feat: replace python-dateutil with stdlib datetime.fromisoformat (#1429)

## Summary

Drop the `python-dateutil` dependency from both the generator and all
generated client packages. Date/datetime parsing now uses Python's
built-in `datetime.fromisoformat()` instead of
`dateutil.parser.isoparse()`.

This removes one runtime dependency from every generated client,
reducing install size and eliminating a dependency that is [unmaintained
upstream and being deprecated by Fedora
45](https://fedoraproject.org/wiki/Changes/DeprecatePython-dateutil) (no
releases since March 2024).

## Changes

**Generator source** (`openapi_python_client/parser/properties/`):
- `datetime.py`: Validate defaults with
`datetime.datetime.fromisoformat()` instead of `isoparse()`. Normalize
`Z` to `+00:00` at generation time so the emitted Python code is clean.
- `date.py`: Validate defaults with `datetime.date.fromisoformat()`
instead of `isoparse().date()`.
- Both: Remove `"from dateutil.parser import isoparse"` from the import
set returned by `get_imports()`.

**Jinja templates**
(`openapi_python_client/templates/property_templates/`):
- `datetime_property.py.jinja`: `isoparse(x)` ->
`datetime.datetime.fromisoformat(x.replace("Z", "+00:00"))`
- `date_property.py.jinja`: `isoparse(x).date()` ->
`datetime.date.fromisoformat(x)`

**Dependency removal** (all 4 metadata formats):
- `pyproject_uv.toml.jinja`, `pyproject_poetry.toml.jinja`,
`pyproject_pdm.toml.jinja`, `setup.py.jinja`: Remove `python-dateutil`
from generated dependencies.
- `pyproject.toml` (generator): Remove `python-dateutil` from runtime
deps and `types-python-dateutil` from dev deps.
- `integration-tests/pyproject.toml`: Remove both as well.

**Tests & golden records**: All regenerated. Unit tests pass (283
passed, 4 skipped).

## Python 3.10 compatibility

`datetime.fromisoformat()` gained full ISO 8601 support (including the
`Z` suffix) in Python 3.11. On Python 3.10, the `Z` suffix raises a
`ValueError`:

```python
# Python 3.10
>>> datetime.datetime.fromisoformat("2024-01-15T10:30:00Z")
ValueError: Invalid isoformat string: '2024-01-15T10:30:00Z'

>>> datetime.datetime.fromisoformat("2024-01-15T10:30:00+00:00")
datetime.datetime(2024, 1, 15, 10, 30, tzinfo=datetime.timezone.utc)  # works
```

```python
# Python 3.11+
>>> datetime.datetime.fromisoformat("2024-01-15T10:30:00Z")
datetime.datetime(2024, 1, 15, 10, 30, tzinfo=datetime.timezone.utc)  # works natively
```

The generated datetime parsing code uses `.replace("Z", "+00:00")` to
normalize `Z` to an explicit UTC offset before calling
`fromisoformat()`, which works on both 3.10 and 3.11+. This is a no-op
on strings without `Z`. Date parsing does not need this since date
strings have no timezone component.

Default values in OpenAPI specs are normalized at generation time (Z ->
+00:00), so the emitted default expressions are clean
`datetime.datetime.fromisoformat("...")` calls.

Author: splch

end_to_end_tests/docstrings-on-attributes-golden-record/pyproject.toml

@@ -13,7 +13,6 @@ include = ["my_test_api_client/py.typed"]
 python = "^3.10"
 httpx = ">=0.23.0,<0.29.0"
 attrs = ">=22.2.0"
-python-dateutil = "^2.8.0"
 
 [build-system]
 requires = ["poetry-core>=2.0.0,<3.0.0"]

end_to_end_tests/golden-record/my_test_api_client/api/defaults/defaults_tests_defaults_post.py

