Make InvocationParams.args a tuple

changelog/6008.improvement.rst

@@ -0,0 +1,2 @@
+``Config.InvocationParams.args`` is now always a ``tuple`` to better convey that it should be
+immutable and avoid accidental modifications.

src/_pytest/config/__init__.py

@@ -169,7 +169,7 @@ def get_config(args=None, plugins=None):
     config = Config(
         pluginmanager,
         invocation_params=Config.InvocationParams(
-            args=args, plugins=plugins, dir=Path().resolve()
+            args=args or (), plugins=plugins, dir=Path().resolve()
         ),
     )
 
@@ -654,7 +654,7 @@ class Config:
 
         Contains the following read-only attributes:
 
-        * ``args``: list of command-line arguments as passed to ``pytest.main()``.
+        * ``args``: tuple of command-line arguments as passed to ``pytest.main()``.
         * ``plugins``: list of extra plugins, might be None.
         * ``dir``: directory where ``pytest.main()`` was invoked from.
     """
@@ -667,13 +667,13 @@ class InvocationParams:
 
         .. note::
 
-            Currently the environment variable PYTEST_ADDOPTS is also handled by
-            pytest implicitly, not being part of the invocation.
+            Note that the environment variable ``PYTEST_ADDOPTS`` and the ``addopts``
+            ini option are handled by pytest, not being included in the ``args`` attribute.
 
             Plugins accessing ``InvocationParams`` must be aware of that.
         """
 
-        args = attr.ib()
+        args = attr.ib(converter=tuple)
         plugins = attr.ib()
         dir = attr.ib(type=Path)
 
@@ -938,7 +938,6 @@ def parse(self, args, addopts=True):
         assert not hasattr(
             self, "args"
         ), "can only parse cmdline args at most once per Config object"
-        assert self.invocation_params.args == args
         self.hook.pytest_addhooks.call_historic(
             kwargs=dict(pluginmanager=self.pluginmanager)
         )

testing/test_config.py

@@ -7,6 +7,7 @@
 import pytest
 from _pytest.compat import importlib_metadata
 from _pytest.config import _iter_rewritable_modules
+from _pytest.config import Config
 from _pytest.config.exceptions import UsageError
 from _pytest.config.findpaths import determine_setup
 from _pytest.config.findpaths import get_common_ancestor
@@ -456,7 +457,7 @@ def test_invocation_params_args(self, _sys_snapshot):
 
         config = Config.fromdictargs(option_dict, args)
         assert config.args == ["a", "b"]
-        assert config.invocation_params.args == args
+        assert config.invocation_params.args == tuple(args)
         assert config.option.verbose == 4
         assert config.option.capture == "no"
 
@@ -1235,14 +1236,18 @@ class DummyPlugin:
     call = calls[0]
     config = call.item.config
 
-    assert config.invocation_params.args == [p, "-v"]
+    assert config.invocation_params.args == (p, "-v")
     assert config.invocation_params.dir == Path(str(testdir.tmpdir))
 
     plugins = config.invocation_params.plugins
     assert len(plugins) == 2
     assert plugins[0] is plugin
     assert type(plugins[1]).__name__ == "Collect"  # installed by testdir.inline_run()
 
+    # args cannot be None
+    with pytest.raises(TypeError):
+        Config.InvocationParams(args=None, plugins=None, dir=Path())
+
 
 @pytest.mark.parametrize(
     "plugin",