How do I write to the middle of a text file while reading its contents?

Question

First of all thanks for helping me out with moving files and for helping me in with tcl script.

Small doubt i had with python code.. as below..

impor         


        

Answer 1

Actually your method works but your fout is open in append mode. So this is why you can only write at the end. Here is an working example.

# creating a file for reference
ff = open("infiletest","w")
pos_file = {}
for i in range(3):
    pos_file[i] = ff.tell()
    ff.write("%s   21/1/1983\n" % i)
ff.close()

# modify the second line
ff = open("infiletest","r+")
ff.seek(pos_file[2])
ff.write("%s   00/0/0000\n" % 2)
ff.close()

Note that that you overwritte the content of the file.

Answer 2

A simple way to add data in the middle of a file is to use fileinput module:

import fileinput

for line in fileinput.input(r'C:\Sanity_Automation\....tcl', inplace=1):
    print line, # preserve old content
    if x in line:
       print data # insert new data

From the fileinput docs:

Optional in-place filtering: if the keyword argument inplace=1 is passed to fileinput.input() or to the FileInput constructor, the file is moved to a backup file and standard output is directed to the input file (if a file of the same name as the backup file already exists, it will be replaced silently). This makes it possible to write a filter that rewrites its input file in place. If the backup parameter is given (typically as backup='.'), it specifies the extension for the backup file, and the backup file remains around; by default, the extension is '.bak' and it is deleted when the output file is closed.

To insert data into filename file while reading it without using fileinput:

import os
from tempfile import NamedTemporaryFile

dirpath = os.path.dirname(filename)
with open(filename) as file, \
     NamedTemporaryFile("w", dir=dirpath, delete=False) as outfile:
    for line in file:
        print >>outfile, line, # copy old content
        if x in line:
           print >>outfile, data # insert new data

os.remove(filename) # rename() doesn't overwrite on Windows
os.rename(outfile.name, filename)

Answer 3

you dont all you can do is read in the file, insert the text you want, and write it back out

with open("some_file","r") as f:
     data = f.read()
some_index_you_want_to_insert_at = 122
some_text_to_insert = "anything goes here"

new_data = data[:some_index_you_want_to_insert_at] + some_text_to_insert + data[some_index_you_want_to_insert_at:]

with open("some_file","w") as f:
     f.write(new_data)

print "all done!"