The module importing seems failing when cross-importing.

My `prog.py` file:

```
import sys
sys.path.append(".")
from m1 import f1

```

And `m1.py`:

```
from m2 import f2

def f1():
  pass

```

And `m2.py`:

```
from m1 import f1

def f2():
  pass   

```

My module m1 needs to use some functions in module 2, and module 2 needs to use some functions in module 1, so I import them the way above. But Python (python3) doesn't let me do so. Here's the exeption:

```
Traceback (most recent call last):
  File "prog.py", line 3, in <module>
    from m1 import f1
  File "/temp/m1.py", line 1, in <module>
    from m2 import f2
  File "/temp/m2.py", line 1, in <module>
    from m1 import f1
ImportError: cannot import name 'f1'

```

I know it is cross-importing, but how to solve this problem?