file tell in a for-loop

[Magdoll]

I was trying to map various locations in a file to a dictionary. At
first I read through the file using a for-loop, but tell() gave back
weird results, so I switched to while, then it worked.

The for-loop version was something like:
d = {}
for line in f:
if line.startswith('>'): d[line] = f.tell()

And the while version was:
d = {}
while 1:
line = f.readline()
if len(line) == 0: break
if line.startswith('>'): d[line] = f.tell()


In the for-loop version, f.tell() would sometimes return the same
result multiple times consecutively, even though the for-loop
apparently progressed the file descriptor. I don't have a clue why
this happened, but I switched to while loop and then it worked.

Does anyone have any ideas as to why this is so?

Thanks,
Magdoll

 

[Justin Ezequiel]

I was trying to map various locations in a file to a dictionary. At
first I read through the file using a for-loop, but tell() gave back
weird results, so I switched to while, then it worked.

The for-loop version was something like:
                d = {}
                for line in f:
                         if line.startswith('>'): d[line] = f.tell()

And the while version was:
                d = {}
                while 1:
                        line = f.readline()
                        if len(line) == 0: break
                        if line.startswith('>'): d[line] = f.tell()

In the for-loop version, f.tell() would sometimes return the same
result multiple times consecutively, even though the for-loop
apparently progressed the file descriptor. I don't have a clue why
this happened, but I switched to while loop and then it worked.

Does anyone have any ideas as to why this is so?

Thanks,
Magdoll

got bitten by that too a while back
the for line in f reads ahead so your f.tell would not be the position
of the end of the line
had to use a while True loop instead also

 

[Tim Chase]

I was trying to map various locations in a file to a dictionary. At
first I read through the file using a for-loop, but tell() gave back
weird results, so I switched to while, then it worked.

The for-loop version was something like:
d = {}
for line in f:
if line.startswith('>'): d[line] = f.tell()

And the while version was:
d = {}
while 1:
line = f.readline()
if len(line) == 0: break
if line.startswith('>'): d[line] = f.tell()


In the for-loop version, f.tell() would sometimes return the same
result multiple times consecutively, even though the for-loop
apparently progressed the file descriptor. I don't have a clue why
this happened, but I switched to while loop and then it worked.

Does anyone have any ideas as to why this is so?

I suspect that at least the iterator version uses internal
buffering, so the tell() call returns the current buffer
read-location, not the current read location. I've also had
problems with tell() returning bogus results while reading
through large non-binary files (in this case about a 530 meg
text-file) once the file-offset passed some point I wasn't able
to identify. It may have to do with newline translation as this
was python2.4 on Win32. Switching to "b"inary mode resolved the
issue for me.

I created the following generator to make my life a little easier:

def offset_iter(fp):
assert 'b' in fp.mode.lower(), \
"offset_iter must have a binary file"
while True:
addr = fp.tell()
line = fp.readline()
if not line: break
yield (addr, line.rstrip('\n\r'))

That way, I can just use

f = file('foo.txt', 'b')
for offset, line in offset_iter(f):
if line.startswith('>'): d[line] = offset

This bookmarks the *beginning* (I think your code notes the
*end*) of each line that starts with ">"

-tkc

 

[Magdoll]

Gotcha. Thanks!

Magdoll

I was trying to map various locations in a file to a dictionary. At
first I read through the file using a for-loop, buttell() gave back
weird results, so I switched to while, then it worked.

The for-loop version was something like:
                d = {}
                for line in f:
                         if line.startswith('>'): d[line] = f.tell()

And the while version was:
                d = {}
                while 1:
                        line = f.readline()
                        if len(line) == 0: break
                        if line.startswith('>'): d[line] = f.tell()

In the for-loop version, f.tell() would sometimes return the same
result multiple times consecutively, even though the for-loop
apparently progressed the file descriptor. I don't have a clue why
this happened, but I switched to while loop and then it worked.

Does anyone have any ideas as to why this is so?

I suspect that at least the iterator version uses internal
buffering, so thetell() call returns the current buffer
read-location, not the current read location.  I've also had
problems withtell() returning bogus results while reading
through large non-binary files (in this case about a 530 meg
text-file) once the file-offset passed some point I wasn't able
to identify.  It may have to do with newline translation as this
was python2.4 on Win32.  Switching to "b"inary mode resolved the
issue for me.

I created the following generator to make my life a little easier:

   def offset_iter(fp):
     assert 'b' in fp.mode.lower(), \
       "offset_iter must have a binary file"
     while True:
       addr = fp.tell()
       line = fp.readline()
       if not line: break
       yield (addr, line.rstrip('\n\r'))

That way, I can just use

   f = file('foo.txt', 'b')
   for offset, line in offset_iter(f):
     if line.startswith('>'): d[line] = offset

This bookmarks the *beginning* (I think your code notes the
*end*) of each line that starts with ">"

-tkc