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

• Type: Bug
• Status: Resolved
• Priority: Blocker
• Resolution: Fixed
• Affects Version/s: qpid-python-1.38.0
• Fix Version/s: qpid-python-1.38.0
• Component/s: Python Client
• Assignee: Jiri Daněk
• Reporter: Jiri Daněk
• Created: 10/Apr/23 14:26
• Updated: 10/Apr/23 17:18
• Resolved: 10/Apr/23 17:18

Description

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.

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

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