Psycopg2 install with pip works but cannot import module on OS X 10.9

Question

I've installed psycopg2 with

pip install psycopg2

and it worked just fine. The install output had a couple of warning along the lines of

In file included from ./psycopg/psycopg.h:33:
./psycopg/config.h:71:13: warning: unused function 'Dprintf' [-Wunused-function]
static void Dprintf(const char *fmt, ...) {}
            ^
1 warning generated.

but in the end it says

Successfully installed psycopg2

and it also appears when I run pip list.

Now when I try to import it in python I get an error:

$ python
Python 2.7.5 (default, Aug 25 2013, 00:04:04)
[GCC 4.2.1 Compatible Apple LLVM 5.0 (clang-500.0.68)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import psycopg2
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ImportError: No module named psycopg2

Why can't Python import the module if it was successfully installed?

(Python 2.7.5 was installed with Homebrew. psycopg2 was installed with pip.)

Answer 1

OS X comes with Python 2.7.5 already; when you install Python with Homebrew, it puts a newer version in a different place without touching the built-in one. Homebrew's Python also comes with Pip, whereas OS X's doesn't. What's going on here is, you're using Homebrew's pip to install psycopg2, then running OS X's python and trying to import it.

Run /usr/local/bin/python (the full path to Homebrew's Python), and try import psycopg2 from there.

If that works, you need to put /usr/local/bin before /usr/bin in your PATH variable, so that your shell finds Homebrew's Python before the OS X one every time. If you use Bash (the default shell in OS X), you can do this by putting the following in your .bash_profile:

export PATH=/usr/local/bin:$PATH

To make sure that you're running the right Python in scripts, use the following shebang line:

#!/usr/bin/env python

env will search PATH for Python and run the script with the first one it finds, the same as typing python from your shell. Bash scripts and such should inherit the PATH variable and find the right python without changes.

You can also hardcode the path to Homebrew Python in your shebang line (#!/usr/local/bin/python), but this means your script will only work on OS X machines with a Homebrew Python installed, and is best avoided.