change vars in place w/loop or list comprehension

[Peter Hansen]

ken.boss at dnr.state.mn.us wrote:
> I am a python newbie, and am grappling with a fundamental concept.  I
> want to
> modify a bunch of variables in place.  Consider the following:
> 
[snip]
>>>>a = 'one'
>>>>b = 'two'
>>>>c = 'three'
>>>>[a, b, c] = [s.upper() for s in [a, b, c]]
> 
> Both of these accomplish what I'm after; I prefer the second for its
> brevity.
> But either approach requires that I spell out my list of vars-to-alter
> [a, b, c] twice.  This strikes me as awkward, and would be difficult
> with longer lists. Any suggestions for a better way to do this?

While the real question is why you think you need to modify a bunch of 
variables in place**, here's one solution to the immediate problem:

for name in 'a b c'.split():
     globals()[name] = globals()[name].upper()

That, of course, will work only if they really are globals, and not if 
they are local variables inside a function.  If that's the case, you 
really want to rethink this anyway (or use Raymond's approach, though it 
still appears to require spelling out your list of names twice).

** Why you might not want to do this at all:

It's very rare in real code to need to work with a large number of 
variables as a set like that.  More generally you should probably be 
storing the strings in something like a list and manipulating them 
there.  This would look like this:

shtuff = ['a', 'b', 'c']
for i,item in enumerate(shtuff):
     shtuff[i] = item.upper()

Or, even more likely:

newList = []
for item in shtuff:
     newList.append(item.upper())

shtuff = newList


If you think you really need to do this anyway, consider posting an 
example of real code from the program you are trying to write, 
explaining the actual use case, and we can analyze the real situation 
instead of debating contrived examples.

-Peter