# up-type-checking-lazy-import (SMP306)
SMP306🔧 Fix sometimes availableImports and dependenciesAll rules
Flags an `if TYPE_CHECKING:` block that holds only imports and no `else`, where Python 3.15 can spell each import as a
`lazy` import. The guard is then only needed for names that are not imported at runtime, which `lazy` covers.
## Why is this bad?
The guard keeps the imports out of the running program, so the names they bind exist only for type checkers, and code
that reads them at runtime fails. A `lazy` import binds the name at module level and runs the import on first use
([PEP 810](https://peps.python.org/pep-0810/)), so the guard is no longer needed and a name used at runtime resolves.
## Example
### Example: fix
`pyproject.toml`
```toml
[project]
name = "demo"
requires-python = ">=3.15"
```
Before
```python
# Demo module.
from typing import TYPE_CHECKING
if TYPE_CHECKING:
from collections.abc import Sequence
def first(values: Sequence[int]) -> int:
return values[0]
```
After
```python
# Demo module.
lazy from collections.abc import Sequence
def first(values: Sequence[int]) -> int:
return values[0]
```
## Configuration
- The Python version the code targets, taken from
[`target-version`](https://docs.astral.sh/ruff/settings/#target-version) in `[tool.ruff]`, falling back to
[`project.requires-python`](https://packaging.python.org/en/latest/specifications/pyproject-toml/#requires-python);
see [Python version](../../configuration/project.html.md#python-version)
### Example: configuration
`pyproject.toml`
```toml
[tool.ruff]
target-version = "py314"
```
```python
from typing import TYPE_CHECKING
if TYPE_CHECKING:
from collections.abc import Sequence
```
## What it does
### Active on
- **`if TYPE_CHECKING:` (or `typing.TYPE_CHECKING`) at module level**, with a body of `import` and `from ... import`
statements only and no `else` or `elif`. The report sits on the condition. The fix is unsafe: a `lazy` import loads
the module on first use, so a name a program never touches costs nothing, while a name it does touch runs the import
then. An import cycle or a module missing at runtime, such as `_typeshed`, then surfaces at that point instead of
never.
### Example: fix
`pyproject.toml`
```toml
[project]
name = "demo"
requires-python = ">=3.15"
```
Before
```python
# Demo module.
from typing import TYPE_CHECKING
if TYPE_CHECKING:
from os import PathLike
def path(value: PathLike) -> None:
pass
```
After
```python
# Demo module.
lazy from os import PathLike
def path(value: PathLike) -> None:
pass
```
### Exceptions
- **Blocks with an `else`, an `elif`, or other statements**: the rule needs a block that only imports.
### Example: not reported
`pyproject.toml`
```toml
[project]
name = "demo"
requires-python = ">=3.15"
```
```python
from typing import TYPE_CHECKING
if TYPE_CHECKING:
from os import PathLike
else:
PathLike = str
```
- **Imports of `from __future__` and `from x import *`**: `lazy` has no form for them.
### Example: not reported
`pyproject.toml`
```toml
[project]
name = "demo"
requires-python = ">=3.15"
```
```python
from typing import TYPE_CHECKING
if TYPE_CHECKING:
from os import *
```
- **Blocks inside another statement**, such as a `try` or a function: `lazy` imports are not allowed there, so the rule
does not look at them.
### Example: not reported
`pyproject.toml`
```toml
[project]
name = "demo"
requires-python = ">=3.15"
```
```python
from typing import TYPE_CHECKING
try:
if TYPE_CHECKING:
from os import PathLike
except ImportError:
pass
```
### Fix
The fix is unsafe, as described under “Active on” above. It replaces the block with one `lazy` import per statement,
and, when `TYPE_CHECKING` is then unused, removes its `from typing import TYPE_CHECKING` line. A block with a comment in
it is reported without a fix, because the fix would drop the comment. A block importing `_typeshed` is too.
### Example: fix
`pyproject.toml`
```toml
[project]
name = "demo"
requires-python = ">=3.15"
```
Before
```python
# Demo module.
from typing import TYPE_CHECKING
if TYPE_CHECKING:
from os import PathLike
```
After
```python
# Demo module.
lazy from os import PathLike
```
## Other tools
- **[Ruff](https://github.com/astral-sh/ruff):** open issue [#29378](https://github.com/astral-sh/ruff/issues/29378).
[TID254](https://docs.astral.sh/ruff/rules/lazy-import-mismatch/), with `require-lazy` set, adds `lazy` to the imports
in the block but keeps the `if TYPE_CHECKING:` guard this rule removes.