[docs] [issue23484] SemLock acquire() keyword arg 'blocking' is invalid

Davin Potts 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)
    >>> ml.release()
    >>> 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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