Python ImportError No module named X

There are sometimes when you want to run a python script from a package directly, and you may encounter “ImportError: No module named X” error. This blog shares why and how to solve it.

Example:

vagrant@stock:/stock/test_python$ tree  .

.

├── p0.py

└── pack1

    ├── __init__.py

    ├── p1.py

    ├── p2.py

vagrant@stock:/stock/test_python$ cat p0.py

print("this is p0")

import pack1.p1

vagrant@stock:/stock/test_python$ cat ./pack1/p1.py

print("this is p1")

import pack1.p2

vagrant@stock:/stock/test_python$ cat ./pack1/p2.py

print("this is p2")

Running p0.py is ok:

vagrant@stock:/stock/test_python$ python ./p0.py

this is p0

this is p1

this is p2

But if you want to run p1.py directly, you will run into the import error:

vagrant@stock:/stock/test_python$ python ./pack1/p1.py

this is p1

Traceback (most recent call last):

  File "./pack1/p1.py", line 7, in <module>

    import pack1.p2

ImportError: No module named 'pack1'

To find out why, we can print python runtime’s path, adding the highlighted into ./pack1/p1.py:

vagrant@stock:/stock/test_python$ cat ./pack1/p1.py

import sys

print('\n'.join(sys.path))

print("this is p1")

And rerun it:

vagrant@stock:/stock/test_python$ python ./pack1/p1.py

/stock/test_python/pack1

/home/vagrant/anaconda3/lib/python3.5

…

this is p1

Traceback (most recent call last):

  File "./pack1/p1.py", line 7, in <module>

    import pack1.p2

ImportError: No module named 'pack1'

You can see Python add the directory where the script being run is under to the runtime paths, there is no /stock/test_python/pack1/pack1/p1.py, and the error is thrown out. 

To solve this problem, export the current directory:

vagrant@stock:/stock/test_python$ export PYTHONPATH='.'

vagrant@stock:/stock/test_python$ python ./pack1/p1.py

/stock/test_python/pack1

/stock/test_python

/home/vagrant/anaconda3/lib/python3.5

…

this is p1

this is p2