♻️ Refactor internal check_file_field(), rename to `ensure_multipar…

…t_is_installed()` to clarify its purpose (fastapi#12106)

fastapi/dependencies/utils.py

@@ -80,25 +80,23 @@
 )
 
 
-def check_file_field(field: ModelField) -> None:
-    field_info = field.field_info
-    if isinstance(field_info, params.Form):
+def ensure_multipart_is_installed() -> None:
+    try:
+        # __version__ is available in both multiparts, and can be mocked
+        from multipart import __version__  # type: ignore
+
+        assert __version__
         try:
-            # __version__ is available in both multiparts, and can be mocked
-            from multipart import __version__  # type: ignore
-
-            assert __version__
-            try:
-                # parse_options_header is only available in the right multipart
-                from multipart.multipart import parse_options_header  # type: ignore
-
-                assert parse_options_header
-            except ImportError:
-                logger.error(multipart_incorrect_install_error)
-                raise RuntimeError(multipart_incorrect_install_error) from None
+            # parse_options_header is only available in the right multipart
+            from multipart.multipart import parse_options_header  # type: ignore
+
+            assert parse_options_header
         except ImportError:
-            logger.error(multipart_not_installed_error)
-            raise RuntimeError(multipart_not_installed_error) from None
+            logger.error(multipart_incorrect_install_error)
+            raise RuntimeError(multipart_incorrect_install_error) from None
+    except ImportError:
+        logger.error(multipart_not_installed_error)
+        raise RuntimeError(multipart_not_installed_error) from None
 
 
 def get_param_sub_dependant(
@@ -336,6 +334,7 @@ def analyze_param(
     if annotation is not inspect.Signature.empty:
         use_annotation = annotation
         type_annotation = annotation
+    # Extract Annotated info
     if get_origin(use_annotation) is Annotated:
         annotated_args = get_args(annotation)
         type_annotation = annotated_args[0]
@@ -355,6 +354,7 @@ def analyze_param(
             )
         else:
             fastapi_annotation = None
+        # Set default for Annotated FieldInfo
         if isinstance(fastapi_annotation, FieldInfo):
             # Copy `field_info` because we mutate `field_info.default` below.
             field_info = copy_field_info(
@@ -369,9 +369,10 @@ def analyze_param(
                 field_info.default = value
             else:
                 field_info.default = Required
+        # Get Annotated Depends
         elif isinstance(fastapi_annotation, params.Depends):
             depends = fastapi_annotation
-
+    # Get Depends from default value
     if isinstance(value, params.Depends):
         assert depends is None, (
             "Cannot specify `Depends` in `Annotated` and default value"
@@ -382,6 +383,7 @@ def analyze_param(
             f" default value together for {param_name!r}"
         )
         depends = value
+    # Get FieldInfo from default value
     elif isinstance(value, FieldInfo):
         assert field_info is None, (
             "Cannot specify FastAPI annotations in `Annotated` and default value"
@@ -391,11 +393,13 @@ def analyze_param(
         if PYDANTIC_V2:
             field_info.annotation = type_annotation
 
+    # Get Depends from type annotation
     if depends is not None and depends.dependency is None:
         # Copy `depends` before mutating it
         depends = copy(depends)
         depends.dependency = type_annotation
 
+    # Handle non-param type annotations like Request
     if lenient_issubclass(
         type_annotation,
         (
@@ -411,6 +415,7 @@ def analyze_param(
         assert (
             field_info is None
         ), f"Cannot specify FastAPI annotation for type {type_annotation!r}"
+    # Handle default assignations, neither field_info nor depends was not found in Annotated nor default value
     elif field_info is None and depends is None:
         default_value = value if value is not inspect.Signature.empty else Required
         if is_path_param:
@@ -428,7 +433,9 @@ def analyze_param(
             field_info = params.Query(annotation=use_annotation, default=default_value)
 
     field = None
+    # It's a field_info, not a dependency
     if field_info is not None:
+        # Handle field_info.in_
         if is_path_param:
             assert isinstance(field_info, params.Path), (
                 f"Cannot use `{field_info.__class__.__name__}` for path param"
@@ -444,6 +451,8 @@ def analyze_param(
             field_info,
             param_name,
         )
+        if isinstance(field_info, params.Form):
+            ensure_multipart_is_installed()
         if not field_info.alias and getattr(field_info, "convert_underscores", None):
             alias = param_name.replace("_", "-")
         else:
@@ -786,7 +795,6 @@ def get_body_field(*, dependant: Dependant, name: str) -> Optional[ModelField]:
     embed = getattr(field_info, "embed", None)
     body_param_names_set = {param.name for param in flat_dependant.body_params}
     if len(body_param_names_set) == 1 and not embed:
-        check_file_field(first_param)
         return first_param
     # If one field requires to embed, all have to be embedded
     # in case a sub-dependency is evaluated with a single unique body field
@@ -825,5 +833,4 @@ def get_body_field(*, dependant: Dependant, name: str) -> Optional[ModelField]:
         alias="body",
         field_info=BodyFieldInfo(**BodyFieldInfo_kwargs),
     )
-    check_file_field(final_field)
     return final_field