@@ -3,7 +3,6 @@
 from typing import Any
 
 import httpx
-from dateutil.parser import isoparse
 
 from ... import errors
 from ...client import AuthenticatedClient, Client
@@ -17,7 +16,7 @@ def _get_kwargs(
     *,
     string_prop: str = "the default string",
     string_with_num: str = "1",
-    date_prop: datetime.date = isoparse("1010-10-10").date(),
+    date_prop: datetime.date = datetime.date.fromisoformat("1010-10-10"),
     float_prop: float = 3.14,
     float_with_int: float = 3.0,
     int_prop: int = 7,
@@ -121,7 +120,7 @@ def sync_detailed(
     client: AuthenticatedClient | Client,
     string_prop: str = "the default string",
     string_with_num: str = "1",
-    date_prop: datetime.date = isoparse("1010-10-10").date(),
+    date_prop: datetime.date = datetime.date.fromisoformat("1010-10-10"),
     float_prop: float = 3.14,
     float_with_int: float = 3.0,
     int_prop: int = 7,
@@ -138,7 +137,7 @@ def sync_detailed(
     Args:
         string_prop (str):  Default: 'the default string'.
         string_with_num (str):  Default: '1'.
-        date_prop (datetime.date):  Default: isoparse('1010-10-10').date().
+        date_prop (datetime.date):  Default: datetime.date.fromisoformat('1010-10-10').
         float_prop (float):  Default: 3.14.
         float_with_int (float):  Default: 3.0.
         int_prop (int):  Default: 7.
@@ -186,7 +185,7 @@ def sync(
     client: AuthenticatedClient | Client,
     string_prop: str = "the default string",
     string_with_num: str = "1",
-    date_prop: datetime.date = isoparse("1010-10-10").date(),
+    date_prop: datetime.date = datetime.date.fromisoformat("1010-10-10"),
     float_prop: float = 3.14,
     float_with_int: float = 3.0,
     int_prop: int = 7,
@@ -203,7 +202,7 @@ def sync(
     Args:
         string_prop (str):  Default: 'the default string'.
         string_with_num (str):  Default: '1'.
-        date_prop (datetime.date):  Default: isoparse('1010-10-10').date().
+        date_prop (datetime.date):  Default: datetime.date.fromisoformat('1010-10-10').
         float_prop (float):  Default: 3.14.
         float_with_int (float):  Default: 3.0.
         int_prop (int):  Default: 7.
@@ -246,7 +245,7 @@ async def asyncio_detailed(
     client: AuthenticatedClient | Client,
     string_prop: str = "the default string",
     string_with_num: str = "1",
-    date_prop: datetime.date = isoparse("1010-10-10").date(),
+    date_prop: datetime.date = datetime.date.fromisoformat("1010-10-10"),
     float_prop: float = 3.14,
     float_with_int: float = 3.0,
     int_prop: int = 7,
@@ -263,7 +262,7 @@ async def asyncio_detailed(
     Args:
         string_prop (str):  Default: 'the default string'.
         string_with_num (str):  Default: '1'.
-        date_prop (datetime.date):  Default: isoparse('1010-10-10').date().
+        date_prop (datetime.date):  Default: datetime.date.fromisoformat('1010-10-10').
         float_prop (float):  Default: 3.14.
         float_with_int (float):  Default: 3.0.
         int_prop (int):  Default: 7.
@@ -309,7 +308,7 @@ async def asyncio(
     client: AuthenticatedClient | Client,
     string_prop: str = "the default string",
     string_with_num: str = "1",
-    date_prop: datetime.date = isoparse("1010-10-10").date(),
+    date_prop: datetime.date = datetime.date.fromisoformat("1010-10-10"),
     float_prop: float = 3.14,
     float_with_int: float = 3.0,
     int_prop: int = 7,
@@ -326,7 +325,7 @@ async def asyncio(
     Args:
         string_prop (str):  Default: 'the default string'.
         string_with_num (str):  Default: '1'.
-        date_prop (datetime.date):  Default: isoparse('1010-10-10').date().
+        date_prop (datetime.date):  Default: datetime.date.fromisoformat('1010-10-10').
         float_prop (float):  Default: 3.14.
         float_with_int (float):  Default: 3.0.
         int_prop (int):  Default: 7.

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

@@ -6,7 +6,6 @@
 from uuid import UUID
 
 from attrs import define as _attrs_define
-from dateutil.parser import isoparse
 
 from ..models.an_all_of_enum import AnAllOfEnum
 from ..models.an_enum import AnEnum
@@ -269,28 +268,28 @@ def _parse_a_camel_date_time(data: object) -> datetime.date | datetime.datetime:
             try:
                 if not isinstance(data, str):
                     raise TypeError()
-                a_camel_date_time_type_0 = isoparse(data)
+                a_camel_date_time_type_0 = datetime.datetime.fromisoformat(data.replace("Z", "+00:00"))
 
                 return a_camel_date_time_type_0
             except (TypeError, ValueError, AttributeError, KeyError):
                 pass
             if not isinstance(data, str):
                 raise TypeError()
-            a_camel_date_time_type_1 = isoparse(data).date()
+            a_camel_date_time_type_1 = datetime.date.fromisoformat(data)
 
             return a_camel_date_time_type_1
 
         a_camel_date_time = _parse_a_camel_date_time(d.pop("aCamelDateTime"))
 
