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Multiprocessing returns Array, RawArray of wrong length python 3.10.4 · Issue #98447 · python/cpython · GitHub
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Multiprocessing returns Array, RawArray of wrong length python 3.10.4 #98447

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Multiprocessing returns Array, RawArray of wrong length python 3.10.4#98447
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topic-multiprocessingtype-bugAn unexpected behavior, bug, or error

Description

@tendermonster
tendermonster
opened on Oct 19, 2022

Bug report

When initializing Array or RawArray of type int, wrong length of the arrays is returned

Reproducible example:

from multiprocessing import RawArray, RawValue, Array, Value
a = Array('d',12)
X = np.frombuffer(a.get_obj())
print(X) # [0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]

a = Array('i',12)
X = np.frombuffer(a.get_obj())
print(X) # [0. 0. 0. 0. 0. 0.]

a = RawArray('d',12)
X = np.frombuffer(a)
print(X) # [0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0. 0.]

a = RawArray('i',12)
X = np.frombuffer(a)
print(X) # [0. 0. 0. 0. 0. 0.]

Your environment

working with vscode
conda-forge python 3.10.4
using ubuntu uname-a: 5.11.0-46-generic #51~20.04.1-Ubuntu SMP Fri Jan 7 06:51:40 UTC 2022

  • CPython versions tested on: conda-forge python 3.10.4
  • Operating system and architecture: using ubuntu uname-a: 5.11.0-46-generic 51~20.04.1-Ubuntu

Activity

  1. tendermonster
    added
    type-bugAn unexpected behavior, bug, or error
    on Oct 19, 2022
  2. AlexWaygood
    added
    topic-multiprocessing
    on Oct 19, 2022
  3. AlexWaygood
    added this to Multiprocessing issueson Oct 19, 2022

  4. mdboom commented on Oct 19, 2022

    @mdboom
    mdboom
    on Oct 19, 2022
    Contributor

    The buffer interface doesn't retain type information about the items in the array, so using np.frombuffer defaults to double (8 bytes per item), even when the type in the array is i (4 bytes per item) since converting to a buffer throws that information out.

    I think you want to use np.asarray(a) instead to get the behaviour you want -- it will correctly look at the type of the array (using the memoryview API), and it likewise avoids the memory copy.

  5. mdboom
    closed this as completedon Oct 19, 2022
  6. mdboom
    moved this to Done in Multiprocessing issueson Oct 19, 2022

  7. tendermonster commented on Oct 20, 2022

    @tendermonster
    tendermonster
    on Oct 20, 2022
    Author

    Thanks for quick response

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