Original issue: Subclassing threading Lock/RLock: better Exception messages · Issue #94077 · python/cpython · GitHub
Right now threading.Lock and threading.RLock are defined as functions. Here’s how the exception messages look if we try to subclass them:
from threading import Lock, RLock
class SubclassLock(Lock):
pass
# TypeError: cannot create 'builtin_function_or_method' instances
class SubclassRLock(RLock):
pass
# TypeError: function() argument 'code' must be code, not str
Here’s my proposal:
def Lock():
"""Factory function that returns a new lock."""
return _allocate_lock()
Lock.__mro_entries__ = lambda bases: (_thread.lock,)
# ...
RLock.__mro_entries__ = lambda bases: (
_PyRLock if _CRLock is None else _CRLock,
)
This makes Lock and RLock functions subclassable:
>>> from threading import RLock
>>> class My(RLock): ...
...
>>> My.__mro__
(<class '__main__.My'>, <class '_thread.RLock'>, <class 'object'>)
>>>
Should I make this change? How often people actually subclass Lock and RLock?
Right now, it can be solved via this hack:
_LockType = type(threading.Lock())
class MyLock(_LockType): ...
Any feedback is welcome! 