Eu tenho um arquivo de texto que se parece com isso:

blah blah
foo1 bar1
foo1 bar2
foo1 bar3
foo2 bar4
foo2 bar5
blah blah

Agora quero inserir'foo bar' entre'foo1 bar3' e'foo2 bar4'.

Foi assim que eu fiz:

import shutil

txt = '1.txt'
tmptxt = '1.txt.tmp'

with open(tmptxt, 'w') as outfile:
    with open(txt, 'r') as infile:
        flag = 0
        for line in infile:
            if not line.startswith('foo1') and flag == 0:
                outfile.write(line)
                continue
            if line.startswith('foo1') and flag == 0:
                flag = 1
                outfile.write(line)
                continue
            if line.startswith('foo1') and flag == 1:
                outfile.write(line)
                continue
            if not line.startswith('foo1') and flag == 1:
                outfile.write('foo bar\n')
                outfile.write(line)
                flag = 2
                continue
            if not line.startswith('foo1') and flag == 2:
                outfile.write(line)
                continue

shutil.move(tmptxt, txt)

Isso funciona para mim, mas parece bastante feio.