Python static type checker (mypy)

mypy is a compile-time static type checker for Python, allowing optional, gradual typing of Python code. Zulip is using mypy’s Python 2 compatible syntax for type annotations, which means that type annotations are written inside comments that start with # type:. Here’s a brief example of the mypy syntax we’re using in Zulip:

user_dict = {} # type: Dict[str, UserProfile]

def get_user_profile_by_email(email):
    # type: (str) -> 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 ‘static-analysis’ build.

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, ...}.

Zulip goals

Zulip is hoping to reach 100% of the codebase annotated with mypy static types, and then enforce that it stays that way. Our current coverage is shown in Coveralls.

Installing mypy

If you installed Zulip’s development environment correctly, mypy should already be installed inside the Python 3 virtualenv at zulip-py3-venv (mypy only supports Python 3). If it isn’t installed (e.g. because you haven’t reprovisioned recently), you can run tools/install-mypy to install it.

Running mypy on Zulip’s code locally

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


It will output 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")

If you need help interpreting or debugging mypy errors, please feel free to mention @sharmaeklavya2 or @timabbott on your pull request (or email to get help; we’d love to both build a great troubleshooting guide in this doc and also help contribute improvements to error messages upstream.

Since mypy is a new tool under rapid development and occasionally makes breaking changes, Zulip is using a pinned version of mypy from its git repository rather than tracking the (older) latest mypy release on PyPI.

Excluded files

Since several Python files in Zulip’s code don’t pass mypy’s checks (even for unannotated code) right now, a list of files to be excluded from the check for CI is present in tools/run-mypy.

To run mypy on all Python files, ignoring the exclude list, you can pass the --all option to tools/run-mypy.

tools/run-mypy --all

If you type annotate some of those files so that they pass without errors, please remove them from the exclude list.

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. 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 need help understanding an issue, please feel free to mention @sharmaeklavya2 or @timabbott on the relevant pull request or issue on GitHub.

If you think you have found a bug in Zulip or mypy, inform the zulip developers by opening an issue on Zulip’s GitHub repository or posting on zulip-devel. If it’s indeed a mypy bug, we can help with reporting it upstream.

Annotating strings

In Python 3, strings can have non-ASCII characters without any problems. Such characters are required to support languages which use non-latin scripts like Japanese and Hindi. They are also needed to support special characters like mathematical symbols, musical symbols, etc. In Python 2, however, str generally doesn’t work well with non-ASCII characters. That’s why unicode was introduced in Python 2.

But there are problems with the unicode and str system. Implicit conversions between str and unicode use the ascii codec, which fails on strings containing non-ASCII characters. Such errors are hard to detect by people who always write in English. To minimize such implicit conversions, we should have a strict separation between str and unicode in Python 2. It might seem that using unicode everywhere will solve all problems, but unfortunately it doesn’t. This is because some parts of the standard library and the Python language (like keyword argument unpacking) insist that parameters passed to them are str.

To make our code work correctly in Python 2, we have to identify strings which contain data which could come from non-ASCII sources like stream names, people’s names, domain names, content of messages, emails, etc. These strings should be unicode. We also have to identify strings which should be str like Exception names, attribute names, parameter names, etc.

Mypy can help with this. We just have to annotate each string as either str or unicode and mypy’s static type checking will tell us if we are incorrectly mixing the two. However, unicode is not defined in Python 3. We want our code to be Python 3 compatible in the future. This can be achieved using ‘six’, a Python 2 and 3 compatibility library.

six.text_type is defined as str in Python 3 and as unicode in Python 2. We’ll be using text_type (instead of unicode) and str to annotate strings in Zulip’s code. We follow the style of doing from six import text_type and using text_type for annotation instead of doing import six and using six.text_type for annotation, because text_type is used so extensively for type annotations that we don’t need to be that verbose.

Sometimes you’ll find that you have to convert strings from one type to another. zerver/lib/ has utility functions to help with that. It also has documentation (in docstrings) which explains the right way to use them.