[New-bugs-announce] [issue15097] Improving wording on the thread-safeness of import

[Merlijn van Deen]

New submission from Merlijn van Deen <valhallasw at gmail.com>:

http://docs.python.org/library/threading.html#importing-in-threaded-code

Currently, the documentation states
"Firstly, other than in the main module, an import should not have the side effect of spawning a new thread and then waiting for that thread in any way. Failing to abide by this restriction can lead to a deadlock if the spawned thread directly or indirectly attempts to import a module."

which, I think, fails to make the main point: a call to import acquires the import lock. A call to import from a second thread will thus block.

As such, I would suggest rephrasing it to something like:
"Firstly, an import acquires the import lock for that thread. Therefore, the import should not have the side effect of waiting for a different thread in any way, as this can lead to a deadlock if that thread directly or indirectly attempts to import a module."

There are two additional points that might be interesting to note:
(1) Any module can be imported. If the import causes a deadlock, that is a bad thing. Every module *will* be imported by tools such as nosetests.
(1b) so: never, ever, have code that causes locks in a different thread  in module level code witout 'if __name__=="__main__" ' blocks?

(2) The lock is also acquired if a module has already been imported. For instance, in

import sys # (1)
def f():
    import sys # (2)

the import lock is acquired in (1) /and/ (2).


Adding example code and/or a flow diagram might be a bit too much, but it does clarify how easy it is to make this mistake. See the attached for an example (both a simple example script, as well as a flow diagram explaining what happens).

----------
assignee: docs at python
components: Documentation
files: deadlock.py
messages: 163068
nosy: docs at python, valhallasw
priority: normal
severity: normal
status: open
title: Improving wording on the thread-safeness of import
type: enhancement
Added file: http://bugs.python.org/file26037/deadlock.py

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<http://bugs.python.org/issue15097>
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