How to import some module from another project

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Python2.7

Project1
----package1
------__init__.py
------module1_1.py
------module1_2.py
----project2
------package2
--------__init__.py
--------module2_1.py
--------module2_2.py

The structure is similar as the above, I want to import module2_1 in module1_1.py, is there any way?

I tried to add __init__.py under project2, it does work for import module1_1 in module 2_1, but in project2, such as in module2_2.py, that is unable to import module2_1. Anyone knows any other way? BTW, my python version is 2.7.