circular python imports

gistfile1.py

# a.py
print '1a'
import b
print '2a'
x = b.x + 1

# b.py
print '1b'
import a
print '2b'
x = 1

"""
Rules for dealing with cyclical imports:

* 'import' and 'from x import y' are executable statements. They execute 
when the program reaches their line.

* If a module is not in sys.modules, then an import creates a new entry 
in sys.modules and executes the code in the module. It does not return 
control to the calling module until the execution has completed.

* If a module does exist in sys.modules, then an import returns it, 
regardless of whether the module has completed executing. This results
in modules which appear to be partly empty.

* The executing script runs in a module named __main__. Importing the 
script under its own name will create a new module unrelated to __main__.

"""

# ./a.py
1a
# load b into sys.modules, run b
1b
# load a into sys.modules, run a
1a
# find b in sys.modules, return it
2a
# AttributeError: 'module' object has no attribute 'x'

# ./b.py
1b
# load a into sys.modules, run a
1a
# load b into sys.modules, run b
1b
# find a in sys.modules, return it
2b
x = 1
2a
x = b.x + 1
2b # See note below
x = 1

# Note: the reason that the '2b' is printed again (and x = 1 is executed)
# is due to rule 2. It's at this point that control finally 'unwinds' to 
# where module b first imports module a.