From 95e0e2ba8ce842fa01b017901379d6c4891f8cca Mon Sep 17 00:00:00 2001
From: Kevin Santana <kevinsantana11@gmail.com>
Date: Tue, 7 Dec 2021 21:44:10 -0500
Subject: [PATCH] throw if addoption called after preparse with no default

---
 AUTHORS                          |  1 +
 changelog/9303.bugfix.rst        |  2 ++
 src/_pytest/config/argparsing.py |  5 +++++
 testing/test_pluginmanager.py    | 15 +++++++++++++++
 4 files changed, 23 insertions(+)
 create mode 100644 changelog/9303.bugfix.rst

diff --git a/AUTHORS b/AUTHORS
index 25b55e5ae..f6e50003b 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -185,6 +185,7 @@ Katerina Koukiou
 Keri Volans
 Kevin Cox
 Kevin J. Foley
+Kevin Santana
 Kodi B. Arfer
 Kostis Anagnostopoulos
 Kristoffer Nordström
diff --git a/changelog/9303.bugfix.rst b/changelog/9303.bugfix.rst
new file mode 100644
index 000000000..cad5abf1f
--- /dev/null
+++ b/changelog/9303.bugfix.rst
@@ -0,0 +1,2 @@
+`Parser.addoption` now throws a value error if an attempt is made at adding an option
+after the pre_parsing process without including a default.
diff --git a/src/_pytest/config/argparsing.py b/src/_pytest/config/argparsing.py
index b0bb3f168..bd8b1ceb8 100644
--- a/src/_pytest/config/argparsing.py
+++ b/src/_pytest/config/argparsing.py
@@ -362,6 +362,11 @@ class OptionGroup:
         results in help showing ``--two-words`` only, but ``--twowords`` gets
         accepted **and** the automatic destination is in ``args.twowords``.
         """
+        if getattr(self.parser, "after_preparse", False) and "default" not in attrs:
+            raise ValueError(
+                "Cannot add options without default after initial conftest discovery"
+            )
+
         conflict = set(optnames).intersection(
             name for opt in self.options for name in opt.names()
         )
diff --git a/testing/test_pluginmanager.py b/testing/test_pluginmanager.py
index 9fe23d177..3ae997e92 100644
--- a/testing/test_pluginmanager.py
+++ b/testing/test_pluginmanager.py
@@ -78,6 +78,21 @@ class TestPytestPluginInteractions:
         )
         assert config.option.test123
 
+    def test_do_option_postinit_nodefault(self, pytester: Pytester) -> None:
+        config = pytester.parseconfigure()
+        assert not hasattr(config.option, "deadbeef")
+        p = pytester.makepyfile(
+            """
+            def pytest_addoption(parser):
+                parser.addoption('--deadbeef', action="store_true")
+        """
+        )
+        with pytest.raises(ValueError):
+            config.pluginmanager._importconftest(
+                p, importmode="prepend", rootpath=pytester.path
+            )
+        assert not hasattr(config.option, "deadbeef")
+
     def test_configure(self, pytester: Pytester) -> None:
         config = pytester.parseconfig()
         values = []