Develop Reference

Using PYTHONPATH to use a virtualenv

I have a virtualenv in a structure like this:
venv/
src/
project_files
I want to run a makefile (which calls out to Python) in the project_files, but I want to run it from a virtual environment. Because of the way my deployment orchestration works, I can't simply do a source venv/bin/activate.
Instead, I've tried to export PYTHONPATH={project_path}/venv/bin/python2.7. When I try to run the makefile, however, the python scripts aren't finding the dependencies installed in the virtualenv. Am I missing something obvious?

The PYTHONPATH environmenbt variable is not used to select the path of the Python executable - which executable is selected depends, as in all other cases, on the shell's PATH environment variable. PYTHONPATH is used to augment the search list of directories (sys.path in Python) in which Python will look for modules to satisfy imports.
Since the interpreter puts certain directories on sys.path before it actions PYTHONPATH precisely to ensure that replacement modules with standard names do not shadow the standard library names. So any standard library module will be imported from the library associated with the interpreter it was installed with (unless you do some manual furkling, which I wouldn't recommend).
venv/bin/activate does a lot of stuff that needs to be handled in the calling shell's namespace, which can make tailoring code rather difficult if you can't find a way to source the script..

You can actually just call the Python interpreter in your virtual environment. So, in your Makefile, instead of calling python, call venv/bin/python.

To run a command in a virtualenv, you could use vex utility:
$ vex venv make
You could also check, whether make PYTHON=venv/bin/python would be enough in your case.
PYTHONPATH adjusts sys.path list. It doesn't change python binary. Don't use it here.

What is the purpose of the environmental variable PYTHONPATH

On windows 7, I currently don't have a python path. Can I safely make one? If so, how do I do it?
Upon making this variable, I can no longer load Spyder (IDE) without it crashing. Does anyone know why?
I would like to edit my existing python path if possible, but just don't know why it isn't already there in environmental variables.
I would ultimately like to be able to run "python myscript.py" and have myscript be in a different directory from the call directory.

PYTHONPATH adds new paths to the ones Python uses by default. The path in total determines where Python will look for modules when you import them.
Look at sys.path to see the combination of the defaults with your PYTHONPATH environment variable. It's likely that Spyder is loading a module that exists in two different places and the wrong one comes first.

When you import modules in python, python searches for the module in the directories in PYTHONPATH, in addition to some other directories.
In order to be able to run your script as > myscript.py, you want to put your script somewhere on PATH (here are some instructions for viewing or updating PATH), this is where the OS looks for scripts and programs when you give it a command. I believe that in windows the .py extension must be associated with python for windows to know that myscript.py should be run using python. This should happen automatically when python in installed, but maybe someone with more windows knowledge can comment on this.

it has role similar to path. this variable tells the python interpreter where to
locate the module files imported into a program. it should include the python source library directory and the directories contain in python source code

How to import modules from alternate locations when using Python IDLE?

I've been trying to figure this out for more than 2 days, screening the internet and the tutorial, but yet I don't have solved my problem. I'm a real newb and don't yet really know what I'm doing..
Software I use:
Mac OS X 10.6
Python v3.2.2
Interactive interpreter (IDLE)
Problem:
IDLE's default directory is /Users/ME/Documents/. Files with the extention .py can only be opened when located in this directory. However, I made a folder where I would like to save all the .py files etc that have to do with this software. Currently, IDLE cannot load .py files from the chosen directory by me.
What I did first was I added to IDLE:
import sys.
sys.path.append('Users/Mydir/')
sys.path
However, in an already existing thread from 2010 I read sys.path is for the Interpreter ONLY, and that if I am to change this I need to modify the PYTHONPATH environment variable:
PYTHONPATH="/Me/Documents/mydir:$PYTHONPATH"
export PYTHONPATH
However, I'm confused how to use this and cannot find answers to my following questions:
1) PYTHONPATH (.py?) is already existing on my computer when I installed the program?
If YES, where is it? I cannot find it anywhere.
If NO, I need to create one. But where and what should be the content so that IDLE can load files from a non-default directory? Should it contain only the words in bold?
I hope I made my problem clear.
Cheers

