Fix exception chaining on PyPy (#712)

* Fix exception chaining on PyPy

Fixes #703

* Add newsfragment

* Blankly exclude PyPy from coverage reporting

* Manually add default no cover marker

Author: hynek

changelog.d/703.change.rst

@@ -0,0 +1 @@
+``raise from`` now works on frozen classes on PyPy.

changelog.d/712.change.rst

@@ -0,0 +1 @@
+``raise from`` now works on frozen classes on PyPy.

pyproject.toml

@@ -13,6 +13,11 @@ source = ["src", ".tox/*/site-packages"]
 
 [tool.coverage.report]
 show_missing = true
+exclude_lines = [
+    "pragma: no cover",
+    # PyPy is unacceptably slow under coverage.
+    "if PYPY:",
+]
 
 
 [tool.black]

src/attr/_compat.py

@@ -91,7 +91,7 @@ def metadata_proxy(d):
         res.data.update(d)  # We blocked update, so we have to do it like this.
         return res
 
-    def just_warn(*args, **kw):  # pragma: nocover
+    def just_warn(*args, **kw):  # pragma: no cover
         """
         We only warn on Python 3 because we are not aware of any concrete
         consequences of not setting the cell on Python 2.
@@ -132,7 +132,7 @@ def make_set_closure_cell():
     """
     # pypy makes this easy. (It also supports the logic below, but
     # why not do the easy/fast thing?)
-    if PYPY:  # pragma: no cover
+    if PYPY:
 
         def set_closure_cell(cell, value):
             cell.__setstate__((value,))

src/attr/_make.py

@@ -12,6 +12,7 @@
 from . import _config, setters
 from ._compat import (
     PY2,
+    PYPY,
     isclass,
     iteritems,
     metadata_proxy,
@@ -527,11 +528,29 @@ def _transform_attrs(
     return _Attributes((attrs, base_attrs, base_attr_map))
 
 
-def _frozen_setattrs(self, name, value):
-    """
-    Attached to frozen classes as __setattr__.
-    """
-    raise FrozenInstanceError()
+if PYPY:
+
+    def _frozen_setattrs(self, name, value):
+        """
+        Attached to frozen classes as __setattr__.
+        """
+        if isinstance(self, BaseException) and name in (
+            "__cause__",
+            "__context__",
+        ):
+            BaseException.__setattr__(self, name, value)
+            return
+
+        raise FrozenInstanceError()
+
+
+else:
+
+    def _frozen_setattrs(self, name, value):
+        """
+        Attached to frozen classes as __setattr__.
+        """
+        raise FrozenInstanceError()
 
 
 def _frozen_delattrs(self, name):

tests/test_next_gen.py

@@ -4,6 +4,8 @@
 
 import re
 
+from functools import partial
+
 import pytest
 
 import attr
@@ -238,3 +240,32 @@ class B:
             @attr.define(on_setattr=attr.setters.validate)
             class C(A):
                 pass
+
+    @pytest.mark.parametrize(
+        "decorator",
+        [
+            partial(attr.s, frozen=True, slots=True, auto_exc=True),
+            attr.frozen,
+            attr.define,
+            attr.mutable,
+        ],
+    )
+    def test_discard_context(self, decorator):
+        """
+        raise from None works.
+
+        Regression test for #703.
+        """
+
+        @decorator
+        class MyException(Exception):
+            x: str = attr.ib()
+
+        with pytest.raises(MyException) as ei:
+            try:
+                raise ValueError()
+            except ValueError:
+                raise MyException("foo") from None
+
+        assert "foo" == ei.value.x
+        assert ei.value.__cause__ is None