Attempting to read open file a second time gets no data

Question

fin = open(\'/abc/xyz/test.txt\', \'a+\')

def lst():
  return fin.read().splitlines()

print lst()

def foo(in):
  print lst()
  fin.write(str(len(lst()) + in)
  fi         


        

Answer 1

Once you read a complete file into memory, reading some more from that file will result in an empty string being returned:

>>> example = open('foobar.txt')
>>> example.read()
'Foo Bar\n'
>>> example.read()
''

In other words, you have reached the end of the file. You have three alternatives here:

1. Re-open the file for each complete read.

2. Use .seek() to go to the start of the file again:

   >>> example = open('foobar.txt')
   >>> example.read()
   'Foo Bar\n'
   >>> example.seek(0)
   >>> example.read()
   'Foo Bar\n'
   

3. Store the contents of the file in a variable, thus caching it in memory, then use that instead of re-reading the file.