"Frank Millman" <fr...@chagford.com> wrote in message
news:i3ov9e$du...@dough.gmane.org...
Hi all
I know the problems related to circular imports, and I know some of the
techniques to get around them. However, I find that I bump my head into
them from time to time, which means, I guess, that I have not fully
understood how to organise my code so that I avoid them in the first
place.
[...]
So I think my main question is, is this a symptom of a flaw in my
approach, or is this something that all programmers bump into from time to
time?
Thanks for the replies. All good info, but it was Ethan that put me onto the
right track.
I omitted to mention one thing originally, as I did not think it important,
but it turns out to be crucial. My code is organised into three 'packages',
not 'modules'.
To reproduce my situation, I did some tests with the following hierarchy -
top/
a.py
/bb
__init__.py
b.py
/cc
__init__.py
c.py
a.py
----
import bb.b
import cc.c
bb.b.foo()
cc.c.foo()
b.py
----
import cc.c
def foo():
print 'in b.foo, call c.bar'
cc.c.bar()
def bar():
print ' bar in b'
c.py
----
import bb.b
def foo():
print 'in c.foo, call b.bar'
bb.b.bar()
def bar():
print ' bar in c'
If I run 'a.py', I get the correct result -
in b.foo, call c.bar
bar in c
in c.foo, call b.bar
bar in b
I changed 'a.py' -
a.py
----
from bb import b
from cc import c
b.foo()
c.foo()
It still worked.
Next I changed 'b.py' -
b.py
----
from cc import c
def foo():
print 'in b.foo, call c.bar'
c.bar()
def bar():
print ' bar in b'
It still worked.
Then I changed 'c.py' -
c.py
----
from bb import b
def foo():
print 'in b.foo, call c.bar'
b.bar()
def bar():
print ' bar in b'
Now I get the following traceback -
Traceback (most recent call last):
File "F:\dd\a.py", line 1, in <module>
from bb import b
File "F:\dd\bb\b.py", line 1, in <module>
from cc import c
File "F:\dd\cc\c.py", line 1, in <module>
from bb import b
ImportError: cannot import name b
Now that I understand this, I can work around my problem by using
fully-qualified module names.
But it would be interesting to know the underlying reason for this
behaviour.
I am using python 2.6.2.
Thanks for any insights.
Frank
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