Mark some public and to-be-public classes as @final

This indicates at least for people using type checkers that these
classes are not designed for inheritance and we make no stability
guarantees regarding inheritance of them.

Currently this doesn't show up in the docs. Sphinx does actually support
`@final`, however it only works when imported directly from `typing`,
while we import from `_pytest.compat`.

In the future there might also be a `@sealed` decorator which would
cover some more cases.

Author: bluetech

changelog/7780.improvement.rst

@@ -0,0 +1,3 @@
+Public classes which are not designed to be inherited from are now marked `@final <https://docs.python.org/3/library/typing.html#typing.final>`_.
+Code which inherits from these classes will trigger a type-checking (e.g. mypy) error, but will still work in runtime.
+Currently the ``final`` designation does not appear in the API Reference but hopefully will in the future.

src/_pytest/_code/code.py

@@ -38,6 +38,7 @@
 from _pytest._io.saferepr import safeformat
 from _pytest._io.saferepr import saferepr
 from _pytest.compat import ATTRS_EQ_FIELD
+from _pytest.compat import final
 from _pytest.compat import get_real_func
 from _pytest.compat import overload
 from _pytest.compat import TYPE_CHECKING
@@ -414,6 +415,7 @@ def recursionindex(self) -> Optional[int]:
 _E = TypeVar("_E", bound=BaseException, covariant=True)
 
 
+@final
 @attr.s(repr=False)
 class ExceptionInfo(Generic[_E]):
     """Wraps sys.exc_info() objects and offers help for navigating the traceback."""

src/_pytest/_io/terminalwriter.py

@@ -7,6 +7,7 @@
 from typing import TextIO
 
 from .wcwidth import wcswidth
+from _pytest.compat import final
 
 
 # This code was initially copied from py 1.8.1, file _io/terminalwriter.py.
@@ -36,6 +37,7 @@ def should_do_markup(file: TextIO) -> bool:
     )
 
 
+@final
 class TerminalWriter:
     _esctable = dict(
         black=30,

src/_pytest/cacheprovider.py

@@ -21,6 +21,7 @@
 from .reports import CollectReport
 from _pytest import nodes
 from _pytest._io import TerminalWriter
+from _pytest.compat import final
 from _pytest.compat import order_preserving_dict
 from _pytest.config import Config
 from _pytest.config import ExitCode
@@ -50,6 +51,7 @@
 """
 
 
+@final
 @attr.s
 class Cache:
     _cachedir = attr.ib(type=Path, repr=False)

src/_pytest/capture.py

@@ -17,6 +17,7 @@
 from typing import Union
 
 import pytest
+from _pytest.compat import final
 from _pytest.compat import TYPE_CHECKING
 from _pytest.config import Config
 from _pytest.config.argparsing import Parser
@@ -498,6 +499,7 @@ def writeorg(self, data):
 # pertinent parts of a namedtuple. If the mypy limitation is ever lifted, can
 # make it a namedtuple again.
 # [0]: https://github.com/python/mypy/issues/685
+@final
 @functools.total_ordering
 class CaptureResult(Generic[AnyStr]):
     """The result of :method:`CaptureFixture.readouterr`."""

src/_pytest/compat.py

@@ -19,7 +19,6 @@
 
 import attr
 
-from _pytest._io.saferepr import saferepr
 from _pytest.outcomes import fail
 from _pytest.outcomes import TEST_OUTCOME
 
@@ -297,6 +296,8 @@ def get_real_func(obj):
             break
         obj = new_obj
     else:
+        from _pytest._io.saferepr import saferepr
+
         raise ValueError(
             ("could not find real function of {start}\nstopped at {current}").format(
                 start=saferepr(start_obj), current=saferepr(obj)
@@ -357,6 +358,19 @@ def overload(f):  # noqa: F811
         return f
 
 
+if TYPE_CHECKING:
+    if sys.version_info >= (3, 8):
+        from typing import final as final
+    else:
+        from typing_extensions import final as final
+elif sys.version_info >= (3, 8):
+    from typing import final as final
+else:
+
+    def final(f):  # noqa: F811
+        return f
+
+
 if getattr(attr, "__version_info__", ()) >= (19, 2):
     ATTRS_EQ_FIELD = "eq"
 else:

src/_pytest/config/__init__.py

@@ -43,6 +43,7 @@
 from _pytest._code import ExceptionInfo
 from _pytest._code import filter_traceback
 from _pytest._io import TerminalWriter
+from _pytest.compat import final
 from _pytest.compat import importlib_metadata
 from _pytest.compat import TYPE_CHECKING
 from _pytest.outcomes import fail
@@ -76,6 +77,7 @@
 hookspec = HookspecMarker("pytest")
 
 
+@final
 class ExitCode(enum.IntEnum):
     """Encodes the valid exit codes by pytest.
 
@@ -322,6 +324,7 @@ def _prepareconfig(
         raise
 
 
+@final
 class PytestPluginManager(PluginManager):
     """A :py:class:`pluggy.PluginManager <pluggy.PluginManager>` with
     additional pytest-specific functionality:
@@ -815,6 +818,7 @@ def _args_converter(args: Iterable[str]) -> Tuple[str, ...]:
     return tuple(args)
 
 
+@final
 class Config:
     """Access to configuration values, pluginmanager and plugin hooks.
 
@@ -825,6 +829,7 @@ class Config:
         invocation.
     """
 
+    @final
     @attr.s(frozen=True)
     class InvocationParams:
         """Holds parameters passed during :func:`pytest.main`.

src/_pytest/config/argparsing.py

@@ -16,6 +16,7 @@
 import py
 
 import _pytest._io
+from _pytest.compat import final
 from _pytest.compat import TYPE_CHECKING
 from _pytest.config.exceptions import UsageError
 
@@ -26,6 +27,7 @@
 FILE_OR_DIR = "file_or_dir"
 
 
+@final
 class Parser:
     """Parser for command line arguments and ini-file values.
 

src/_pytest/config/exceptions.py

@@ -1,3 +1,7 @@
+from _pytest.compat import final
+
+
+@final
 class UsageError(Exception):
     """Error in pytest usage or invocation."""
 

src/_pytest/fixtures.py

@@ -32,6 +32,7 @@
 from _pytest._io import TerminalWriter
 from _pytest.compat import _format_args
 from _pytest.compat import _PytestWrapper
+from _pytest.compat import final
 from _pytest.compat import get_real_func
 from _pytest.compat import get_real_method
 from _pytest.compat import getfuncargnames
@@ -730,6 +731,7 @@ def __repr__(self) -> str:
         return "<FixtureRequest for %r>" % (self.node)
 
 
+@final
 class SubRequest(FixtureRequest):
     """A sub request for handling getting a fixture from a test function/fixture."""
 
@@ -796,6 +798,7 @@ def scope2index(scope: str, descr: str, where: Optional[str] = None) -> int:
         )
 
 
+@final
 class FixtureLookupError(LookupError):
     """Could not return a requested fixture (missing or invalid)."""
 
@@ -952,6 +955,7 @@ def _eval_scope_callable(
     return result
 
 
+@final
 class FixtureDef(Generic[_FixtureValue]):
     """A container for a factory definition."""
 
@@ -1161,6 +1165,7 @@ def result(*args, **kwargs):
     return result
 
 
+@final
 @attr.s(frozen=True)
 class FixtureFunctionMarker:
     scope = attr.ib(type="Union[_Scope, Callable[[str, Config], _Scope]]")