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[fastapi] Handle parameters with Depends correctly (FAST003) #15364

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[fastapi] Handle parameters with Depends correctly (FAST003) #15364

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[`fastapi`] Handle parameters with `Depends` correctly (`FAST003`)
InSyncWithFoo Jan 9, 2025
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34 changes: 33 additions & 1 deletion crates/ruff_linter/resources/test/fixtures/fastapi/FAST003.py
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
from typing import Annotated

from fastapi import FastAPI, Path
from fastapi import Depends, FastAPI, Path

app = FastAPI()

Expand Down Expand Up @@ -144,3 +144,35 @@ async def read_thing(query: str):
@app.get("/things/{thing_id=}")
async def read_thing(query: str):
return {"query": query}


# https://github.com/astral-sh/ruff/issues/13657
def dependable(thing_id): ...
def not_so_dependable(lorem): ...
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from foo import unknown_imported
unknown_not_function = unknown_imported()


### Errors
@app.get("/things/{thing_id}")
async def single(other: Annotated[str, Depends(not_so_dependable)]): ...
@app.get("/things/{thing_id}")
async def double(other: Annotated[str, Depends(not_so_dependable), Depends(dependable)]): ...
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Just for my understanding. Is this an error because fastapi only respects the first Depends? Do you have a reference that documents this behavior or did you find it out by try and error?

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@InSyncWithFoo InSyncWithFoo Jan 9, 2025 •
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The documentation doesn't mention what happens when two Depends are used in a path operation function, only when multiple of them are used in a path operation decorator.

Testing this again, I think I made a mistake. FastAPI seems to respect the last rather than the first. The code in question:

from fastapi import FastAPI, Depends
from typing import Annotated

app = FastAPI()

def foo(a: str) -> str:
    print(f'foo: {a}')
    return 'lorem'
    
def bar(b: str) -> int:
    print(f'bar: {b}')
    return 42
    
def baz(c: str) -> bytes:
    print(f'baz: {c}')
    return b''

@app.get("/things/{c}")  # `a`/`b`
async def read_thing(other: Annotated[str, Depends(baz), Depends(bar), Depends(foo)]):
    return other
$ uv run fastapi run hello.py
      ...
      INFO   1.2.3.4:5 - "GET /things/ipsum HTTP/1.1" 422

{
    "detail": [
        {
            "type": "missing",
            "loc": ["query", "a"],
            "msg": "Field required",
            "input": null
        }
    ]
}

It would be best if we could ask someone with FastAPI expertise to have a look.

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Thanks for the detailed investigation.

@charliermarsh do you know someone we could ping on this?

I'd otherwise suggest to bail (ignore the rule) if multiple Depends are seen

@app.get("/things/{thing_id}")
async def default(other: str = Depends(not_so_dependable)): ...


### No errors
@app.get("/things/{thing_id}")
async def single(other: Annotated[str, Depends(dependable)]): ...
@app.get("/things/{thing_id}")
async def double(other: Annotated[str, Depends(dependable), Depends(not_so_dependable)]): ...
@app.get("/things/{thing_id}")
async def default(other: str = Depends(dependable)): ...
@app.get("/things/{thing_id}")
async def unknown_1(other: str = Depends(unknown_unresolved)): ...
@app.get("/things/{thing_id}")
async def unknown_2(other: str = Depends(unknown_not_function)): ...
@app.get("/things/{thing_id}")
async def unknown_3(other: str = Depends(unknown_imported)): ...
162 changes: 148 additions & 14 deletions crates/ruff_linter/src/rules/fastapi/rules/fastapi_unused_path_parameter.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,8 +6,8 @@ use ruff_diagnostics::Fix;
use ruff_diagnostics::{Diagnostic, FixAvailability, Violation};
use ruff_macros::{derive_message_formats, ViolationMetadata};
use ruff_python_ast as ast;
use ruff_python_ast::{Expr, Parameter, ParameterWithDefault};
use ruff_python_semantic::{Modules, SemanticModel};
use ruff_python_ast::{Expr, ExprCall, ExprSubscript, Parameter, ParameterWithDefault};
use ruff_python_semantic::{BindingKind, Modules, ScopeKind, SemanticModel};
use ruff_python_stdlib::identifiers::is_identifier;
use ruff_text_size::{Ranged, TextSize};

