Printing to a file from a list of lists in Python

Question

I am trying to print to a file that will look like:

'A'
'1'
'B'
'2'
'C'
'3' 

Given the code below, however, the result is :

['A']
['B']
['C']

This is probably a 'softball' question, but what am I doing wrong here?

l1 = ['1']
l2 = ['A']
l3 = ['2']
l4 = ['B']
l5 = ['3']
l6 = ['C']

listoflists = [l1,l2,l3,l4,l5,l6]
itr = iter(listoflists)

f = open ('order.txt','w')

while True: 
    try:
           itr.next()
           s = str(itr.next())
           f.write(str('\n'))
           f.write(s)

    except StopIteration:
        break
f.close()

Answer 1

Your code could be a lot simpler:

for list in listoflists:
    f.write(str(list))
    f.write('\n')

But, this is going to print something like ['1']. It seems like you want something more like:

for list in listoflists:
    f.write(str(list[0]))
    f.write('\n')

Also, why do you have a bunch of single-element lists? Couldn't you put all the elements into one list?

Answer 2

First of all, don't use iter and next(), that's what for is for. Secondly, you are actually writing a list to the file, not its contents. So you could either print the first element of the list (i.e. l1[0]) or iterate through all the inner lists elements.

Your code should look like this:

l1 = ['1']
l2 = ['A']
l3 = ['2']
l4 = ['B']
l5 = ['3']
l6 = ['C']

listoflists = [l1,l2,l3,l4,l5,l6]

f = open ('order.txt','w')

for inner_list in listoflists:
    for element in inner_list:
        f.write(element+'\n')

f.close()

Answer 3

I think the best way to solve this is just with a basic nested loop. Try this:

l1 = ['1']
l2 = ['A']
l3 = ['2']
l4 = ['B']
l5 = ['3']
l6 = ['C']
listoflists = [l1,l2,l3,l4,l5,l6]

f = open("out.txt","w")

# for each list and
# for each item in the list;
# write the item to the file, separated by a comma
for list in listoflists: 
    for item in list: 
        f.write(item+",") 

f.close()

Out.txt now holds:

1,A,2,B,3,C,

---

Oh, and no Python question is complete without a one-liner solution (this also removes the trailing comma from my initial response).

open("out.txt","w").write(",".join(("".join(i) for i in listoflists)))

Out.txt now holds:

1,A,2,B,3,C

Answer 4

Including the quotes in the output is a bit odd, but if you insist:

for entry in listoflists:
  print >>f, repr(entry[0])

You don't specify what will happen if the inner list does not have just one element, so any other possibility is ignored here.

Answer 5

You can simply iterate through all list elements with itertools.chain (documented here):

import itertools

l1 = ['1']
l2 = ['A']
l3 = ['2']
l4 = ['B']
l5 = ['3']
l6 = ['C']

chainedlists = itertools.chain(l1,l2,l3,l4,l5,l6)

with open ('order.txt','wt') as f:
    for element in chainedlists:
        # Change this how you want it to be formatted, it will output
        # a string "a" as 'a' (with the quotes)
        f.write("%s\n" % repr(element))

Answer 6

The simple reason why you are getting the wrong file contents is because you are calling iter twice. Lines 15-16 are:

itr.next()
s = str(itr.next())

For more Pythonic printing semantics, see the other answers