-        a_date = isoparse(d.pop("a_date")).date()
+        a_date = datetime.date.fromisoformat(d.pop("a_date"))
 
         def _parse_a_nullable_date(data: object) -> datetime.date | None:
             if data is None:
                 return data
             try:
                 if not isinstance(data, str):
                     raise TypeError()
-                a_nullable_date_type_0 = isoparse(data).date()
+                a_nullable_date_type_0 = datetime.date.fromisoformat(data)
 
                 return a_nullable_date_type_0
             except (TypeError, ValueError, AttributeError, KeyError):
@@ -414,7 +413,7 @@ def _parse_nullable_model(data: object) -> ModelWithUnionProperty | None:
         if isinstance(_a_not_required_date, Unset):
             a_not_required_date = UNSET
         else:
-            a_not_required_date = isoparse(_a_not_required_date).date()
+            a_not_required_date = datetime.date.fromisoformat(_a_not_required_date)
 
         _a_not_required_uuid = d.pop("a_not_required_uuid", UNSET)
         a_not_required_uuid: UUID | Unset

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

@@ -7,7 +7,6 @@
 
 from attrs import define as _attrs_define
 from attrs import field as _attrs_field
-from dateutil.parser import isoparse
 
 from ..models.an_enum import AnEnum
 from ..types import File
@@ -230,17 +229,17 @@ def from_dict(cls: type[T], src_dict: Mapping[str, Any]) -> T:
         date_properties_ref = []
         _date_properties_ref = d.pop("date_properties_ref")
         for componentsschemas_an_other_array_of_date_item_data in _date_properties_ref:
-            componentsschemas_an_other_array_of_date_item = isoparse(
+            componentsschemas_an_other_array_of_date_item = datetime.date.fromisoformat(
                 componentsschemas_an_other_array_of_date_item_data
-            ).date()
+            )
 
             date_properties_ref.append(componentsschemas_an_other_array_of_date_item)
 
         datetime_properties_ref = []
         _datetime_properties_ref = d.pop("datetime_properties_ref")
         for componentsschemas_an_other_array_of_date_time_item_data in _datetime_properties_ref:
-            componentsschemas_an_other_array_of_date_time_item = isoparse(
-                componentsschemas_an_other_array_of_date_time_item_data
+            componentsschemas_an_other_array_of_date_time_item = datetime.datetime.fromisoformat(
+                componentsschemas_an_other_array_of_date_time_item_data.replace("Z", "+00:00")
             )
 
             datetime_properties_ref.append(componentsschemas_an_other_array_of_date_time_item)
@@ -276,14 +275,18 @@ def from_dict(cls: type[T], src_dict: Mapping[str, Any]) -> T:
         date_properties = []
         _date_properties = d.pop("date_properties")
         for componentsschemas_an_array_of_date_item_data in _date_properties:
-            componentsschemas_an_array_of_date_item = isoparse(componentsschemas_an_array_of_date_item_data).date()
+            componentsschemas_an_array_of_date_item = datetime.date.fromisoformat(
+                componentsschemas_an_array_of_date_item_data
+            )
 
             date_properties.append(componentsschemas_an_array_of_date_item)
 
         datetime_properties = []
         _datetime_properties = d.pop("datetime_properties")
         for componentsschemas_an_array_of_date_time_item_data in _datetime_properties:
-            componentsschemas_an_array_of_date_time_item = isoparse(componentsschemas_an_array_of_date_time_item_data)
+            componentsschemas_an_array_of_date_time_item = datetime.datetime.fromisoformat(
+                componentsschemas_an_array_of_date_time_item_data.replace("Z", "+00:00")
+            )
 
             datetime_properties.append(componentsschemas_an_array_of_date_time_item)
 
@@ -310,9 +313,9 @@ def from_dict(cls: type[T], src_dict: Mapping[str, Any]) -> T:
 
         str_property_ref = d.pop("str_property_ref")
 
-        date_property_ref = isoparse(d.pop("date_property_ref")).date()
+        date_property_ref = datetime.date.fromisoformat(d.pop("date_property_ref"))
 
-        datetime_property_ref = isoparse(d.pop("datetime_property_ref"))
+        datetime_property_ref = datetime.datetime.fromisoformat(d.pop("datetime_property_ref").replace("Z", "+00:00"))
 
         int32_property_ref = d.pop("int32_property_ref")
 

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

@@ -8,7 +8,6 @@
 
 from attrs import define as _attrs_define
 from attrs import field as _attrs_field
-from dateutil.parser import isoparse
 
 from .. import types
 from ..models.different_enum import DifferentEnum
@@ -300,14 +299,14 @@ def _parse_some_nullable_object(data: object) -> BodyUploadFileTestsUploadPostSo
         if isinstance(_a_datetime, Unset):
             a_datetime = UNSET
         else:
-            a_datetime = isoparse(_a_datetime)
+            a_datetime = datetime.datetime.fromisoformat(_a_datetime.replace("Z", "+00:00"))
 
         _a_date = d.pop("a_date", UNSET)
         a_date: datetime.date | Unset
         if isinstance(_a_date, Unset):
             a_date = UNSET
         else:
-            a_date = isoparse(_a_date).date()
+            a_date = datetime.date.fromisoformat(_a_date)
 
         some_number = d.pop("some_number", UNSET)
 

