imports and exec

[Paul Carter]

On May 16, 4:22 pm, Patrick Maupin <pmau... at gmail.com> wrote:
> 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

Thanks, that helped a lot! I was misunderstanding what exec does by
default.

--
Paul