The wrong python interpreter is called

Question

I updated my python interpreter, but I think the old one is still called. When I check for the version I get:

$ python -V
Python 3.0.1

But I believe the old interpreter is still being called. When I run the command:

python myProg.py

The script runs properly. But when I invoke it with the command

./myProg.py

I get the error message:

AttributeError: 'str' object has no attribute 'format'

Which apparently is due to the old interpreter being called. How can I fix this? I run Mac OS X 10.5. Has it something to do with the first line:

#!/usr/bin/python

I just started out with python and am not very familiar with interpreted languages, so I am not too sure what is going on.

Answer 1

According to the first line of the script, #!/usr/bin/python, you are calling the Python interpreter at /usr/bin/python (which is most likely the one that ships with Mac OS X). You have to change that path to the path where you installed your Python 3 interpreter (likely /usr/local/bin/python or /opt/local/bin/python); or you can just change that line to read #!/usr/bin/env python, which will call the python listed first in your PATH variable (which seems to be the newer version you installed).

Answer 2

Firstly, the recommended shebang line is:

#!/usr/bin/env python

This will make sure the python interpreter that is invoked when you ./foo.py is the same interpreter that is invoked when you invoke python from the command line.

From your description, I suspect that if you did:

which python

It would not give you /usr/bin/python. It would give you something else, which is where the python 3 interpreter lives. You can either modify your shebang line to the above, or replace the path to the python interpreter with the path returned by which.

Answer 3

Try which python. I will tell you which python interpreter is used in your environment. If it is not /usr/bin/python like in the script, then your suspicion is confirmed.

Answer 4

It's very possibly what you suspect, that the shebang line is calling the older version. Two things you might want to check:

1) what version is the interpreter at /usr/bin/python:

/usr/bin/python -V

2) where is the python 3 interpreter you installed:

which python

If you get the correct one from the command line, then replace your shebang line with this:

#!/usr/bin/env python

Addendum: You could also replace the older version of python with a symlink to python 3, but beware that any major OS X updates (ie: 10.5.6 to 10.5.7) will likely break this:

sudo mv /usr/bin/python /usr/bin/python25
sudo ln -s /path/to/python/3/python /usr/bin/python