Altering my python path: helloworld.py returns command not found—

Question

Massive apologies for this embarrassing question—

I'm using my MacBook Pro, running snow leopard, and using Python 2.7.1. Trying to run my first script and all the first pages of all my tutorials are laughing at me:

Let me preface with:

$ whereis python  
/usr/bin/python  
$ which python
/Library/Frameworks/Python.framework/Versions/2.7/bin/python

(Is this my issue?)

I wrote helloworld.py to /users/charles in vim:

$ vim helloworld.py  
#!/usr/bin/python  
# Hello World Python Program  

print "Hello World!";

When trying to run it from terminal:

$ helloworld.py
-bash: helloworld.py: command not found

When trying to run it from python:

$ python
>>> helloworld.py
Traceback (most recent call last):
  File :<stdin>", line 1, in <module>
NameError: name 'helloworld' is not defined

From Dive Into Python (not sure if this is pertinent):

$ python
>>> import sys,os
>>> print 'sys.argv[0] =',sys.argv[0]
sys.argv[0]=
>>> pathname=os.path.dirname(sys.argv[0])
>>> print 'path=',pathname
path=
>>> print 'full path=',os.path.abspath(pathname)
full path= /Users/charles

I'm befuddled! Do I need to alter one of my paths so it finds my script?

I'm absolutely new to programming, I actually just found out that terminal was something you could use.

Thanks!

Answer 1

Let's start with the first error you received. Understanding error messages is important.

-bash: helloworld.py: command not found

This indicates that helloworld.py is not a command that can be executed. To run the file, you then have two options:

1. Run it using the python interpreter. python helloworld.py
2. Make the file executable and then run it directly. ./helloworld.py

To make files executable in a *nix environment, you have to change their mode to allow execution. In order to do this, you use the chmod command (man chmod for more info).

chmod +x helloworld.py

This assumes that you are in the directory containing the helloworld.py file. If not, cd there first or use the full path.

The ./ is necessary because it tells the shell to run the file located here, not by looking in $PATH. $PATH is a list of possible executable locations. When you try to run helloworld.py directly, the shell tries to look for it in $PATH. You want to run the local file, so you have to prefix it with ./, which means "from here".

As an aside, note the first line of your python script:

#!/usr/bin/python

This is called a shebang line and tells system to use the /usr/bin/python executable to load the file. Internally, that means that the program loader will be doing /user/bin/python helloworld.py.

Finally, when you called python with no arguments, you were dropped into an interactive Python interpreter session. >>> helloworld.py in this environment is not referencing the file of that name, it's just interpreted as python code. Invalid python code. This is why you get your second error, NameError: name 'helloworld' is not defined.