python: combining lists and strings

Question

Here is a problem I seem to be stuck on. I am trying to get python to print every combination of two lists.

# Test use case: This does what I expect:
lista = ['1', '2', '3', '4']
listb = ['a', 'b', 'c']

for x in lista:
  for y in listb:
    print  x, y
##
##  result summary -
## 1 a
## 1 b
## 1 c
## 2 a
## 2 b
## 2 c
## 3 a
## 3 b
## 3 c
## 4 a
## 4 b
## 4 c

# actual use case:
# test files:
##    file_a contents =
##    this is group 1:
##    this is group 2:
##    this is group 3:
##    
##    
##    file_b contents =
##    red,1,1,1
##    blue,2,2,2
##    green,3,3,3
##    yellow,4,4,4
##    
import csv
with open('file_b', 'r') as f:
    reader = csv.reader(f)
    with open('file_a', 'r') as template:
      for line in template:
        for row in reader:
          print line, row[0]

The result starts out like (what I want) above, but it only iterates through the first line of file_a and stops.

Any suggestions? Thoughts on why the behavior is different from case A to B?

I've been trying out itertools also, but it treats each character as an an individual string.

Thanks!!

You have file_a in the inner loop. I think you meant for it to be the outer loop? — user590028
– user590028, Commented Aug 6, 2014 at 21:02
reader is an iterator here, so it'll be exhausted after the first iteration. Write f.seek(0);reader = csv.reader(f) between the two loops. — Ashwini Chaudhary
– Ashwini Chaudhary, Commented Aug 6, 2014 at 21:07

Padraic Cunningham · Accepted Answer · 2014-08-06 21:36:01Z

3

from itertools import product
lista = ['1', '2', '3', '4']
listb = ['a', 'b', 'c']
print (list(product(lista,listb)))

In [8]: lista = ['1', '2', '3', '4']

In [9]: listb = ['a', 'b', 'c']

In [10]: prod = (product(lista,listb))

In [11]: for x,y in prod:
   ....:         print (x,y)
   ....:     
1 a
1 b
1 c
2 a
2 b
2 c
3 a
3 b
3 c
4 a
4 b
4 c

itertools.product will do the work for you.

Cartesian product of input iterables. Equivalent to nested for-loops in a generator expression. For example, product(A, B) returns the same as ((x,y) for x in A for y in B). This is basically the same as doing:

[(x,y) for x in lista for y in listb]

You can put your lines in two lists and do the same:

with open('file_b', 'r') as f,open('file_a', 'r') as template:
    lines = template.readlines()
    lines2 =  list(csv.reader(f))        
    prod = (product(lines,lines2))
    for x,y in prod:
        print (x,y)

edited Aug 6, 2014 at 21:36

answered Aug 6, 2014 at 21:03

Padraic Cunningham

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3 Comments

rev_it_up Over a year ago

Thanks so much for all of the suggestions.

rev_it_up Over a year ago

@Ashwini Chaudhary - your solution works and I did not need to import from another module.

rev_it_up Over a year ago

Padraic Cunningham - very nice solution with product from itertools! I was starting to work in this direction however your solution provided a very powerful answer to the situation at hand.

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