Write list items to a text file in Python

Question

I have a - probably - trivial question that's been bugging me for some time and I still haven't found an answer. Many of my scripts read some files, compare values from them and save these in a list or dictionary from which I then write an output file of some sort. What I always do is, I loop over the list and write the single items to my output, separated by a tab, comma, or line break. What I am always wondering is how I can prevent a separator appearing after the last item on my list has been printed.

Here is an example:

dict1 = {a: [1,2,3], b: [4,5,6], c: [7,8,9]}
for key in dict1:
    outfile.write(key +": ")
    for item in dict1[key]:
        outfile.write(str(item) +", ")
    outfile.write("\n")

The output would then look like this:

a: 1, 2, 3, 
b: 4, 5, 6, 
c: 7, 8, 9,

How do I avoid that last ", "?

wim · Accepted Answer · 2013-03-04 13:59:48Z

3

Use the str.join method and a list comprehesion:

for key, value in dict1.iteritems():
    outfile.write(key +": ")
    outfile.write(', '.join([str(item) for item in value]))
    outfile.write("\n")

answered Mar 4, 2013 at 13:59

wim

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4 Comments

Jon Clements Over a year ago

This works, but what may come as a surprise to the OP is that it won't necessarily be in key order... I get a, c, b for instance... (however they didn't specify it was a definite requirement)

Lilith-Elina Over a year ago

@JonClements Thanks for that remark. Does that happen because of the list comprehension or simply because dictionaries don't preserve order?

msvalkon Over a year ago

Might even go with outfile.write("%s: %s\n" % (key, ", ".join((str(item) for item in value))))

Lilith-Elina Over a year ago

@JonClements They are never in the order I expect, anyway. ;-)

redShadow · Accepted Answer · 2013-03-04 14:02:44Z

1

What about using the standard-library CSV module?

It will handle that, plus any necessary escapint (what if one of your values includes a comma?)

(Then, of course, as general case, use string.join(list) to properly join without the trailing separator)

answered Mar 4, 2013 at 14:02

redShadow

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1 Comment

Lilith-Elina Over a year ago

I didn't think of that because I rarely need CSV format. I will have a look. :-)

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