Fix for cyclic exception context

Author: Zac-HD

hypothesis-python/RELEASE.rst

@@ -1,4 +1,4 @@
 RELEASE_TYPE: patch
 
-This patch updates our vendored `list of top-level domains <https://www.iana.org/domains/root/db>`__,
-which is used by the provisional :func:`~hypothesis.provisional.domains` strategy.
+This patch fixes an internal error when the ``__context__``
+attribute of a raised exception leads to a cycle (:issue:`4115`).

hypothesis-python/src/hypothesis/internal/escalation.py

@@ -13,9 +13,10 @@
 import sys
 import textwrap
 import traceback
+from functools import partial
 from inspect import getframeinfo
 from pathlib import Path
-from typing import Dict, NamedTuple, Optional, Type
+from typing import Dict, NamedTuple, Optional, Tuple, Type
 
 import hypothesis
 from hypothesis.errors import _Trimmable
@@ -107,20 +108,27 @@ def __str__(self) -> str:
         return f"{self.exc_type.__name__} at {self.filename}:{self.lineno}{ctx}{group}"
 
     @classmethod
-    def from_exception(cls, exception: BaseException, /) -> "InterestingOrigin":
+    def from_exception(
+        cls, exception: BaseException, /, seen: Tuple[BaseException, ...] = ()
+    ) -> "InterestingOrigin":
         filename, lineno = None, None
         if tb := get_trimmed_traceback(exception):
             filename, lineno, *_ = traceback.extract_tb(tb)[-1]
+        seen = (*seen, exception)
+        make = partial(cls.from_exception, seen=seen)
+        context: "InterestingOrigin | tuple[()]" = ()
+        if exception.__context__ is not None and exception.__context__ not in seen:
+            context = make(exception.__context__)
         return cls(
             type(exception),
             filename,
             lineno,
             # Note that if __cause__ is set it is always equal to __context__, explicitly
             # to support introspection when debugging, so we can use that unconditionally.
-            cls.from_exception(exception.__context__) if exception.__context__ else (),
+            context,
             # We distinguish exception groups by the inner exceptions, as for __context__
             (
-                tuple(map(cls.from_exception, exception.exceptions))
+                tuple(make(exc) for exc in exception.exceptions if exc not in seen)
                 if isinstance(exception, BaseExceptionGroup)
                 else ()
             ),

hypothesis-python/tests/cover/test_escalation.py

@@ -70,3 +70,26 @@ def test_handles_groups():
     assert "ExceptionGroup at " in str(origin)
     assert "child exception" in str(origin)
     assert "ValueError at " in str(origin)
+
+
+def make_exceptions_with_cycles():
+    err = ValueError()
+    err.__context__ = err
+    yield err
+
+    err = TypeError()
+    err.__context__ = BaseExceptionGroup("msg", [err])
+    yield err
+
+    inner = LookupError()
+    err = BaseExceptionGroup("msg", [inner])
+    inner.__context__ = err
+    yield err
+
+    inner = OSError()
+    yield BaseExceptionGroup("msg", [inner, inner, inner])
+
+
+@pytest.mark.parametrize("err", list(make_exceptions_with_cycles()))
+def test_handles_cycles(err):
+    esc.InterestingOrigin.from_exception(err)