Newshot 
gawain requires type annotations on the variables, parameters, returns and class attributes of Python code, then runs a type checker, ty or pyrefly, to check that the types are correct.
Type checkers such as mypy, Pyright, ty and pyrefly infer the type of a variable that has no annotation. ruff's ANN rules and flake8-annotations ask for annotations on functions only. gawain also asks for them on variables and class attributes.
Why
I built gawain only because I hate dict[str, Any] and wanted to do something about it. If your answer to this is "but Python already infers that" or "but my IDE infers it for me", then don't use it, little padawan.
What it reports
Given shop.py:
PORT = 8080
retries = 3
total = compute()
def send(to, body):
print(f"to {to}: {body}")
class Order:
def __init__(self, items: list[str]) -> None:
self.items = items
gawain check shop.py prints:
[presence]
shop.py:1:1 GW001 variable 'PORT' has no type annotation
shop.py:3:1 GW001 variable 'total' has no type annotation
shop.py:6:5 GW003 function 'send' has no return type annotation
shop.py:6:10 GW002 parameter 'to' of function 'send' has no type annotation
shop.py:6:14 GW002 parameter 'body' of function 'send' has no type annotation
shop.py:12:14 GW004 attribute 'items' is not declared in the body of class 'Order'
6 errors (presence). Type check skipped until presence errors are fixed.
retries = 3 passes, because a literal has an obvious type. PORT is a constant, so it needs Final[int].
When every annotation is in place, gawain runs the type checker on the same files and prints what it reports. With ty, the default, for prices.py:
def total(prices: list[int]) -> int:
return "0"
[types]
prices.py:2:12 ty[invalid-return-type] Return type does not match returned value: expected `int`, found `Literal["0"]`
1 error (types).
Install
Install gawain with the extra of the type checker you want. ty is the default checker, and the ty extra pins it to 0.0.100:
uv tool install "gawain[ty]"
# or
pip install "gawain[ty]"
To use pyrefly instead, install the pyrefly extra, which pins pyrefly 1.4.1 and does not install ty:
uv tool install "gawain[pyrefly]"
# or
pip install "gawain[pyrefly]"
Then choose it with checker = "pyrefly" in [tool.gawain], or with --checker pyrefly. docs/configuration.md explains the switch.
pip install gawain without an extra installs only the gawain command. It then needs a checker on PATH, or --no-types to run the annotation checks only.
Use gawain with care. ty is in beta, and its diagnostics may change between versions. If that is a problem for your project, use pyrefly, or --no-types to run the annotation checks only.
Usage
gawain check src/ # annotations, then types
gawain check src/ --no-types # annotations only
gawain check src/ --checker pyrefly # pyrefly instead of ty
gawain check src/ --mode strict # an annotation on every assignment
gawain check src/ --format json # one JSON object, for tools
gawain exits with 0 when nothing is reported, 1 when it reports an error, and 2 on a usage error or when the type checker cannot run. The type checker's warnings are printed but do not fail the check.
Rules
| Code | Reports |
|---|---|
| GW001 | A variable assigned with no annotation. |
| GW002 | A parameter with no annotation. |
| GW003 | A function with no return annotation. |
| GW004 | A class attribute with no annotation, or a self.x that the class body does not declare. |
| GW010 | A constant that is not annotated Final[T] . |
| GW011 | A bare Final , with no type. |
| GW020 | Any in an annotation. |
GW000 marks a file gawain cannot read, and GW900 and GW901 mark a broken or unused suppression. To silence a line, name the code:
value = load() # gawain: ignore[GW001]
docs/rules.md has the full rules, the exemptions and the suppression syntax.
Modes
The default mode is obvious. A name may go without an annotation when its value has an obvious type and the name is never assigned again: a literal (count = 0), a container of literals (ids = [1, 2, 3]) or a constructor call (user = User("ada")). strict asks for an annotation on every assignment:
[tool.gawain]
mode = "strict"
docs/rules.md has the exact rules of each mode.
pre-commit
repos:
- repo: https://github.com/matheuzgomes/gawain
rev: v0.3.0
hooks:
- id: gawain
pre-commit installs the hook from this repository's source, so building it needs Rust 1.97 or newer. docs/configuration.md covers [tool.gawain], paths and the hook in detail.
Performance
On the CPython 3.14 standard library (1,847 files, 974,586 lines), median of 30 runs (5 for flake8-annotations):
| Tool | Checks | Median |
|---|---|---|
| gawain, obvious mode | variables, parameters, returns, attributes | 0.46 s |
| gawain, strict mode | variables, parameters, returns, attributes | 0.36 s |
| ruff --select ANN | parameters, returns | 0.35 s |
| flake8-annotations | parameters, returns | 11.4 s |
gawain ran with --no-types. This is one benchmark on one machine (AMD Ryzen 7 3700X, WSL2). docs/benchmarks.md has the versions and the commands to run it again.
Documentation
- docs/rules.md: rules, modes, exemptions and suppression.
- docs/configuration.md: paths, output, exit codes, [tool.gawain] and pre-commit.
- docs/benchmarks.md: how the numbers above were measured.
Acknowledgements
gawain reads Python with ruff_python_parser and checks types with ty, both by Astral, or with pyrefly, by Meta.
Name
I don't know, I just like the Arthurian tales. Gawain is a knight of the Round Table, and the mascot is the Green Knight's axe with the green girdle from Sir Gawain and the Green Knight.
License
MIT. See LICENSE.