New submission from Larry Hastings <la...@hastings.org>:
It's considered good hygiene to use functools.update_wrapper() to make your wrapped functions look like the original. However, when using functools.partial() to pre-supply arguments to a function, if you then call functools.update_wrapper() to update that partial object, inspect.signature() returns the *original* function's signature, not the *wrapped* function's signature. To be precise: if you wrap a function with functools.partial() to pre-provide arguments, then immediately call inspect.signature() on that partial object, it returns the correct signature with the pre-filled parameters removed. If you then call functools.update_wrapper() to update the partial from the original function, inspect.signature() now returns the *wrong* signature. I looked into it a little. The specific thing changing inspect.signature()'s behavior is the '__wrapped__' attribute added by functools.update_wrapper(). By default inspect.signature() will unwrap partial objects, but only if it has a '__wrapped__' attribute. This all looks pretty deliberate. And it seems like there was some thought given to this wrinkle; inspect.signature() takes a "follow_wrapper_chains" parameter the user can supply to control this behavior. But the default is True, meaning that by default it unwraps partial objects if they have a '__wrapped__'. I admit I don't have any context for this. Why do we want inspect.signature() to return the wrong signature by default? ---------- components: Library (Lib) files: update_wrapper.breaks.partial.signature.test.py messages: 413299 nosy: larry priority: normal severity: normal stage: test needed status: open title: functools.update_wrapper breaks the signature of functools.partial objects type: behavior versions: Python 3.10, Python 3.11, Python 3.7, Python 3.8, Python 3.9 Added file: https://bugs.python.org/file50625/update_wrapper.breaks.partial.signature.test.py _______________________________________ Python tracker <rep...@bugs.python.org> <https://bugs.python.org/issue46761> _______________________________________ _______________________________________________ Python-bugs-list mailing list Unsubscribe: https://mail.python.org/mailman/options/python-bugs-list/archive%40mail-archive.com