Expand Down Expand Up @@ -137,16 +137,29 @@ pub(crate) fn fastapi_unused_path_parameter(
let path_params = PathParamIterator::new(path.to_str());

// Extract the arguments from the function signature
let named_args: Vec<_> = function_def
.parameters
.args
.iter()
.chain(&function_def.parameters.kwonlyargs)
.map(|ParameterWithDefault { parameter, .. }| {
parameter_alias(parameter, checker.semantic())
.unwrap_or_else(|| parameter.name.as_str())
})
.collect();
let mut named_args = vec![];

for parameter_with_default in non_posonly_non_variadic_parameters(function_def) {
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let ParameterWithDefault { parameter, .. } = parameter_with_default;

if let Some(alias) = parameter_alias(parameter, checker.semantic()) {
named_args.push(alias.to_string());
} else {
named_args.push(parameter.name.to_string());
}

let Some(dependency) =
Dependency::from_parameter(parameter_with_default, checker.semantic())
else {
continue;
};

if let Some(parameter_names) = dependency.parameter_names() {
named_args.extend(parameter_names);
} else {
return;
}
}

// Check if any of the path parameters are not in the function signature.
let mut diagnostics = vec![];
Expand All @@ -156,8 +169,9 @@ pub(crate) fn fastapi_unused_path_parameter(
continue;
}

// If the path parameter is already in the function signature, we don't need to do anything.
if named_args.contains(&path_param) {
// If the path parameter is already in the function or the dependency signature,
// we don't need to do anything.
if named_args.contains(&path_param.to_string()) {
continue;
}

Expand Down Expand Up @@ -194,6 +208,126 @@ pub(crate) fn fastapi_unused_path_parameter(
checker.diagnostics.extend(diagnostics);
}

fn non_posonly_non_variadic_parameters(
function_def: &ast::StmtFunctionDef,
) -> impl Iterator<Item = &ParameterWithDefault> {
function_def
.parameters
.args
.iter()
.chain(&function_def.parameters.kwonlyargs)
}

#[derive(Debug, is_macro::Is)]
enum Dependency {
/// Not defined in the same file, or otherwise cannot be determined to be a function.
Unknown,
/// A function defined in the same file, whose parameter names are as given.
Function(Vec<String>),
}

impl Dependency {
fn parameter_names(self) -> Option<Vec<String>> {
match self {
Self::Unknown => None,
Self::Function(parameter_names) => Some(parameter_names),
}
}
}

impl Dependency {
/// Return `foo` in the first metadata `Depends(foo)`,
/// or that of the default value:
///
/// ```python
/// def _(
/// a: Annotated[str, Depends(foo), Depends(bar)],
/// # ^^^^^^^^^^^^
/// a: str = Depends(foo),
/// # ^^^^^^^^^^^^
/// ): ...
/// ```
fn from_parameter(
parameter_with_default: &ParameterWithDefault,
semantic: &SemanticModel,
) -> Option<Self> {
let ParameterWithDefault {
parameter, default, ..
} = parameter_with_default;

if let Some(default) = default {
if let Some(dependency) = Self::from_call(default.as_ref(), semantic) {
return Some(dependency);
};
}

let Expr::Subscript(ExprSubscript { value, slice, .. }) =
&parameter.annotation.as_deref()?
else {
return None;
};

if !semantic.match_typing_expr(value, "Annotated") {
return None;
}

let Expr::Tuple(tuple) = slice.as_ref() else {
return None;
};

tuple
.elts
.iter()
.skip(1)
.find_map(|metadata| Self::from_call(metadata, semantic))
}

fn from_call(expr: &Expr, semantic: &SemanticModel) -> Option<Self> {
let Expr::Call(ExprCall {
func, arguments, ..
}) = expr
else {
return None;
};

if !is_fastapi_depends(func.as_ref(), semantic) {
return None;
}

let Some(Expr::Name(name)) = arguments.find_argument_value("dependency", 0) else {
return None;
};

let Some(binding) = semantic.only_binding(name).map(|id| semantic.binding(id)) else {
return Some(Self::Unknown);
};

let BindingKind::FunctionDefinition(scope_id) = binding.kind else {
return Some(Self::Unknown);
};

let scope = &semantic.scopes[scope_id];

let ScopeKind::Function(function_def) = scope.kind else {
return Some(Self::Unknown);
};

let parameter_names = non_posonly_non_variadic_parameters(function_def)
.map(|ParameterWithDefault { parameter, .. }| parameter.name.to_string())
.collect::<Vec<_>>();

