config: remove after_preparse

The `after_preparse` hack was used to decide between two behaviors of
`--help`:

1. Just set `help` on the Namespace and continue parsing
2. Interrupt the parsing

The `after_preparse` was used to make 1 happen before `_preparse` and 2
after. But that's not what we want, the timing shouldn't really matter,
only the "mode" in which we parse:

- `Parser.parse_known_[and_unknown_]arguments` -- permissive mode, want
  behavior 1.
- `Parser.parse` -- actual parsing, want behavior 2.

Make it so.

Author: bluetech

src/_pytest/config/__init__.py

@@ -1528,7 +1528,6 @@ def parse(self, args: list[str], addopts: bool = True) -> None:
             kwargs=dict(pluginmanager=self.pluginmanager)
         )
         self._preparse(args, addopts=addopts)
-        self._parser.after_preparse = True  # type: ignore
         try:
             self._parser.parse(args, namespace=self.option)
         except PrintHelp:

src/_pytest/config/argparsing.py

@@ -130,15 +130,21 @@ def parse(
         """Parse the arguments.
 
         Unlike ``parse_known_args`` and ``parse_known_and_unknown_args``,
-        raises UsageError on unknown flags.
+        raises PrintHelp on `--help` and UsageError on unknown flags
 
         :meta private:
         """
         from _pytest._argcomplete import try_argcomplete
 
         try_argcomplete(self.optparser)
         strargs = [os.fspath(x) for x in args]
-        return self.optparser.parse_intermixed_args(strargs, namespace=namespace)
+        if namespace is None:
+            namespace = argparse.Namespace()
+        try:
+            namespace._raise_print_help = True
+            return self.optparser.parse_intermixed_args(strargs, namespace=namespace)
+        finally:
+            del namespace._raise_print_help
 
     def parse_known_args(
         self,

src/_pytest/helpconfig.py

@@ -14,19 +14,21 @@
 from _pytest.config import ExitCode
 from _pytest.config import PrintHelp
 from _pytest.config.argparsing import Parser
-from _pytest.config.argparsing import PytestArgumentParser
 from _pytest.terminal import TerminalReporter
 import pytest
 
 
 class HelpAction(argparse.Action):
-    """An argparse Action that will raise an exception in order to skip the
-    rest of the argument parsing when --help is passed.
+    """An argparse Action that will raise a PrintHelp exception in order to skip
+    the rest of the argument parsing when --help is passed.
 
-    This prevents argparse from quitting due to missing required arguments
-    when any are defined, for example by ``pytest_addoption``.
-    This is similar to the way that the builtin argparse --help option is
-    implemented by raising SystemExit.
+    This prevents argparse from raising UsageError when `--help` is used along
+    with missing required arguments when any are defined, for example by
+    ``pytest_addoption``. This is similar to the way that the builtin argparse
+    --help option is implemented by raising SystemExit.
+
+    To opt in to this behavior, the parse caller must set
+    `namespace._raise_print_help = True`. Otherwise it just sets the option.
     """
 
     def __init__(
@@ -50,9 +52,7 @@ def __call__(
     ) -> None:
         setattr(namespace, self.dest, self.const)
 
-        # We should only skip the rest of the parsing after preparse is done.
-        assert isinstance(parser, PytestArgumentParser)
-        if getattr(parser._parser, "after_preparse", False):
+        if getattr(namespace, "_raise_print_help", False):
             raise PrintHelp
 
 

testing/test_helpconfig.py

@@ -83,6 +83,14 @@ def pytest_addoption(parser):
     result.stdout.fnmatch_lines(lines, consecutive=True)
 
 
+def test_parse_known_args_doesnt_quit_on_help(pytester: Pytester) -> None:
+    """`parse_known_args` shouldn't exit on `--help`, unlike `parse`."""
+    config = pytester.parseconfig()
+    # Doesn't raise or exit!
+    config._parser.parse_known_args(["--help"])
+    config._parser.parse_known_and_unknown_args(["--help"])
+
+
 def test_hookvalidation_unknown(pytester: Pytester) -> None:
     pytester.makeconftest(
         """