[Tutor] Question on lists

[John Miller]

On Friday, September 12, 2003, at 03:45 PM, "james roy"  
<james_royus at eudoramail.com> wrote:
> Hello,
> I am new to programming with python. I dont know I am having my  
> problem like this:
> I am having a few lists that i know in no dynamically.
> For example:
> List1 = ['1','2','3']
> List2 = ['/']
> List3 = ['4','5','6']
> I want to make the combinations like this:
>
> '1' / '4'
> '1' / '5'
> '1' / '6'
> '2' / '4'
> '2' / '5'
> '2' / '6'
> '3' / '4'
> '3' / '5'
> '3' / '6'
>
> I want to genearlise this as:
> I can have 'n' no of elements in 'N' no of lists and i want to make  
> the combinations like i have written.

I found this to be an intriguing problem. I, too, am having trouble  
generalizing a solution. *IF* I know ahead of time how many lists there  
will be, I can simply nest a bunch of for loops like so:

'''
all_lists=[['a','b','c'],['*','/'],[1,2,3,4,5],['+','- 
'],['w','x','y','z']]
P = [] #all the resulting permutations
for h in all_lists[0]:
     for i in all_lists[1]:
         for j in all_lists[2]:
             for k in all_lists[3]:
                 for l in all_lists[4]:
                     P.append((h,i,j,k,l))
print P
'''

But the question was 'N' number of lists. Can I somehow count the  
number of lists and use that to construct on the fly 'N' for loops that  
are properly indented? Is this a case where you need to use something  
like 'exec'? Hmmm, (coding out loud):

'''
all_lists=[['a','b','c'],['*','/'],[1,2,3,4,5],['+','- 
'],['w','x','y','z']]
#this 'var_dict' is cheating; I'm assuming a number of variables to  
cover
#the largest possible number of lists could be constructed;
#other approaches?
var_dict = {0:'h',1:'i',2:'j',3:'k',4:'l'}
codelines = {} #a dictionary of lines of code keyed by 'line number'
j = 0 #to keep track of the number of lines to add the final line  
outside the loop
P = [] #all the resulting permutations
for i in range(len(all_lists)):
     codelines[i] = '%sfor %s in all_lists[%s]:' % ('    ' * i,  
var_dict[i], i)
     j += 1
codelines[j]='%sP.append((h,i,j,k,l))' % ('    ' * j)
codestrings = []
for k in range(len(codelines)):
     codestrings.append(codelines[k])
code = '\n'.join(codestrings)
exec code
print P
'''
The problem is, this doesn't quite work (although I think it's close).  
So, basically, I have two questions:

1. Is this (using exec) the right approach to the OP's question?
2. Can anyone see how to make 'exec code' work?

Thanks!

John Miller