Some(Self::Function(parameter_names))
}
}

fn is_fastapi_depends(expr: &Expr, semantic: &SemanticModel) -> bool {
let Some(qualified_name) = semantic.resolve_qualified_name(expr) else {
return false;
};

matches!(qualified_name.segments(), ["fastapi", "Depends"])
}

/// Extract the expected in-route name for a given parameter, if it has an alias.
/// For example, given `document_id: Annotated[str, Path(alias="documentId")]`, returns `"documentId"`.
fn parameter_alias<'a>(parameter: &'a Parameter, semantic: &SemanticModel) -> Option<&'a str> {
Expand Down
63 changes: 62 additions & 1 deletion ...pshots/ruff_linter__rules__fastapi__tests__fast-api-unused-path-parameter_FAST003.py.snap
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Expand Up @@ -340,4 +340,65 @@ FAST003.py:87:18: FAST003 [*] Parameter `name` appears in route path, but not in
88 |+async def read_thing(name, *, author: Annotated[str, Path(alias="author_name")], title: str):
89 89 | return {"author": author, "title": title}
90 90 |
91 91 |
91 91 |

FAST003.py:158:19: FAST003 [*] Parameter `thing_id` appears in route path, but not in `single` signature
|
157 | ### Errors
158 | @app.get("/things/{thing_id}")
| ^^^^^^^^^^ FAST003
159 | async def single(other: Annotated[str, Depends(not_so_dependable)]): ...
160 | @app.get("/things/{thing_id}")
|
= help: Add `thing_id` to function signature

ℹ Unsafe fix
156 156 |
157 157 | ### Errors
158 158 | @app.get("/things/{thing_id}")
159 |-async def single(other: Annotated[str, Depends(not_so_dependable)]): ...
159 |+async def single(other: Annotated[str, Depends(not_so_dependable)], thing_id): ...
160 160 | @app.get("/things/{thing_id}")
161 161 | async def double(other: Annotated[str, Depends(not_so_dependable), Depends(dependable)]): ...
162 162 | @app.get("/things/{thing_id}")

FAST003.py:160:19: FAST003 [*] Parameter `thing_id` appears in route path, but not in `double` signature
|
158 | @app.get("/things/{thing_id}")
159 | async def single(other: Annotated[str, Depends(not_so_dependable)]): ...
160 | @app.get("/things/{thing_id}")
| ^^^^^^^^^^ FAST003
161 | async def double(other: Annotated[str, Depends(not_so_dependable), Depends(dependable)]): ...
162 | @app.get("/things/{thing_id}")
|
= help: Add `thing_id` to function signature

ℹ Unsafe fix
158 158 | @app.get("/things/{thing_id}")
159 159 | async def single(other: Annotated[str, Depends(not_so_dependable)]): ...
160 160 | @app.get("/things/{thing_id}")
161 |-async def double(other: Annotated[str, Depends(not_so_dependable), Depends(dependable)]): ...
161 |+async def double(other: Annotated[str, Depends(not_so_dependable), Depends(dependable)], thing_id): ...
162 162 | @app.get("/things/{thing_id}")
163 163 | async def default(other: str = Depends(not_so_dependable)): ...
164 164 |

FAST003.py:162:19: FAST003 [*] Parameter `thing_id` appears in route path, but not in `default` signature
|
160 | @app.get("/things/{thing_id}")
161 | async def double(other: Annotated[str, Depends(not_so_dependable), Depends(dependable)]): ...
162 | @app.get("/things/{thing_id}")
| ^^^^^^^^^^ FAST003
163 | async def default(other: str = Depends(not_so_dependable)): ...
|
= help: Add `thing_id` to function signature

ℹ Unsafe fix
160 160 | @app.get("/things/{thing_id}")
161 161 | async def double(other: Annotated[str, Depends(not_so_dependable), Depends(dependable)]): ...
162 162 | @app.get("/things/{thing_id}")
163 |-async def default(other: str = Depends(not_so_dependable)): ...
163 |+async def default(thing_id, other: str = Depends(not_so_dependable)): ...
164 164 |
165 165 |
166 166 | ### No errors
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