ty: Check Python Code

ty reads code without running it and reports mistakes.

ty is a Python type checker from Astral, the makers of uv and ruff. Run it as a project chore:

uv run ty check

A clean run ends with:

All checks passed!

Type Hints

A type hint is an optional note in code about what kind of value a function takes or returns, such as def average(values: list[float]) -> float:. Python runs fine without them, and you can add them as your projects grow.

ty reasons possible types from the code (for example, given name = "ada" it knows that name is text).

ty Catches Some Things Even Without Hints

Run on this code, which has no hints:

import pandass


def add(a, b):
    return a + b


add(1)

name = "ada"
name.uppercase()

print(undefined_name)

ty reports:

error[unresolved-import]: Cannot resolve imported module `pandass`
error[missing-argument]: No argument provided for required parameter `b` of function `add`
error[unresolved-attribute]: Object of type `Literal["ada"]` has no attribute `uppercase`
error[unresolved-reference]: Name `undefined_name` used when not defined

These are mistakes we might make frequently: a misspelled import, a missing argument, a method that does not exist, and a name that was never created.

What ty Misses Without Hints

def average(values):
    return sum(values) / len(values)


print(average([1, 2, 3]).upper())

ty passes this, even though a number has no .upper(). With hints, ty can follow the types through:

def average(values: list[float]) -> float:
    return sum(values) / len(values)


print(average([1, 2, 3]).upper())

error[unresolved-attribute]: Object of type `float` has no attribute `upper`

So hints are not required, but they let ty (and your readers) catch more. Add them where they help most: functions other people will call, and functions that take or return data.

Read a ty Message

error[missing-argument]: No argument provided for required parameter `b` of function `add`
  --> demo.py:13:1
   |
13 | add(1)
   | ^^^^^^

error or warning: how serious it is. Fix both.
[missing-argument]: the name of the rule that was broken.
demo.py:13:1: the file, line, and column.
The ^^^^^^ marks the code at fault.

Common Messages

Rule	Usual cause	What to try
`unresolved-import`	Misspelled name, or the package is not installed	Check the spelling. Then run `uv sync`, or `uv add <package>`.
`missing-argument`	A function was called with too few values	Compare the call to the `def` line.
`unresolved-attribute`	A method or attribute does not exist on that kind of value	Check the spelling and the library documentation.
`unresolved-reference`	A name is used before it is created, or is misspelled	Check the spelling, and where the name is defined.
`invalid-assignment` / `invalid-argument-type`	A value does not match a declared type hint	Fix the value, or the hint.

It is best to fix what the message points to. It's finding bugs that can show up down the road. We don't recommend adding comments to silence the checks.

Using modern tools to build high quality code is a differentiator.

WHY?

Python reports many mistakes only when the faulty line runs, which can be after a long data load. A static type checker can find errors early, in seconds.

Running ty every time builds good professional habits: read the tool's message, fix the cause, run it again.

Verification

[ ] uv run ty check ends with All checks passed!.
[ ] You can explain what one ty message means and which line it points to.

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