using python to encapsulate part of a string after 3 commas

I am trying to create a python script that adds quotations around part of a string, after 3 commas

So if the input data looks like this:

1234,1,1/1/2010,This is a test. One, two, three.

I want python to convert the string to:

1234,1,1/1/2010,"This is a test. One, two, three."

The quotes will always need to be added after 3 commas

I am using Python 3.1.2 and have the following so far:

i_file=open("input.csv","r")
o_file=open("output.csv","w")

for line in i_file:
        tokens=line.split(",")
        count=0
        new_line=""
        for element in tokens:
                if count = "3":
                        new_line = new_line + '"' + element + '"'
                        break
                else:
                        new_line = new_line + element + ","
                        count=count+1

        o_file.write(new_line + "\n")
        print(line, " -> ", 开发者_JAVA技巧new_line)

i_file.close()
o_file.close()

The script closes immediately when I try to run it and produces no output

Can you see what's wrong?

Thanks

Having addressed the two issues mentioned in my comment above I've just tested that the code below (edit: ALMOST works; see very short code sample below for a fully tested and working version) for your test input.

i_file=open("input.csv","r")
o_file=open("output.csv","w")

for line in i_file:
    tokens=line.split(",")
    count=0
    new_line=""
    for element in tokens:
        if count == 3:
            new_line = new_line + '"' + element + '"'
            break
        else:
            new_line = new_line + element + ","
            count=count+1

    o_file.write(new_line + "\n")
    print(line, " -> ", new_line)

i_file.close()
o_file.close()

Side note:
A relatively new feature in Python is the with statement. Below is an example of how you might take advantage of that more-robust method of coding (note that you don't need to add the close() calls at the end of processing):

with open("input.csv","r") as i_file, open("output.csv","w") as o_file:
    for line in i_file:
        tokens = line.split(",", 3)
        if len(tokens) > 3:
            o_file.write(','.join(tokens[0:3]))
            o_file.write('"{0}"'.format(tokens[-1].rstrip('\n')))

Shorter but untested:

i_file=open("input.csv","r")
o_file=open("output.csv","w")

comma = ','
for line in i_file:
    tokens=line.split(",")
    new_line = comma.join(tokens[:3]+['"'+comma.join(tokens[3:])+'"'])
    o_file.write(new_line+'\n')
    print(line, " -> ", new_line)

i_file.close()
o_file.close()

Perhaps you should consider using a regular expression to do this? Something like

import re
t = "1234,1,1/1/2010,This is a test. One, two, three."
first,rest = re.search(r'([^,]+,[^,]+,[^,]+,)(.*)',t).groups()
op = '%s"%s"'%(first,rest)
print op

1234,1,1/1/2010,"This is a test. One, two, three."

Does this satisfy your requirements?

>>> import re
>>> s
'1234,1,1/1/2010,This is a test. One, two, three.'
>>> re.sub("(.[^,]*,.[^,]*,.[^,]*,)(.*)" , '\\1\"\\2"' , s)
'1234,1,1/1/2010,"This is a test. One, two, three."'

import re
o=open("output.csv","w")
for line in open("input.csv"):
    line=re.sub("(.[^,]*,.[^,]*,.[^,]*,)(.*)" , '\\1\"\\2"' , line)
    o.write(line)
o.close()