fix: Treat leading underscore as a sign of invalid identifier

There are 2 reasons for this change:

- leading underscores have special meaning and therefore we should prefix
those properties to avoid unexpected semantics
- in my particular use case my team is working with a schema which has
the following fields in an object: "field" and "_field" and current code
strips leading underscore and produces invalid data object with 2 fields
of the same name

Author: maxkomarychev

README.md

@@ -129,6 +129,8 @@ package_name_override: my_extra_special_package_name
 
 When generating properties, the `name` attribute of the OpenAPI schema will be used. When the `name` is not a valid
 Python identifier (e.g. begins with a number) this string will be prepended. Defaults to "field\_".
+It will also be used to prefix fields in schema starting with "_" in order to avoid
+ambigous semantics.
 
 Example:
 

end_to_end_tests/golden-record/my_test_api_client/models/a_model.py

@@ -35,6 +35,7 @@ class AModel:
         a_nullable_date (Optional[datetime.date]):
         a_not_required_date (Union[Unset, datetime.date]):
         attr_1_leading_digit (Union[Unset, str]):
+        attr_leading_underscore (Union[Unset, str]):
         required_nullable (Optional[str]):
         not_required_nullable (Union[Unset, None, str]):
         not_required_not_nullable (Union[Unset, str]):
@@ -62,6 +63,7 @@ class AModel:
     nested_list_of_enums: Union[Unset, List[List[DifferentEnum]]] = UNSET
     a_not_required_date: Union[Unset, datetime.date] = UNSET
     attr_1_leading_digit: Union[Unset, str] = UNSET
+    attr_leading_underscore: Union[Unset, str] = UNSET
     not_required_nullable: Union[Unset, None, str] = UNSET
     not_required_not_nullable: Union[Unset, str] = UNSET
     not_required_one_of_models: Union["FreeFormModel", "ModelWithUnionProperty", Unset] = UNSET
@@ -123,6 +125,7 @@ def to_dict(self) -> Dict[str, Any]:
             a_not_required_date = self.a_not_required_date.isoformat()
 
         attr_1_leading_digit = self.attr_1_leading_digit
+        attr_leading_underscore = self.attr_leading_underscore
         required_nullable = self.required_nullable
         not_required_nullable = self.not_required_nullable
         not_required_not_nullable = self.not_required_not_nullable
@@ -207,6 +210,8 @@ def to_dict(self) -> Dict[str, Any]:
             field_dict["a_not_required_date"] = a_not_required_date
         if attr_1_leading_digit is not UNSET:
             field_dict["1_leading_digit"] = attr_1_leading_digit
+        if attr_leading_underscore is not UNSET:
+            field_dict["_leading_underscore"] = attr_leading_underscore
         if not_required_nullable is not UNSET:
             field_dict["not_required_nullable"] = not_required_nullable
         if not_required_not_nullable is not UNSET:
@@ -313,6 +318,8 @@ def _parse_one_of_models(data: object) -> Union["FreeFormModel", "ModelWithUnion
 
         attr_1_leading_digit = d.pop("1_leading_digit", UNSET)
 
+        attr_leading_underscore = d.pop("_leading_underscore", UNSET)
+
         required_nullable = d.pop("required_nullable")
 
         not_required_nullable = d.pop("not_required_nullable", UNSET)
@@ -447,6 +454,7 @@ def _parse_not_required_nullable_one_of_models(
             a_nullable_date=a_nullable_date,
             a_not_required_date=a_not_required_date,
             attr_1_leading_digit=attr_1_leading_digit,
+            attr_leading_underscore=attr_leading_underscore,
             required_nullable=required_nullable,
             not_required_nullable=not_required_nullable,
             not_required_not_nullable=not_required_not_nullable,

end_to_end_tests/openapi.json

@@ -1327,6 +1327,10 @@
             "title": "Leading Digit",
             "type": "string"
           },
+          "_leading_underscore": {
+            "title": "Leading Underscore",
+            "type": "string"
+          },
           "required_nullable": {
             "title": "Required AND Nullable",
             "type": "string",

openapi_python_client/utils.py

@@ -12,7 +12,7 @@ class PythonIdentifier(str):
     def __new__(cls, value: str, prefix: str) -> "PythonIdentifier":
         new_value = fix_reserved_words(snake_case(sanitize(value)))
 
-        if not new_value.isidentifier():
+        if not new_value.isidentifier() or value[0] == "_":
             new_value = f"{prefix}{new_value}"
         return str.__new__(cls, new_value)