Insert character at a fixed position of lines

[Peter Otten]

Francesco Pietra wrote:

> How to insert letter "A" on each line (of a very long list of lines)
> at position 22, i.e., one space after "LEU", leaving all other
> characters at the same position as in the original example:
> 
> 
> ATOM      1  N   LEU     1     146.615  40.494 103.776  1.00 73.04      
> 1SG   2
> 
> In all lines"ATOM" is constant as to both position and string, while
> "LEU" is constant as to position only, i.e., "LEU" may be replaced by
> three different uppercase letters. Therefore, the most direct
> indication would be position 22.

You insert a string into another one using slices

line = line[:22] + " " + line[22:]

(Python's strings are immutable, so you are not really modifying the old
string but creating a new one)

> Should the script introduce blank lines, no problem. That I know how
> to correct with a subsequent script.

You are probably printing lines read from a file. These lines already end
with a newline, and print introduces a second one. Use the file's write()
method instead of print to avoid that, e. g.:

import sys

for line in sys.stdin:
    line = line[:22] + " " + line[22:]
    sys.stdout.write(line)

Peter