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allow failed import of Pool from multiprocessing #8162

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allow failed import of Pool from multiprocessing #8162

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allow failed import of Pool from multiprocessing
landfillbaby Apr 28, 2020
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landfillbaby Apr 28, 2020
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1 change: 1 addition & 0 deletions news/8161.bugfix
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
Fix ``list --outdated`` and ``list --uptodate` on platforms without ``sem_open``
28 changes: 17 additions & 11 deletions src/pip/_internal/commands/list.py
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Original file line number Diff line number Diff line change
Expand Up @@ -210,17 +210,23 @@ def latest_info(dist):
dist.latest_filetype = typ
return dist

# This is done for 2x speed up of requests to pypi.org
# so that "real time" of this function
# is almost equal to "user time"
pool = Pool(DEFAULT_POOLSIZE)

for dist in pool.imap_unordered(latest_info, packages):
if dist is not None:
yield dist

pool.close()
pool.join()
try:
# This is done for 2x speed up of requests to pypi.org
# so that "real time" of this function
# is almost equal to "user time"
pool = Pool(DEFAULT_POOLSIZE)

for dist in pool.imap_unordered(latest_info, packages):
if dist is not None:
yield dist

pool.close()
pool.join()
except ImportError:
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@CrafterKolyan CrafterKolyan Apr 28, 2020 •
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I am not an expert of when Python actually imports class Pool and raises ImportError but in my opinion it should be in the actual importing code:

from multiprocessing.dummy import Pool

In CPython they wrap importing of module in try except clause.
(https://github.com/python/cpython/blob/d9a43e20facdf4ad10186f820601c6580e1baa80/Lib/multiprocessing/synchronize.py#L24-L33)

So the except part will never be executed in current implementation.

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Also it might be cleaner to move both for loops in a fetch_latest_info_multithreaded and fetch_latest_info_single_threaded, and then include the from multiprocessing.dummy import Pool in the fetch_latest_info_multithreaded function

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Yeah, I think the ImportError block needs to be put on the from multiprocessing.dummy line instead.

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@landfillbaby landfillbaby Apr 28, 2020 •
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yeah I was wondering if this reliance on delayed import would be allowed, I'll change it if it's really necessary
I'm not sure what you mean by "the except part will never be executed" though @CrafterKolyan, since that exception is exactly what caused this issue; raising from within an except block still raises.

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I'm not exactly sure from the original bug report which statement is triggering the ImportError. We should probably determine that, and just wrap that in the try...except (maybe setting a flag that we test later to choose the right code path).

Also, should this code have a test? Otherwise there's a (small) risk that the two implementations get out of line.

for dist in packages:
dist = latest_info(dist)
if dist is not None:
yield dist

def output_package_listing(self, packages, options):
packages = sorted(
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