in the root of the project, letting mypy know that it’s third-party code, or add type stubs to the stubs/ directory, which has type stubs that mypy can use to type-check calls into that third-party module.

It’s easy to add new stubs! Just read the docs, look at some of existing examples to see how they work, and remember to remove the ignore_missing_imports entry in mypy.ini when you add them.

For any third-party modules that don’t have stubs, mypy treats everything in the third-party module as an Any, which is the right model (one certainly wouldn’t want to need stubs for everything just to use mypy!), but means the code can’t be fully type-checked.

zerver/lib/ has a useful decorator print_types. It prints the types of the parameters of the decorated function and the return type whenever that function is called. This can help find out what parameter types a function is supposed to accept, or if parameters with the wrong types are being passed to a function.

Here is an example using the interactive console:

>>> from zerver.lib.type_debug import print_types
>>> @print_types
... def func(x, y):
...     return x + y
>>> func(1.0, 2)
func(float, int) -> float
>>> func('a', 'b')
func(str, str) -> str
>>> func((1, 2), (3,))
func((int, int), (int,)) -> (int, int, int)
(1, 2, 3)
>>> func([1, 2, 3], [4, 5, 6, 7])
func([int, ...], [int, ...]) -> [int, ...]
[1, 2, 3, 4, 5, 6, 7]

print_all prints the type of the first item of lists. So [int, ...] represents a list whose first element’s type is int. Types of all items are not printed because a list can have many elements, which would make the output too large.

Similarly in dicts, one key’s type and the corresponding value’s type are printed. So {1: 'a', 2: 'b', 3: 'c'} will be printed as {int: str, ...}.

Using @overload to accurately describe variations

Sometimes, a function’s type is most precisely expressed as a few possibilites, and which possibility can be determined by looking at the arguments. You can express that idea in a way mypy understands using @overload. For example, check_list returns a Validator function that verifies that an object is a list, raising an exception if it isn’t.

It supports being passed a sub_validator, which will verify that each element in the list has a given type as well. One can express the idea “If sub_validator validates that something is a ResultT, check_list(sub_validator) validators that something is a List[ResultT] as follows:

def check_list(sub_validator: None, length: Optional[int]=None) -> Validator[List[object]]:
def check_list(sub_validator: Validator[ResultT],
               length: Optional[int]=None) -> Validator[List[ResultT]]:
def check_list(sub_validator: Optional[Validator[ResultT]]=None,
               length: Optional[int]=None) -> Validator[List[ResultT]]:

The first overload expresses the types for the case where no sub_validator is passed, in which case all we know is that it returns a Validator[List[object]]; whereas the second defines the type logic for the case where we are passed a sub_validator.

See the mypy overloading documentation for more details.

Troubleshooting advice

All of our linters, including mypy, are designed to only check files that have been added in git (this is by design, since it means you have untracked files in your Zulip checkout safely). So if you get a mypy error like this after adding a new file that is referenced by the existing codebase:

mypy | zerver/ note: Import of 'zerver.lib.markdown_wrappers' ignored
mypy | zerver/ note: (Using --follow-imports=error, module not passed on command line)

The problem is that you need to git add the new file.