Guido van Rossum added the comment:
So to save others the bother to check what's in the zip file, the contents of
mytest/mod1.py is as follows:
import mytest.mod2 as mod
def func():
print('mod1.func called')
mod.func()
There's no __init__.py (so mytest is a namespace package, PEP 420) but adding
an empty __init__.py makes no difference. The problem occurs with both Python 2
and 3.
The root cause is the import cycle. I played around with dis and found the
following (I suspect others have already found this but the thread was hard to
follow for me initially):
>>> dis.dis('import a.b')
1 0 LOAD_CONST 0 (0)
2 LOAD_CONST 1 (None)
4 IMPORT_NAME 0 (a.b)
6 STORE_NAME 1 (a)
8 LOAD_CONST 1 (None)
10 RETURN_VALUE
>>>
compared to
>>> dis.dis('import a.b as c')
1 0 LOAD_CONST 0 (0)
2 LOAD_CONST 1 (None)
4 IMPORT_NAME 0 (a.b)
6 LOAD_ATTR 1 (b) <-- error here
8 STORE_NAME 2 (c)
10 LOAD_CONST 1 (None)
12 RETURN_VALUE
>>>
What this shows is that the implementation of "import a.b" and "import a.b as
c" are different. The former calls __import__('a.b', ...) which returns the
module 'a' and stores that in the variable 'a'. In the OP's case, because of
the import cycle, while sys.modules['a.b'] exists, module 'a' does not yet have
the attribute 'b'. That's the reason that in the latter example, the LOAD_ATTR
opcode fails.
I'm inclined not to attempt to fix this. The reason that we don't pull 'a.b'
out of sys.modules at this point is that if later in the execution of a/b.py we
get an exception, the import will be cancelled, and sys.modules['a.b'] will be
*deleted*, and then the variable 'c' would have a reference to a half-loaded
module.
The semantics of imports in the case of cycles are somewhat complex but clearly
defined and there are only a few rules to consider, and from these rules it is
possible to reason out whether any particular case is valid or not. I would
prefer to keep it this way rather than add more special cases. There's a good
reason why, *in general* (regardless of import cycles), "import a.b as c" is
implemented as a getattr operation on a, not as an index operation on
sys.modules (it is possible for module a to override its attribute b without
updating sys.modules) and I'd rather keep those semantics than give them up for
this particular edge case. Cyclic imports are hard. If they don't work for you,
avoid them.
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<http://bugs.python.org/issue30024>
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