go to specific line in text file

[Patrick David]

Hello NG,

I am searching for a way to jump to a specific line in a text file, let's
say to line no. 9000.
Is there any method like file.seek() which leads me to a given line instead
of a given byte?

Hope for help
Patrick

 

[John Machin]

Hello NG,

I am searching for a way to jump to a specific line in a text file, let's
say to line no. 9000.
Is there any method like file.seek() which leads me to a given line instead
of a given byte?

If by "jump" you mean without reading the preceding 8999 lines, and by
"text file" you mean variable length records without a separate index
structure, that method hasn't been implemented yet. AFAIK it hasn't
been implemented in any other language either

The linecache module may be what you want, depending on how closely
your access patterns match those the module was designed for.

Cheers,
John

 

[Hrvoje Niksic]

I am searching for a way to jump to a specific line in a text file,
let's say to line no. 9000. Is there any method like file.seek()
which leads me to a given line instead of a given byte?

You can simulate it fairly easily, but it will internally read the
file line by line and will take the time roughly proportional to the
size of the file.

from itertools import islice
def seek_to_line(f, n):
for ignored_line in islice(f, n - 1):
pass # skip n-1 lines

f = open('foo')
seek_to_line(f, 9000) # seek to line 9000

# print lines 9000 and later
for line in f:
print line

 

[John Machin]

You can simulate it fairly easily, but it will internally read the
file line by line and will take the time roughly proportional to the
size of the file.

from itertools import islice
def seek_to_line(f, n):

The OP gave no impression that he'd restrict himself to one
seek_to_line call per open. Perhaps you need
f.seek(0)
here otherwise
seek_to_line(f, 20)
seek_to_line(f, 10)
will give something unexpected (like jumping forwards instead of
backwards).

 

[Jonathan Gardner]

I am searching for a way to jump to a specific line in a text file, let's
say to line no. 9000.
Is there any method like file.seek() which leads me to a given line instead
of a given byte?

As others have said, no. But if you're wondering how other file
formats do this, like BDB or PostgreSQL data files, which absolutely
have to jump to the magical spot in the file where the data is, they
keep an index at the beginning of the file that points to what byte
the data they are looking for is in.

So if it's really important to be able to do this, consider that.