Allow extending lists with --override foo+=bar

Allow appending to a list with += syntax, instead of replacing the
existing value.

Fixes: #3087

docs/changelog/3087.feature.rst

@@ -0,0 +1,3 @@
+``--override`` can now take options in the form of ``foo+=bar`` which
+will append ``bar`` to the end of an existing list, rather than
+replacing it.

src/tox/config/loader/api.py

@@ -24,6 +24,12 @@ def __init__(self, value: str) -> None:
         if not equal:
             msg = f"override {value} has no = sign in it"
             raise ArgumentTypeError(msg)
+
+        self.append = False
+        if key.endswith("+"):  # key += value appends to a list
+            key = key[:-1]
+            self.append = True
+
         self.namespace, _, self.key = key.rpartition(".")
 
     def __repr__(self) -> str:
@@ -117,10 +123,14 @@ def load(  # noqa: PLR0913
         :param args: the config load arguments
         :return: the converted type
         """
-        if key in self.overrides:
-            return _STR_CONVERT.to(self.overrides[key].value, of_type, factory)
+        override = self.overrides.get(key)
+        if override and not override.append:
+            return _STR_CONVERT.to(override.value, of_type, factory)
         raw = self.load_raw(key, conf, args.env_name)
-        return self.build(key, of_type, factory, conf, raw, args)
+        converted = self.build(key, of_type, factory, conf, raw, args)
+        if override and override.append:
+            converted += _STR_CONVERT.to(override.value, of_type, factory)
+        return converted
 
     def build(  # noqa: PLR0913
         self,

tests/config/loader/test_loader.py

@@ -28,6 +28,18 @@ def test_override_add(flag: str) -> None:
     assert value.key == "magic"
     assert value.value == "true"
     assert not value.namespace
+    assert value.append is False
+
+
+@pytest.mark.parametrize("flag", ["-x", "--override"])
+def test_override_append(flag: str) -> None:
+    parsed, _, __, ___, ____ = get_options(flag, "magic+=true")
+    assert len(parsed.override) == 1
+    value = parsed.override[0]
+    assert value.key == "magic"
+    assert value.value == "true"
+    assert not value.namespace
+    assert value.append is True
 
 
 def test_override_equals() -> None:

tests/config/test_main.py

@@ -2,7 +2,7 @@
 
 import os
 from pathlib import Path
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, List
 
 from tox.config.loader.api import Override
 from tox.config.loader.memory import MemoryLoader
@@ -64,6 +64,16 @@ def test_config_override_wins_memory_loader(tox_ini_conf: ToxIniCreator) -> None
     assert conf["c"] == "ok"
 
 
+def test_config_override_appends(tox_ini_conf: ToxIniCreator) -> None:
+    example = """
+    [testenv]
+    passenv = foo
+    """
+    conf = tox_ini_conf(example, override=[Override("testenv.passenv+=bar")]).get_env("testenv")
+    conf.add_config("passenv", of_type=List[str], default=[], desc="desc")
+    assert conf["passenv"] == ["foo", "bar"]
+
+
 def test_args_are_paths_when_disabled(tox_project: ToxProjectCreator) -> None:
     ini = "[testenv]\npackage=skip\ncommands={posargs}\nargs_are_paths=False"
     project = tox_project({"tox.ini": ini, "w": {"a.txt": "a"}})