[Python-3000] cyclic imports worked in py2.6, fail in 3.0

[Ondrej Certik]

Hi,

I am trying to get SymPy working on python 3.0, to see if there are
possible bugs. I must say that I am impressed by the 2to3 tool, that
is now able to translate the whole sympy!
Many thanks to Benjamin for fixing several bugs very quickly in it.

However, I am having some troubles with cyclic imports. Currently the
way we handle them in sympy is that we have this code at the end of
each module (only in sympy core):

# /cyclic/
import basic as _
_.Add       = Add
del _

import mul as _
_.Add       = Add
del _

import power as _
_.Add       = Add
del _


What it does is that it injects the classes defined in this particular
module to the other modules. First problem: 2to3 tool doesn't convert
this correctly, I had to manually conver this
to:

from . import basic as _
_.Add       = Add
del _

from . import mul as _
_.Add       = Add
del _

from . import power as _
_.Add       = Add
del _

and second problem is that it still fails to import in python3.0:

$ python3.0 q.py
Traceback (most recent call last):
  File "q.py", line 1, in <module>
    import sympy
  File "/home/ondra/repos/sympy/sympy/__init__.py", line 16, in <module>
    from sympy.core import *
  File "/home/ondra/repos/sympy/sympy/core/__init__.py", line 6, in <module>
    from .numbers import Number, Real, Rational, Integer, igcd, ilcm,
RealNumber, \
  File "/home/ondra/repos/sympy/sympy/core/numbers.py", line 13, in <module>
    from .power import integer_nthroot
  File "/home/ondra/repos/sympy/sympy/core/power.py", line 631, in <module>
    from . import mul as _
  File "/home/ondra/repos/sympy/sympy/core/mul.py", line 657, in <module>
    from . import add as _
  File "/home/ondra/repos/sympy/sympy/core/add.py", line 384, in <module>
    from . import mul as _
ImportError: cannot import name mul


However, this works in python2.4, 2.5 and 2.6. Notice, that "from .
import mul as _" worked in power.py, but failed in add.py 3 lines
below. This is weird, isn't it?

So my questions are:

* is our "hack" supported at all? If not, how would you suggest us to
handle cyclic imports? Basically, we want Add and Mul classes to be
defined in separate modules, however the methods of both classes need
access to the other --- so the only other option that I can see is to
locally import the other module in each method, which is slow and not
so clean. Another option is to import the other class to the module at
runtime using some dynamic features of Python.

* if it is supposed to work, is this a bug in python3.0?

Thanks,
Ondrej