STINNER Victor <vstin...@python.org> added the comment:
Guido: "I'm still worried about the change in semantics where globals["__builtins__"] is assigned a different dict after the function object has been created (...)" Well, there is a semantics change of Python 3.10 documented at: https://docs.python.org/dev/whatsnew/3.10.html#other-language-changes "Functions have a new __builtins__ attribute which is used to look for builtin symbols when a function is executed, instead of looking into __globals__['__builtins__']. (Contributed by Mark Shannon in bpo-42990.)" And the function __builtins__ attribute is read-only. Your example is not affected by PR 24564 because the globals has the "__builtins__" key. In Python 3.10, you can modify func.__builtins__ (new attribute): --- def foo(s): return len(s) code = foo.__code__ FunctionType = type(foo) f = FunctionType(code, {"__builtins__": {"len": len}}) print(f("abc")) f.__builtins__.clear() print(f("abc")) --- Output: --- 3 Traceback (most recent call last): (...) NameError: name 'len' is not defined --- Mark: "Because globals['__builtins__'] is cached for each function activation, executing functions don't see updates." In Python 3.10, if someone wants to hack builtins while the function is running, modifying the builtins namespace in-place works as expected: --- def f(): print(len("test")) builtins_ns = f.__globals__['__builtins__'].__dict__ #builtins_ns = f.__builtins__ builtins_ns['len'] = lambda x: 7 print(len("test")) f() --- Output: --- 4 7 --- It also works with "builtins_ns = f.__builtins__". Guido: "I realize this is a pretty esoteric, but it does show the change in semantics (from later to earlier binding). Should we care? I like early binding because it allows more optimizations[1], but traditionally Python's semantics use late binding." Modifying built-in functions/types is commonly done in tests. Example: --- import unittest.mock def func(): with unittest.mock.patch('builtins.chr', return_value='mock'): return chr(65) print(func()) --- The expected output is: "mock". Overriding an attribute of the builtins module immediately updates func.__builtins__. It works because func.__builtins__ is builtins.__dict__. In FAT Python, I implemented an optimization which copies builtin functions to constants, replace LOAD_GLOBAL with LOAD_CONST: https://fatoptimizer.readthedocs.io/en/latest/optimizations.html#copy-builtin-to-constant This optimization breaks this Python semantics, it is no longer possible to override builtin functions in tests: https://fatoptimizer.readthedocs.io/en/latest/semantics.html#builtin-functions-replaced-in-the-middle-of-a-function ---------- _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue42990> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
