how to reload a Class in python shell?

I finally found the answer:

import MyPak
from MyPak import MyMod

after editing MyPak/MyMod.py file, to reload the class MyMod in the file MyMod.py, one needs to

import sys
del sys.modules['MyPak.MyMod'] 
reload(MyPak)
from MyPak import MyMod

Caveats:

Executing del MyPak or del MyMod or del MyPak.MyMod does not solve the problem since it simply removes the name binding. Python only searches sys.modules to see whether the modules had already been imported. Check out the discussion in the post module name in sys.modules and globals().
When reloading MyPak, python tries to execute the line from MyMod import MyMod in MyPak/__init__.py. However, it finds MyPak.MyMod in sys.modules, thus it will NOT reload MyMod although MyPak/MyMod.py has been updated. And you will find that no new MyPak/MyMod.pyc is generated.

On Python 3 only, import the reload function:

>>> from importlib import reload

On both Python 2.x, and 3.x, you can then simply call reload on the module:

>>> import MyPak
>>> reload(MyPak)
>>> from MyPak import MyMod

However, instances of the old class will not be updated (there's simply no code that describes the update mechanism).

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