Reading and writing a file in python

Question

Im trying to read a file line by line. I want to replace key with value if found in the dictionary and write the contents to the new file. Here is the logic:

fout = open(output_file,"w+")

with open(input_file, 'r') as fin:
    for line in fin:
        for key in sorted(Db):
            if re.match(key,line):
                line = re.sub(key,Db[key],line) ## line 246 
                fout.write(line)
                break
            else:
                fout.write(line)

Whenever i try to run this file, I'm getting the following tracebacks:

Traceback (most recent call last):
  File "final.py", line 246, in <module>
    if re.match(key,line):
  File "c:\Python33\lib\re.py", line 156, in match
    return _compile(pattern, flags).match(string)
  File "c:\Python33\lib\functools.py", line 258, in wrapper
    result = user_function(*args, **kwds)
  File "c:\Python33\lib\re.py", line 274, in _compile
    return sre_compile.compile(pattern, flags)
  File "c:\Python33\lib\sre_compile.py", line 493, in compile
    p = sre_parse.parse(p, flags)
  File "c:\Python33\lib\sre_parse.py", line 724, in parse
    p = _parse_sub(source, pattern, 0)
  File "c:\Python33\lib\sre_parse.py", line 347, in _parse_sub
    itemsappend(_parse(source, state))
  File "c:\Python33\lib\sre_parse.py", line 552, in _parse
    raise error("nothing to repeat")
sre_constants.error: nothing to repeat

Kindly let me know if I'm missing something. Thanks in advance.

Thanks, Anand

Answer 1

I think you should try and debug this problem yourself. Here is what I would do. add a print statement in your script before line 246:

print key, 
print Db[key]
print line

Depending on the output, take action. To test what would work, you can use the python interpreter. Assuming you get out of the print above:

key
foo
key 123

you can test it:

line = 'key 123'
re.sub('key', 'foo', line)
'foo 123'

In this case it works. I'm sure you'll soon find out what the problem is. Good luck!