I’m a complete Python noob. How can I remove two characters from the beginning of each line in a file? I was trying something like this:


import re

f = open('M:/file.txt')

for line in lines:
    line = line.strip()     
    #do something here

You were off to a good start. Try this in your loop:

for line in lines:
    line = line[2:]
    # do something here

The [2:] is called “slice” syntax, it essentially says “give me the part of this sequence which begins at index 2 and continues to the end (since no end point was specified after the colon).

String slicing will help you:

>>> a="Some very long string"
>>> a[2:]
'me very long string'

Just as a tip, you can shorten your program to

for line in open('M:/file.txt'):
    line = line[2:]

And if you need to carry the line number too, use

for i, line in enumerate(open('M:/file.txt.')):
    line = line[2:]

Instead of using a for loop, you might be happier with a a list comprehension:

[line[2:] for line in lines]

Just as a curiosity, do check the cut unix tool.

$ cut -c2- filename

The slicing syntax for -c is quite similar to python’s.

If you want to modify a file’s contents, not just process the string, try fileinput‘s inplace parameter:

# strip_2_chars.py
import fileinput
for line in fileinput.input(inplace=1):
  print line[2:]

Then, on the command line:

python strip_2_chars.py m:\file.txt

It may be interesting to know that there is a subtle, yet important difference between:

file = open( filename )
lines = file.readlines()
for line in lines:
   do something


file = open( filename )
for line in file:
    do something

The first solution (with readlines) will load the whole contents of the file in memory and return a python list (of strings). On the other hand, the second solution makes use of something that’s called a iterator. This will in fact move the pointer in the file as needed and return a string. This has an important benefit: The file is not loaded into memory. For small files, both approaches are OK. But as long as you only work with the file line-by-line, I suggest to use the iterator behavior directly.

So my solution would be:

infile = open( filename )
outfile = open( "%s.new" % filename, "w" )
for line in infile:
    outfile.write( line[2:] )

Come to think of it: If it’s a non-ascii file (for example latin-1 encoded), consider using codecs.open. Otherwise you may have a nasty surprise as you may accidentally cut a multibyte character in half 😉

However, if you don’t need python, and the only thing you need to do is crop the first two characters from the file, then the most efficient way to do it is kch’s suggestion and use cut:

cat filename | cut -d2- > newfile

For these kinds of quick-and-dirty file operations I always have cygwin installed on my non-linux boxen. But I believe there’s a set of windows binaries for these tools as well which would perform faster than in cygwin iirc.

for line in open("file"):
    print line[2:]

line = line[2:]

You’ll find python has some great ways to deal with strings. Some other useful string methods you might want to check out are ones like split(), replace(), and startswith()/endswith().

Just use line[2:]

Since you are learning Python, I would like to add that, given the tools offered by Python (slicing, split, replace and all the other which have been mention), you will find that for many tasks regex are overkill. So the

import re

at the beginning of your example may or may not be strictly needed.

Not very efficient but yes it does work. Looks pretty complex.

print line[-(len(line)-2):]