Python unable to read lines from text file after read()

Question

I have a problem whereby I am trying to first check a text file for the existence of a known string, and based on this, loop over the file and insert a different line.

For some reason, after calling file.read() to check for the test string, the for loop appears not to work. I have tried calling file.seek(0) to get back to the start of the file, but this has not helped.

My current code is as follows:

try:
  f_old = open(text_file)
  f_new = open(text_file + '.new','w')
except:
  print 'Unable to open text file!'
  logger.info('Unable to open text file, exiting')
  sys.exit()
wroteOut = False

# first check if file contains an test string
if '<dir>' in f_old.read():
  #f_old.seek(0)  # <-- do we need to do this??
  for line in f_old: # loop thru file
    print line
    if '<test string>' in line:
      line = '    <found the test string!>'
    if '<test string2>' in line: 
      line = '  <found test string2!>' 
    f_new.write(line) # write out the line
  wroteOut = True # set flag so we know it worked
f_new.close()
f_old.close()

pls check your indentation it's not possible to read this code correctly — poko
– poko, Commented Nov 17, 2015 at 10:46

Martijn Pieters · Accepted Answer · 2015-11-17 10:46:11Z

5

You already know the answer:

#f_old.seek(0)  # <-- do we need to do this??

Yes, you need to seek back to the start of the file before you can read the contents again.

All file operations work with the current file position. Using file.read() reads all of the file, leaving the current position set to the end of the file. If you wanted to re-read data from the start of the file, a file.seek(0) call is required. The alternatives are to:

Not read the file again, you just read all of the data, so use that information instead. File operations are slow, using the same data from memory is much, much faster:
```
contents = f_old.read()
if '<dir>' in contents:
for line in contents.splitlines():
    # ....
```
Re-open the file. Opening a file in read mode puts the current file position back at the start.

edited Nov 17, 2015 at 10:46

answered Nov 17, 2015 at 10:44

Martijn Pieters

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2 Comments

btongeorge Over a year ago

This looks good. If I want to loop over contents again, do I need to do some equivalent of .seek(0)?

Martijn Pieters Over a year ago

@btongeorge: just reuse contents. It is a string with the contents of the file.

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