imports and exec

[Patrick Maupin]

On May 16, 1:51 pm, Paul Carter <pacman... at gmail.com> wrote:
> We are using python for our build system. Each subproject dir has a
> python script that builds it. Parent dirs have python scripts that
> recurse into their children and use exec to invoke the python scripts.
> Recently we discovered that one of the python scripts works when
> invoked directly, but fails when invoked with exec. I've created a
> very simple pair of python scripts that demonstrates the problem.
> (We're using python 2.6)
>
> Can someone explain or point to a good explanation of why this problem
> occurs?
>
> I have two python files: prim.py and sec.py.
> prim.py:
> -----------------------------------------
> #!/usr/bin/env python
>
> ##sec2 = open('sec.py')
> ##exec sec2
>
> def invoke():
>   sec = open('sec.py')
>   exec sec
>
> invoke()
> ------------------------------
>
> and sec.py:
> ----------------------------------
> import sys
>
> def sec():
>   print('Inside sec(): ' +  str(sys.argv))
>
> print('Outside sec(): ' + str(sys.argv))
> sec()
> --------------------------------------
>
> When I run prim.py, I get an error:
> --------------------------------
> Outside sec(): ['./prim.py']
> Traceback (most recent call last):
>   File "./prim.py", line 10, in <module>
>     invoke()
>   File "./prim.py", line 8, in invoke
>     exec sec
>   File "sec.py", line 7, in <module>
>     sec()
>   File "sec.py", line 4, in sec
>     print('Inside sec(): ' +  str(sys.argv))
> NameError: global name 'sys' is not defined
> ----------------------------------
>
> I don't understand why the sys import is not visible in the sec()
> function. I can fix this by adding a:
>   global sys
> or
>   import sys
> inside the sec() function. But I would like to understand why this is
> necessary, especially since sys is visible in sec.py outside of the
> sec() function. I found this page discussing exec
>
> http://docs.python.org/reference/executionmodel.html
>
> but I'm afraid I couldn't quite follow it. It makes me think that a
> free variable is involved, but I don't see how. It did made me try
> invoking exec in prim.py outside of the invoke() function (see the
> commented out code). Everything worked fine when I did that.
>
> Can someone set me straight? Suggestions on avoiding this problem are
> welcome as well!
>
> Thanks in advance.

Yes.  Since you did not pass any dictionaries to your exec of "sec",
sec inherited the globals and locals from the calling function.  So
the import of sys happened in that functions locals (bad practice in
any case) and the globals used by sec() are the globals of your main
module.

Especially if invoked from inside a function, you should always pass a
dict to exec.  Just "exec sec() in {}" will do the trick.  But if you
want to use your caller's dictionaries, you can do "exec sec() in
globals()"  But you should *not* use a functions locals directly if
you are going to modify them (as you are currently doing).

Regards,
Pat