Allow appending to deps with -x testenv.deps+=foo

Fixes: #3256

docs/changelog/3256.bugfix.rst

@@ -0,0 +1 @@
+Allow appending to ``deps`` with ``--override testenv.deps+=foo`` - by :user:`stefanor`.

src/tox/config/loader/api.py

@@ -5,6 +5,7 @@
 from typing import TYPE_CHECKING, Any, List, Mapping, TypeVar
 
 from tox.plugin import impl
+from tox.tox_env.python.pip.req_file import PythonDeps
 
 from .convert import Convert, Factory
 from .str_convert import StrConvert
@@ -148,6 +149,8 @@ def load(  # noqa: PLR0913
                 converted.update(converted_override)
             elif isinstance(converted, SetEnv) and isinstance(converted_override, SetEnv):
                 converted.update(converted_override, override=True)
+            elif isinstance(converted, PythonDeps) and isinstance(converted_override, PythonDeps):
+                converted += converted_override
             else:
                 msg = "Only able to append to lists and dicts"
                 raise ValueError(msg)

tests/config/test_main.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import os
+from functools import partial
 from pathlib import Path
 from typing import TYPE_CHECKING, List
 
@@ -9,6 +10,7 @@
 from tox.config.loader.api import Override
 from tox.config.loader.memory import MemoryLoader
 from tox.config.sets import ConfigSet
+from tox.tox_env.python.pip.req_file import PythonDeps
 
 if TYPE_CHECKING:
     from tests.conftest import ToxIniCreator
@@ -98,6 +100,37 @@ def test_config_override_appends_to_empty_setenv(tox_ini_conf: ToxIniCreator) ->
     assert conf["setenv"].load("foo") == "bar"
 
 
+def test_config_override_appends_to_pythondeps(tox_ini_conf: ToxIniCreator, tmp_path: Path) -> None:
+    example = """
+    [testenv]
+    deps = foo
+    """
+    conf = tox_ini_conf(example, override=[Override("testenv.deps+=bar")]).get_env("testenv")
+    conf.add_config(
+        "deps",
+        of_type=PythonDeps,
+        factory=partial(PythonDeps.factory, tmp_path),
+        default=PythonDeps("", root=tmp_path),
+        desc="desc",
+    )
+    assert conf["deps"].lines() == ["foo", "bar"]
+
+
+def test_config_override_appends_to_empty_pythondeps(tox_ini_conf: ToxIniCreator, tmp_path: Path) -> None:
+    example = """
+    [testenv]
+    """
+    conf = tox_ini_conf(example, override=[Override("testenv.deps+=bar")]).get_env("testenv")
+    conf.add_config(
+        "deps",
+        of_type=PythonDeps,
+        factory=partial(PythonDeps.factory, tmp_path),
+        default=PythonDeps("", root=tmp_path),
+        desc="desc",
+    )
+    assert conf["deps"].lines() == ["bar"]
+
+
 def test_config_override_cannot_append(tox_ini_conf: ToxIniCreator) -> None:
     example = """
     [testenv]