Remove work-arounds for setting cell_contents (#1201)

* Remove work-arounds for setting cell_contents

Simple assignment to cell_contents works starting from Python 3.7[1].
This is also the minimum supported version of Python according to
pyproject.toml. It follows that we don't need complex work-arounds that
were needed in the past.

Also tested on pypy 3.8.

[1] https://bugs.python.org/issue30486

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---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Hynek Schlawack <hs@ox.cx>

src/attr/_compat.py

@@ -4,8 +4,6 @@
 import platform
 import sys
 import threading
-import types
-import warnings
 
 from collections.abc import Mapping, Sequence  # noqa: F401
 from typing import _GenericAlias
@@ -26,16 +24,6 @@
     from typing import Protocol  # noqa: F401
 
 
-def just_warn(*args, **kw):
-    warnings.warn(
-        "Running interpreter doesn't sufficiently support code object "
-        "introspection.  Some features like bare super() or accessing "
-        "__class__ will not work with slotted classes.",
-        RuntimeWarning,
-        stacklevel=2,
-    )
-
-
 class _AnnotationExtractor:
     """
     Extract type annotations from a callable, returning None whenever there
@@ -76,101 +64,6 @@ def get_return_type(self):
         return None
 
 
-def make_set_closure_cell():
-    """Return a function of two arguments (cell, value) which sets
-    the value stored in the closure cell `cell` to `value`.
-    """
-    # pypy makes this easy. (It also supports the logic below, but
-    # why not do the easy/fast thing?)
-    if PYPY:
-
-        def set_closure_cell(cell, value):
-            cell.__setstate__((value,))
-
-        return set_closure_cell
-
-    # Otherwise gotta do it the hard way.
-
-    try:
-        if sys.version_info >= (3, 8):
-
-            def set_closure_cell(cell, value):
-                cell.cell_contents = value
-
-        else:
-            # Create a function that will set its first cellvar to `value`.
-            def set_first_cellvar_to(value):
-                x = value
-                return
-
-                # This function will be eliminated as dead code, but
-                # not before its reference to `x` forces `x` to be
-                # represented as a closure cell rather than a local.
-                def force_x_to_be_a_cell():  # pragma: no cover
-                    return x
-
-            # Extract the code object and make sure our assumptions about
-            # the closure behavior are correct.
-            co = set_first_cellvar_to.__code__
-            if co.co_cellvars != ("x",) or co.co_freevars != ():
-                raise AssertionError  # pragma: no cover
-
-            # Convert this code object to a code object that sets the
-            # function's first _freevar_ (not cellvar) to the argument.
-            args = [co.co_argcount]
-            args.append(co.co_kwonlyargcount)
-            args.extend(
-                [
-                    co.co_nlocals,
-                    co.co_stacksize,
-                    co.co_flags,
-                    co.co_code,
-                    co.co_consts,
-                    co.co_names,
-                    co.co_varnames,
-                    co.co_filename,
-                    co.co_name,
-                    co.co_firstlineno,
-                    co.co_lnotab,
-                    # These two arguments are reversed:
-                    co.co_cellvars,
-                    co.co_freevars,
-                ]
-            )
-            set_first_freevar_code = types.CodeType(*args)
-
-            def set_closure_cell(cell, value):
-                # Create a function using the set_first_freevar_code,
-                # whose first closure cell is `cell`. Calling it will
-                # change the value of that cell.
-                setter = types.FunctionType(
-                    set_first_freevar_code, {}, "setter", (), (cell,)
-                )
-                # And call it to set the cell.
-                setter(value)
-
-        # Make sure it works on this interpreter:
-        def make_func_with_cell():
-            x = None
-
-            def func():
-                return x  # pragma: no cover
-
-            return func
-
-        cell = make_func_with_cell().__closure__[0]
-        set_closure_cell(cell, 100)
-        if cell.cell_contents != 100:
-            raise AssertionError  # pragma: no cover
-
-    except Exception:  # noqa: BLE001
-        return just_warn
-    else:
-        return set_closure_cell
-
-
-set_closure_cell = make_set_closure_cell()
-
 # Thread-local global to track attrs instances which are already being repr'd.
 # This is needed because there is no other (thread-safe) way to pass info
 # about the instances that are already being repr'd through the call stack

src/attr/_make.py

@@ -18,7 +18,6 @@
     PY310,
     _AnnotationExtractor,
     get_generic_base,
-    set_closure_cell,
 )
 from .exceptions import (
     DefaultAlreadySetError,
@@ -909,7 +908,7 @@ def _create_slots_class(self):
                     pass
                 else:
                     if match:
-                        set_closure_cell(cell, cls)
+                        cell.cell_contents = cls
 
         return cls
 

tests/test_slots.py

@@ -5,8 +5,6 @@
 """
 
 import pickle
-import sys
-import types
 import weakref
 
 from unittest import mock
@@ -15,7 +13,7 @@
 
 import attr
 
-from attr._compat import PYPY, just_warn, make_set_closure_cell
+from attr._compat import PYPY
 
 
 # Pympler doesn't work on PyPy.
@@ -478,36 +476,6 @@ def statmethod():
 
         assert D.statmethod() is D
 
-    @pytest.mark.skipif(PYPY, reason="set_closure_cell always works on PyPy")
-    @pytest.mark.skipif(
-        sys.version_info >= (3, 8),
-        reason="can't break CodeType.replace() via monkeypatch",
-    )
-    def test_code_hack_failure(self, monkeypatch):
-        """
-        Keeps working if function/code object introspection doesn't work
-        on this (nonstandard) interpreter.
-
-        A warning is emitted that points to the actual code.
-        """
-        # This is a pretty good approximation of the behavior of
-        # the actual types.CodeType on Brython.
-        monkeypatch.setattr(types, "CodeType", lambda: None)
-        func = make_set_closure_cell()
-
-        with pytest.warns(RuntimeWarning) as wr:
-            func()
-
-        w = wr.pop()
-        assert __file__ == w.filename
-        assert (
-            "Running interpreter doesn't sufficiently support code object "
-            "introspection.  Some features like bare super() or accessing "
-            "__class__ will not work with slotted classes.",
-        ) == w.message.args
-
-        assert just_warn is func
-
 
 @pytest.mark.skipif(PYPY, reason="__slots__ only block weakref on CPython")
 def test_not_weakrefable():