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

@@ -7,7 +7,6 @@
 
 from attrs import define as _attrs_define
 from attrs import field as _attrs_field
-from dateutil.parser import isoparse
 
 from ..models.an_all_of_enum import AnAllOfEnum
 from ..models.an_enum import AnEnum
@@ -276,28 +275,28 @@ def _parse_a_camel_date_time(data: object) -> datetime.date | datetime.datetime:
             try:
                 if not isinstance(data, str):
                     raise TypeError()
-                a_camel_date_time_type_0 = isoparse(data)
+                a_camel_date_time_type_0 = datetime.datetime.fromisoformat(data.replace("Z", "+00:00"))
 
                 return a_camel_date_time_type_0
             except (TypeError, ValueError, AttributeError, KeyError):
                 pass
             if not isinstance(data, str):
                 raise TypeError()
-            a_camel_date_time_type_1 = isoparse(data).date()
+            a_camel_date_time_type_1 = datetime.date.fromisoformat(data)
 
             return a_camel_date_time_type_1
 
         a_camel_date_time = _parse_a_camel_date_time(d.pop("aCamelDateTime"))
 
-        a_date = isoparse(d.pop("a_date")).date()
+        a_date = datetime.date.fromisoformat(d.pop("a_date"))
 
         def _parse_a_nullable_date(data: object) -> datetime.date | None:
             if data is None:
                 return data
             try:
                 if not isinstance(data, str):
                     raise TypeError()
-                a_nullable_date_type_0 = isoparse(data).date()
+                a_nullable_date_type_0 = datetime.date.fromisoformat(data)
 
                 return a_nullable_date_type_0
             except (TypeError, ValueError, AttributeError, KeyError):
@@ -421,7 +420,7 @@ def _parse_nullable_model(data: object) -> ModelWithUnionProperty | None:
         if isinstance(_a_not_required_date, Unset):
             a_not_required_date = UNSET
         else:
-            a_not_required_date = isoparse(_a_not_required_date).date()
+            a_not_required_date = datetime.date.fromisoformat(_a_not_required_date)
 
         _a_not_required_uuid = d.pop("a_not_required_uuid", UNSET)
         a_not_required_uuid: UUID | Unset

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

@@ -6,7 +6,6 @@
 
 from attrs import define as _attrs_define
 from attrs import field as _attrs_field
-from dateutil.parser import isoparse
 
 from ..types import UNSET, Unset
 
@@ -44,7 +43,7 @@ def from_dict(cls: type[T], src_dict: Mapping[str, Any]) -> T:
         if isinstance(_datetime_, Unset):
             datetime_ = UNSET
         else:
-            datetime_ = isoparse(_datetime_)
+            datetime_ = datetime.datetime.fromisoformat(_datetime_.replace("Z", "+00:00"))
 
         model_with_date_time_property = cls(
             datetime_=datetime_,

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

@@ -6,7 +6,6 @@
 
 from attrs import define as _attrs_define
 from attrs import field as _attrs_field
-from dateutil.parser import isoparse
 
 from ..models.model_with_merged_properties_string_to_enum import ModelWithMergedPropertiesStringToEnum
 from ..types import UNSET, Unset
@@ -81,7 +80,7 @@ def from_dict(cls: type[T], src_dict: Mapping[str, Any]) -> T:
         if isinstance(_string_to_date, Unset):
             string_to_date = UNSET
         else:
-            string_to_date = isoparse(_string_to_date).date()
+            string_to_date = datetime.date.fromisoformat(_string_to_date)
 
         number_to_int = d.pop("numberToInt", UNSET)
 

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

@@ -6,7 +6,6 @@
 
 from attrs import define as _attrs_define
 from attrs import field as _attrs_field
-from dateutil.parser import isoparse
 
 T = TypeVar("T", bound="ModelWithPrimitiveAdditionalPropertiesADateHolder")
 
@@ -32,7 +31,7 @@ def from_dict(cls: type[T], src_dict: Mapping[str, Any]) -> T:
 
         additional_properties = {}
         for prop_name, prop_dict in d.items():
-            additional_property = isoparse(prop_dict)
+            additional_property = datetime.datetime.fromisoformat(prop_dict.replace("Z", "+00:00"))
 
             additional_properties[prop_name] = additional_property
 

end_to_end_tests/golden-record/pyproject.toml

@@ -13,7 +13,6 @@ include = ["my_test_api_client/py.typed"]
 python = "^3.10"
 httpx = ">=0.23.0,<0.29.0"
 attrs = ">=22.2.0"
-python-dateutil = "^2.8.0"
 
 [build-system]
 requires = ["poetry-core>=2.0.0,<3.0.0"]