[docs] [issue23484] SemLock acquire() keyword arg 'blocking' is invalid
report at bugs.python.org
Sun Mar 1 22:53:05 CET 2015
Davin Potts added the comment:
In the docs for 2.7, multiprocessing.Lock refers to threading.Lock in which the signature for acquire looks like:
However, in the current code in 2.7, Modules/_multiprocessing/semaphore.c implements multiprocessing.Lock.acquire to support two arguments: 'block' and 'timeout'.
In the docs for 3.4, threading.Lock.acquire now has both 'blocking' and 'timeout' arguments. Notably, it specifies: "It is forbidden to specify a timeout when blocking is false." This is indeed enforced by threading.Lock but not at all by multiprocessing.Lock. In a 3.4.3 shell:
>>> import multiprocessing
>>> import threading
>>> ml = multiprocessing.Lock()
>>> ml.acquire(False, 20.0)
>>> tl = threading.Lock()
>>> tl.acquire(False, 20.0)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: can't specify a timeout for a non-blocking call
In 2.7, acquire similarly permits a supplied value for timeout even when set to be non-blocking.
A further difference: the code in Modules/_multiprocessing/semaphore.c appears to translate a timeout=None into an infinite wait whereas the threading implementation rejects any attempt to supply a None for timeout, demanding a float instead. (In 3.4, threading uses timeout=-1 to request an infinite wait.)
Given these discrepancies between threading's and multiprocessing's implementations for Lock and given the difficulties in renaming an argument that can be supplied as a non-keyword parameter, the right thing to do at this point is to properly document multiprocessing.Lock's acquire as using 'block' and 'timeout'.
assignee: -> docs at python
components: +Documentation -Extension Modules
nosy: +docs at python
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