Replacing a line in a file using Python

Question

I want to replace a line in my file with new line(actually I have to insert some content in the line). The line actually contains AC_CONFIG_FILES([]) I have to replace this line with a new line by adding some makefile arguments based on some list. Then I constructed a new string and did replacement in the file. Is there any efficient way of doing this?

# 'subdirs' is the list which contains makefile arguments
rstring = "AC_CONFIG_FILES([Makefile"
for t in subdirs:
    rstring = rstring+' src/'+t+'/Makefile'
rstring += '])'
print rstring
# 'fname' is the file in which replacement have to be done
#  i is used for indexing in 'insert' function
# 'rline' is the modified line
fname = 'configure.ac'
i = 0
with open(fname,'r') as f:
      modlines=[]
      for line in f.readlines():
          if 'AC_CONFIG_FILES' in line:
                  modlines.insert(i,rstring+'\n')
                  i = i+1
                  continue
          modlines.insert(i,line)
          i = i+1
with open(fname,'w') as out:
      for i in range(len(modlines)):
          out.write(modlines[i])

Side note: your code formatting could usefully be improved: I suggest that you read and apply PEP 8. Furthermore, be careful not to write in Python like you would write in C: Python code is simpler than the code in this question; for instance, you often don't need indexes (see Johannes Charra's answer). — Eric O. Lebigot, Sep 17 '13 at 07:56

score 4 · Accepted Answer · edited Sep 17 '13 at 07:58

4

You could do this without loop/counter variables

modlines = []
with open(fname) as f:
    for line in f:
        if 'AC_CONFIG_FILES' in line:
            modlines.append(rstring + '\n')
        else:
            modlines.append(line)

with open(fname, 'w') as out:
    for line in modlines:
        out.write(line)

edited Sep 17 '13 at 07:58

Eric O. Lebigot

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answered Sep 17 '13 at 06:58

Johannes Charra

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You could replace your second `for` with `out.write(''.join(modlines))`. – ilent2 Sep 17 '13 at 07:09
1

That takes a lot more memory. – simonzack Sep 17 '13 at 07:30
2

Why the down vote? This solution looks very good to me. – Eric O. Lebigot Sep 17 '13 at 07:58
down vote to discourage people from answering duplicate questions answered hundreds of times already. – simonzack Sep 17 '13 at 09:11

Replacing a line in a file using Python

1 Answers1