It's not totally clear to me what you mean by load. That could mean Open and Close files in the IDLE editor. Or it could mean being able to use the Python import statement to load existing Python modules from other files. I'll assume the latter, that by load you mean import.
There are two general ways to launch IDLE on Mac OS X. One is from the command line of a terminal session; if you installed Python 3.2 using the python.org installers, by default typing /usr/local/bin/idle3.2 will work. The other way is by launching IDLE.app from /Applications/Python 3.2, i.e. by double-clicking its icon. Because you say the default directory for files is your Documents folder, I'm assuming you are using the second method because IDLE.app sets Documents as its current working directory, which becomes the default directory for *Open*s and *Save*s and is automatically added as the first directory on Python's sys.path, the list of directories that Python uses to search for modules when importing.
If you want to add other directories to sys.path, as you've noted you can use the PYTHONPATH environment variable to do so. The standard way to do this is to add an export PYTHONPATH=... definition to a shell startup script, like .bash_profile. However, if you use IDLE.app, no shell is involved so commands in .bash_profile have no effect.
While there are ways to modify the environment variables for OS X GUI apps, in this case, a simpler solution is to use the other method to invoke IDLE, from the command line of a shell session, using either /usr/local/bin/idle3.2 or, if you've run the Update Shell Profile command in the /Applications/Python 3.2 folder (and opened a new terminal session), just idle3. Then, a PYTHONPATH environment variable you set up will be inherited by that IDLE.
BTW, there is no direct way to modify the initial current working directory of IDLE.app from Documents other than modifying the code in IDLE. If you start IDLE from a command
line, it inherits the current working directory of the shell.
[UPDATE] But rather than fooling around with defining PYTHONPATH, here is another even simpler, and probably better, approach that should work with either IDLE.app or the command line idle. It takes advantage of Python path configuration (.pth) files and user site-package directories. Assuming you are using a standard Python framework build of 3.2 (like from a python.org installer) on Mac OS X, create a path file for the directory you want to permanently add to sys.path. In a terminal session:
mkdir -p ~/Library/Python/3.2/lib/python/site-packages
cd ~/Library/Python/3.2/lib/python/site-packages
cat >my_paths.pth <<EOF
/Users/YOUR_USER_NAME/path/to/your_additional_python_directory_1
/Users/YOUR_USER_NAME/path/to/your_additional_python_directory_2
EOF
Now, whenever you run that Python 3.2 or IDLE under your user name, the directories you have added to the .pth file will automatically be added to sys.path.
BTW, the exact path location of the user site-packages directory for versions of Python earlier than 3.2 or 2.7 may be slightly different. Also, on other Unix-y systems, the default location for the user site-package directory is ~/.local/lib/python3.2/site-packages.

PYTHONPATH is an environment variable (see here and here). I don't have a Mac, but from the threads I have linked to you would type something like
launchctl setenv PYTHONPATH=/Me/Documents/mydir:$PYTHONPATH
on the command line to allow you to run Python scripts from /Me/Documents/mydir. Alternatively, put this line in a file called .bashrc in your home directory (~) and this path will be set each time each time you open a terminal. See here for a short introduction to .bashrc and other .bash* files. Hope that helps.
EDIT See also this question.

Python Windows: correct virtualenv paths

I'm new to virtualenv and not sure how to set up paths. My paths have been set to something like this:
PYTHONPATH=C:\Python27\
PYTHONSTARTUP=C:\Python27\Scripts\startup.py
PATH=%PYTHONPATH%;...;%PYTHONPATH%\Scripts
Should I remove those paths for virtualenv's activate script to work correctly? If I can keep my paths then how do I call scripts for an env when it has been activated? Do I call scripts by explicitly running them with python.exe instead of simply typing the script name alone?
python myscript.py
Not sure how to handle the paths and I would appreciate a little guidance.

First, you have your paths wrong. PYTHONPATH tells Python in what folders to look for Python modules and normally you don't put Python's installation folder in it. For keeping installation folder of Python there's different environment variable called PYTHONHOME. So instead of PYTHONPATH=C:\Python27\ you should have PYTHONHOME=C:\Python27\. You should change PATH variable to use PYTHONHOME accordingly.
As to how to set environment variables when working with virtualenv; you don't need to do anything because virtualenv stores original values when it's activated, modifies environment variables it needs to modify and then restores original values when it's deactivated.
You can take a look at Using Python on Windows

Think you are fine just get on with virtual-env, (follow docs) but remember you must use cmd shell (NO POINT AND CLICKING!!) Took me a while before I realized that...
Once you have activated And installed what you want to in the virtual env,, you invoke scripts by "python scriptname"

Noob Question: How to import some imodule in Python?

I am using Python 2.5 on CentOS 5.5
I have got a file called MultipartPostHandler.py, I am supposed to use it like this:
import MultipartPostHandler
But I dont know where should I put the file MultipartPostHandler.py so I can use it. Thanks a lot!

http://docs.python.org/tutorial/modules.html#the-module-search-path
When a module named spam is imported,
the interpreter searches for a file
named spam.py in the current
directory, and then in the list of
directories specified by the
environment variable PYTHONPATH. This
has the same syntax as the shell
variable PATH, that is, a list of
directory names. When PYTHONPATH is
not set, or when the file is not found
there, the search continues in an
installation-dependent default path;
on Unix, this is usually
.:/usr/local/lib/python.
Actually, modules are searched in the
list of directories given by the
variable sys.path which is initialized
from the directory containing the
input script (or the current
directory), PYTHONPATH and the
installation- dependent default. This
allows Python programs that know what
they’re doing to modify or replace the
module search path.
So, you have to put it into the current directory or use sys.path to show your program where the searched module is.

The easiest is to put it in the same folder as the code you're writing.

If you got it from somebody who provides an installer, then to be safe, just run the installer.
If it's just something you grabbed specifically for this project, put it in the same folder as the .py files that you're writing.
Otherwise (if you're planning to use it for a few projects and don't want to copy/paste the file), the safest place to put it is probably in the /lib/site-packages sub-directory of the directory where Python is installed.

The easiest - yes, is to put it in the same directory where you have the importer python code. Also, you can export PYTHONPATH environment variable which points to the directory (or directories separated by :) where you have MultipartPostHandler.py. Another option: make it distributable. This will be useful if you are going to publish your code.