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sharkdp merged 1 commit into from
Apr 27, 2026
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update typing-features and faqs #3335

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50 changes: 0 additions & 50 deletions docs/features/type-system.md
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Expand Up @@ -169,55 +169,5 @@ def to_json(person: Person):

(Full example in the [playground](https://play.ty.dev/34a227bb-93d5-405e-86c3-72f57ec5642e))

## Gradual guarantee

ty generally tries to avoid emitting false positive type errors in untyped code. The following
snippet does not produce any type errors when checked with ty (whereas other type checkers make the
assumption that `max_retries` is of type `None`, leading to an error in the attribute assignment):

```py
class RetryPolicy:
max_retries = None

policy = RetryPolicy()
policy.max_retries = 1
```

(Full example in the [playground](https://play.ty.dev/a5286db1-cdfd-45e7-af54-29649ba5c423))

This is achieved by treating `max_retries` as being of type `Unknown | None`, which means that the
type of the attribute is not fully known, but `None` is definitely a possible value.

Users can always opt into stricter checking by adding type annotations (`int | None`, in this case).

!!! info

We are also planning to add a mode for users that prefer to have stricter types inferred by default
in these situations. You can follow [this issue](https://github.com/astral-sh/ty/issues/1240) for
updates.

## Fixpoint iteration

In a situation where a symbol's type cyclically depends on itself, ty uses a mechanism called fixpoint iteration to
be able to infer a type for that symbol. In the `tick` method below, note how the type of `self.value` depends on
`self.value` itself. ty starts by assuming that `self.value` is just `Unknown | Literal[0]` (the type inferred
from the `__init__` method), and then iterates until the type converges to `Unknown | Literal[0, 1, 2, 3, 4]`.
Without the modulo operation, the union would grow indefinitely. In that case, we fall back to `int` after a certain
number of iterations.

```py
class LoopingCounter:
def __init__(self):
self.value = 0

def tick(self):
self.value = (self.value + 1) % 5

# reveals: Unknown | Literal[0, 1, 2, 3, 4]
reveal_type(LoopingCounter().value)
```

(Full example in the [playground](https://play.ty.dev/64400d96-ee1b-48f3-8361-b583dddddf82))

[materializations]: https://typing.python.org/en/latest/spec/concepts.html#materialization
[python typing documentation]: https://typing.python.org/en/latest/spec/index.html
5 changes: 2 additions & 3 deletions docs/reference/typing-faq.md
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Expand Up @@ -18,9 +18,8 @@ from missing_module import MissingClass # error: unresolved-import
reveal_type(MissingClass) # Unknown
```

ty also uses unions with `Unknown` to maintain the
[gradual guarantee](../features/type-system.md#gradual-guarantee), which helps avoid false positive
errors in untyped code while still providing useful type information where possible.
ty also uses unions with `Unknown` to avoid false positive errors in untyped code while still
providing useful type information where possible.

For example, consider the following untyped `Message` class (which could come from a third-party
dependency that you have no control over). ty treats the `data` attribute as having type
Expand Down