Question about circular imports

Frank Millman frank at chagford.com
Sun Feb 26 11:42:18 CET 2012


Hi all

I seem to have a recurring battle with circular imports, and I am trying to 
nail it once and for all.

Let me say at the outset that I don't think I can get rid of circular 
imports altogether. It is not uncommon for me to find that a method in 
Module A needs to access something in Module B, and a method in Module B 
needs to access something in Module A. I know that the standard advice is to 
reorganise the code to avoid this, and I try to do this where possible, but 
for now I would like to address the question of how to handle the situation 
if this is otherwise unavoidable.

The problem is clearly explained in the Python Programming FAQ -

"Circular imports are fine where both modules use the "import <module>" form 
of import. They fail when the 2nd module wants to grab a name out of the 
first ("from module import name") and the import is at the top level. That's 
because names in the 1st are not yet available, because the first module is 
busy importing the 2nd."

Having recently reorganised my code into packages, I find that the same 
problem arises with packages. Assume the following structure, copied from 
the Tutorial -

sound/
    __init__.py
    formats/
        __init__.py
        wavread.py
        wavwrite.py

The following fails -

in wavread.py -
    from formats import wavwrite [this works]

in wavwrite.py -
    from formats import wavread [this fails with ImportError]

I can think of two solutions - one is cumbersome, the other may not be good 
practice.

The first solution is -

in wavread.py -
    import formats.wavwrite

in wavwrite.py -
    import formats.wavread

I then have to use the full path to reference any attribute inside the 
imported module, which I find cumbersome.

The second solution is -

in formats/__init__.py
    import sys
    sys.path.insert(0, __path__[0])

in wavread.py -
    import wavwrite

in wavwrite.py -
    import wavread

This works, but I don't know if it is a good idea to add all the sub-package 
paths to sys.path. I realise that it is up to me to avoid any name clashes. 
Are there any other downsides?

So I guess my question is -

- is there a better solution to my problem?
- if not, is my second solution acceptable?

If not, I seem to be stuck with using full path names to reference any 
attributes in imported modules.

I am using Python3 exclusively now, if that makes any difference.

Any advice will be appreciated.

Frank Millman






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