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.

Aya · Accepted Answer · 2013-04-19T16:08:08.193

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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.

edited Apr 19 '13 at 16:08

answered Apr 19 '13 at 15:12

Aya

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The per-process single command was what I was looking for. Does anyone know a way to do this with Windows too? – Paul Draper Apr 19 '13 at 15:47
@PaulDraper If you mean in `cmd.exe`, I think you have to do it in two calls, i.e. `SET PYTHONPATH=blah` then `python blah`. You could wrap it in a batch file, but it would remain set after the batch file terminated, unless you wrapped the batch file with another batch file which used `CALL` to call it, which, IIRC, creates a new environment. – Aya Apr 19 '13 at 16:00
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@PaulDraper actually, there's an easier way - see updated answer. – Aya Apr 19 '13 at 16:08

score 2 · Answer 2 · answered Apr 19 '13 at 15:11

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.

score 1 · Answer 3 · answered Apr 19 '13 at 15:01

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This is what virtualenv is for.

answered Apr 19 '13 at 15:01

Daniel Roseman

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Yes...I looked at that, but I though I must have mistaken about its use. since nobody would require that, just where a simple command line arg could have sufficed... – Paul Draper Apr 19 '13 at 15:36

How to flexibly change PYTHONPATH

3 Answers3