[jira] [Created] (QPID-8632) [python client] Circular imports in mllib and qpid.connection, qpid.delegates

["Jiri Daněk (Jira)" <ji...@apache.org>]

Jiri Daněk created QPID-8632:
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             Summary: [python client] Circular imports in mllib and qpid.connection, qpid.delegates
                 Key: QPID-8632
                 URL: https://issues.apache.org/jira/browse/QPID-8632
             Project: Qpid
          Issue Type: Bug
          Components: Python Client
    Affects Versions: qpid-python-1.38.0
            Reporter: Jiri Daněk


Circular imports in Python actually do work, and break only when one makes a change that trips this potential minefield. One such change is Python 2/3 compatibility.

What triggers the problem is the situation when two modules import each other, _and_ the other module needs to be evaluated for the first import (when evaluating the first module) to complete. See https://stackoverflow.com/questions/744373/what-happens-when-using-mutual-or-circular-cyclic-imports for discussion and examples.

Picking solution that is compatible with Python 2 as well as Python 3 in the future (QPID-8631) is tricky.

{noformat}
Traceback (most recent call last):
  File "./qpid-python-test", line 29, in <module>
    from qpid.harness import Skipped
  File "/home/runner/work/qpid-python/qpid-python/qpid/__init__.py", line 20, in <module>
    from . import connection
  File "/home/runner/work/qpid-python/qpid-python/qpid/connection.py", line 29, in <module>
    from . import delegates
  File "/home/runner/work/qpid-python/qpid-python/qpid/delegates.py", line 21, in <module>
    from . import connection, session
ImportError: cannot import name connection
{noformat}

{noformat}
Traceback (most recent call last):
  File "./qpid-python-test", line 30, in <module>
    from qpid.harness import Skipped
  File "/home/runner/work/qpid-python/qpid-python/qpid/__init__.py", line 21, in <module>
    from . import connection
  File "/home/runner/work/qpid-python/qpid-python/qpid/connection.py", line 21, in <module>
    from . import datatypes, session
  File "/home/runner/work/qpid-python/qpid-python/qpid/session.py", line 22, in <module>
    from .generator import command_invoker
  File "/home/runner/work/qpid-python/qpid-python/qpid/generator.py", line 23, in <module>
    from .ops import *
  File "/home/runner/work/qpid-python/qpid-python/qpid/ops.py", line 21, in <module>
    import os, mllib, sys
  File "/home/runner/work/qpid-python/qpid-python/mllib/__init__.py", line 27, in <module>
    from . import dom, transforms, parsers
  File "/home/runner/work/qpid-python/qpid-python/mllib/dom.py", line 34, in <module>
    from . import transforms
  File "/home/runner/work/qpid-python/qpid-python/mllib/transforms.py", line 25, in <module>
    from . import dom
ImportError: cannot import name dom
{noformat}



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