zulip/docs/contributing/mypy.md

# Python static type checker (mypy)

[mypy](http://mypy-lang.org/) 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.

As a result, Zulip is in the process of migrating from using mypy's
Python 2 compatible syntax for type annotations (in which type
annotations are written inside comments that start with `# type: `) to
the Python 3 syntax.  Here's a brief example of the mypy syntax we're
using in Zulip:

```
user_dict = {} # type: 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 cheat sheet for Python 3](http://mypy.readthedocs.io/en/latest/cheat_sheet_py3.html)
  is the best resource for quickly understanding how to write the PEP
  484 type annotations used by mypy correctly.

* The
  [Python type annotation spec in PEP 484](https://www.python.org/dev/peps/pep-0484/)

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

## `type_debug.py`

`zerver/lib/type_debug.py` 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
3.0
>>> func('a', 'b')
func(str, str) -> str
'ab'
>>> 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, ...}`.

## 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 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, run the command:

    tools/run-mypy

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:

```
test.py: note: In function "test":
test.py:200: 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.  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](https://github.com/zulip/zulip/issues) or posting on
[zulip-devel](https://groups.google.com/d/forum/zulip-devel).  If it's
indeed a mypy bug, we can help with reporting it upstream.