How to flexibly change PYTHONPATH

Question

To specify the classpath in Java, I use the -cp or -classpath option to java. What is the equivalent option in Python?

I know I can set the OS variable PYTHONPATH but there shouldn't be one PYTHONPATH to rule them all.

I sometimes use PyDev in Eclipse. It can handle multiple source directories. How?

I often have multiple source directories. Sometimes I separate production and testing code. Sometimes I have a Git submodule with with some Python packages.

Answer 1

To specify the classpath in Java, I use the -cp or -classpath option to java. What is the equivalent option in Python?

Well, there's no "equivalent option" in Python as far as I'm aware, but any Unix-like shell will let you set/override it on a per-process basis, if you were to run Python like this...

$ PYTHONPATH=/put/path/here python myscript.py

...a syntax which you could also use for Java with...

$ CLASSPATH=/put/path/here java MyMainClass

The closest Windows equivalent to this would be...

> cmd /c "set PYTHONPATH=\put\path\here && python myscript.py"

...if you don't want the environment variable to be set in the calling cmd.exe.

I sometimes use PyDev in Eclipse. It can handle multiple source directories. How?

When running code, it probably does something similar by setting the variable in the execve(2) call.

Answer 2

To do this programmatically, you use the following code:

import sys
sys.path.append('directory')

If necessary you could specify the directory to append from a command line argument.

Depending on what exactly your aims are, this might not be the best solution, but for small one-off kinds of issues, it works alright.

Answer 3

This is what virtualenv is for.