Circular import

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September 26th, 2012, 05:13 PM

Slhd

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Join Date: Sep 2012

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Circular import

Is it allowed to have a circular import stucture, simplified with the example below?

Code:


# Module A:
import C

# Module B:
import A

# Module C:
import B

# Main :
import A
import B
import C

As I see it, it only requires an extra compilation of A. Is it wrong? What could happen if there are some circular imports? It is very hard, in my case, to get a good program structure that has no circular imports.

September 26th, 2012, 06:43 PM

b49P23TIvg

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It appears that python recursively loads the modules.

Code:

$ ( cd /tmp && python -c 'import A' )
importing A line 1
importing C line 1
importing B line 1
finishing B import
finishing C import
finishing A import
$ ( cd /tmp && for a in A B C ; do echo $a.py && cat $a.py ; done )
A.py
print('importing A line 1')
import C
print('finishing A import')
B.py
print('importing B line 1')
import A
print('finishing B import')
C.py
print('importing C line 1')
import B
print('finishing C import')
$

Now, suppose I stick an error (not a syntax error) into A.

print('importing A line 1')
import C
1/0 # error
print('finishing A import')

Then it turns out that sys.modules contains entries for B and for C but not A .

Code:

$ ( cd /tmp && python -ic 'import sys,A' )
importing A line 1
importing C line 1
importing B line 1
finishing B import
finishing C import
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "A.py", line 3, in <module>
    1/0 # error
ZeroDivisionError: integer division or modulo by zero
>>> print(sys.modules)
{'apt.os': None, ..., 'B': <module 'B' from 'B.pyc'>, ... }

Good luck.

__________________
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September 27th, 2012, 11:55 AM

Slhd

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Circular import, further testing

Thank you for your effort. This inspired me to do some own testing more ressembling my case. In the test there are 3 modules, A.py, B.py and C.py. They are called from main.py. The import structure is circular. The modules has one (or two) functions each, a(), b(), c() and c1(). There are print outs att the beginning of each module and when importing is finished. There are also print outs when each routine starts running and is finished.

Code:

# Module A.py
print 'start module A'
import C
print 'End import C in module A'

def a() :
    print 'A.a() is running'
    C.c1()
    print 'In module A finish C.c1()'
    

# Module B.py
print 'start module B'
import A
print 'End import A in module B'

def b() :
    print 'B.b() is running'
    A.a()
    print 'In module B finish B.b() and A.a()'


# Module C.py
print 'start module C'
import A
import B 
print 'End import A, B in module C'

def c() :
    print 'C.c() is running'
    A.a()
    B.b()
    print 'In module C finish A.a() and B.b()'

def c1() :
    print 'C.c1() is running'


# Module main.py
import sys
print 'start main'
import A
import B
import C
print 'End import A, B and C in module main'
if True  :
    print 'running main'
    A.a()
    B.b()
    C.c()
    print 'In module main finish A.a(), B.b() and C.c()'

#   Finding the modules in 'sys.modules'
    print
    sys_modules = sys.modules
    module_keys = sys_modules.keys()
    for mod in module_keys :
        if mod == 'A' or mod == 'B' or mod == 'C' or mod == '__main__' :
            print mod, ':', sys_modules[ mod ]


### The print out from this code is :

start main
start module A
start module C
start module B
End import A in module B
End import A, B in module C
End import C in module A
End import A, B and C in module main

running main
A.a() is running
C.c1() is running
In module A finish C.c1()
B.b() is running
A.a() is running
C.c1() is running
In module A finish C.c1()
In module B finish B.b() and A.a()
C.c() is running
A.a() is running
C.c1() is running
In module A finish C.c1()
B.b() is running
A.a() is running
C.c1() is running
In module A finish C.c1()
In module B finish B.b() and A.a()
In module C finish A.a() and B.b()
In module main() finish A.a(), B.b() and C.c()

A : <module 'A' from '/Users/Sten/Programmering/Python/Train/Test import/A.pyc'>
B : <module 'B' from '/Users/Sten/Programmering/Python/Train/Test import/B.pyc'>
C : <module 'C' from '/Users/Sten/Programmering/Python/Train/Test import/C.pyc'>
__main__ : <module '__main__' (built-in)>

It seams to me that all works OK – any comments?
Thanks in advance

September 27th, 2012, 12:13 PM

b49P23TIvg

Contributing User

Join Date: Aug 2011

Posts: 3,357

Time spent in forums: 1 Month 2 Weeks 3 Days 9 h 10 m 17 sec
Reputation Power: 383

Spell "seams" as "seems".

More seriously, the test was fairly easy and we learned to "just try it". Actually, I was fairly sure beforehand, in fact rewrote my post after I tried it, that the circular import would work somewhat differently.

Last edited by b49P23TIvg : September 27th, 2012 at 12:15 PM.

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