Make InvocationParams.args a tuple

This avoids mutating the original list to reflect on InvocationParams,
which is supposed to be an immutable snapshot of the state of pytest.main()
at the moment of invocation (see pytest-dev/pytest-xdist#478).

Author: nicoddemus

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",