How to solve "OSError: telling position disabled by next() call"

Question

I am creating a file editing system and would like to make a line based tell() function instead of a byte based one. This function would be used inside of a "with loop" with the open(file) call. This function is part of a class that has:

self.f = open(self.file, 'a+')
# self.file is a string that has the filename in it

The following is the original function (It also has a char setting if you wanted line and byte return):

def tell(self, char=False):
    t, lc = self.f.tell(), 0
    self.f.seek(0)
    for line in self.f:
        if t >= len(line):
            t -= len(line)
            lc += 1
        else:
            break
    if char:
        return lc, t
    return lc

The problem I'm having with this is that this returns an OSError and it has to do with how the system is iterating over the file but I don't understand the issue. Thanks to anyone who can help.

Answer 1

I don't know if this was the original error but you can get the same error if you try to call f.tell() inside of a line-by-line iteration of a file like so:

with open(path, "r+") as f:
  for line in f:
    f.tell() #OSError

which can be easily substituted by the following:

with open(path, mode) as f:
  line = f.readline()
  while line:
    f.tell() #returns the location of the next line
    line = f.readline()