Python can't find module NLTK

Question

I followed these instructions http://www.nltk.org/install.html to install nltk module on my mac (10.6) I have installed python 2.7, but when I open IDLE and type import nltk it gives me this error

Traceback (most recent call last):
  File "<pyshell#0>", line 1, in <module>
    import nltk
ImportError: No module named nltk

The problem is the module is installed in another python version, 2.6. How can I install the package in python version 2.7? I tried some of the solutions suggested in various answers, for example I tried typing this in the terminal

export PYTHONPATH=$PYTHONPATH:/usr/local/lib/python2.7/site-packages

and then installed NLTK again with the command

sudo pip install -U nltk

but I get the message: Requirement already up-to-date in /Library/Python/2.6/. So apparently the command line export PYTHONPATH didn't do anything (it still tries to install the package in 2.6) OR (more likely) I didn't understand the meaning/functioning of that command line. What am I doing wrong?

Answer 1

On OS X you could have multiple installation of Python, so investigate it first:

$ which python python2 python3
/usr/bin/python
/usr/local/bin/python3

$ which pip pip2 pip3
/usr/local/bin/pip
/usr/local/bin/pip2
/usr/local/bin/pip3

All within /usr/bin are built-in and all other in /usr/local/bin are external installed by Homebrew or some other package manager.

If you're using pip or pip3 from /usr/local, then you've to use the same Python instance, otherwise they're different instances.

Just install it via pip:

pip install nltk

or for Python 3:

pip3 install nltk

then run the right Python instance from /usr/local/bin or update your PATH system variable.