I am trying to import a module from a particular directory.

The problem is that if I use sys.path.append(mod_directory) to append the path and then open the python interpreter, the directory mod_directory gets added to the end of the list sys.path. If I export the PYTHONPATH variable before opening the python interpreter, the directory gets added to the start of the list. In the latter case I can import the module but in the former, I cannot.

Can somebody explain why this is happening and give me a solution to add the mod_directory to the start, inside a python script ?

This is working as documented. Any paths specified in PYTHONPATH are documented as normally coming after the working directory but before the standard interpreter-supplied paths. sys.path.append() appends to the existing path. See here and here. If you want a particular directory to come first, simply insert it at the head of sys.path:

import sys
sys.path.insert(0,'/path/to/mod_directory')

That said, there are usually better ways to manage imports than either using PYTHONPATH or manipulating sys.path directly. See, for example, the answers to this question.

You could use:

import os
path = 'the path you want'
os.environ['PATH'] += ':'+path

As to me, i need to caffe to my python path. I can add it’s path to the file /home/xy/.bashrc by add

export PYTHONPATH=/home/xy/caffe-master/python:$PYTHONPATH.

to my /home/xy/.bashrc file.

But when I use pycharm, the path is still not in.

So I can add path to PYTHONPATH variable, by run -> edit Configuration.

Temporarily changing dirs works well for importing:

cwd = os.getcwd()
os.chdir(<module_path>)
import <module>
os.chdir(cwd)

When running a Python script from Powershell under Windows, this should work:

$pathToSourceRoot = "C:/Users/Steve/YourCode"
$env:PYTHONPATH = "$($pathToSourceRoot);$($pathToSourceRoot)/subdirs_if_required"

# Now run the actual script
python your_script.py