CODEWITHSUNDEEP
pythonfilefile-descriptor
Lin
Lin

Reputation: 1202

File descriptor position

I'm trying to use tell() to show my position on a file descriptor but seems that is always pointing to the wrong place.

On code below that I use to read a data.txt file (below the code), during all running the POS is printed to wrong position (for example -10, 9, ...), the last position printed is 93, completely different that it really is.

Then near the end I print the next 5 bytes, and it shows the line after SPACE. Ok, that is correct since I have read the line already (in fact I also tried to fetch the pos before readline(), then fd.seek it.

But to my surprise if I pos=fd.tell() then fd.seek(pos), it prints exactly what I want (but didn't expect).

How that is happenning?

#!/usr/bin/env python

class space:
    def read(self, filename):
        with open( filename, "r") as fd:
            hdr={}
            while True:
                line = fd.readline().split()
                if line[0] == "SPACE":
                    break
                key=line[0]
                for value in line[1:]:
                    hdr.setdefault(key, []).append(value)

                pos = fd.tell()
                print pos,line

            # Building the header
            self.header = {
                    'x0'          : int(hdr['XAXIS'][0]),
                    'dx'          : int(hdr['XAXIS'][1]),
                    'xmax'        : int(hdr['XAXIS'][2]),
                    'y0'          : int(hdr['YAXIS'][0]),
                    'dy'          : int(hdr['YAXIS'][1]),
                    'ymax'        : int(hdr['YAXIS'][2]),
                    'nobjects'    : int(hdr['NOBJECTS'][0]),
                    'objects'     : hdr['OBJECT']
                    }

            # Probably there is a best way to do this
            self.x0          = self.header['x0']
            self.y0          = self.header['y0']
            self.dx          = self.header['dx']
            self.dy          = self.header['dy']
            self.xmax        = self.header['xmax']
            self.ymax        = self.header['ymax']
            self.nobjects    = self.header['nobjects']
            self.objects     = self.header['objects']

            # Storing the POSition of File Descriptor (just for safety)
            pos = fd.tell()
            print pos

            # Why
            print fd.read(5)    # Gives me 1525

            # While
            fd.seek(pos)
            print fd.read(5)    # Gives me SPACE

            # Didn't the fd position on first fd.read(5) was not pointing to correct
            # place?

if __name__ == "__main__":
    sp=space()
    sp.read("data.txt")

Running the code above returns:

 %./spc.py
-10 ['XAXIS', '1525', '2', '1767']
9 ['YAXIS', '1525', '2', '2011']
21 ['NOBJECTS', '5']
35 ['OBJECT', 'YAXIS']
49 ['OBJECT', 'XAXIS']
64 ['OBJECT', 'XCOORD']
79 ['OBJECT', 'YCOORD']
93 ['OBJECT', 'DEPTH']
114
1525
SPACE

This is the data.txt file

XAXIS 1525 2 1767
YAXIS 1525 2 2011
NOBJECTS 5
OBJECT YAXIS
OBJECT XAXIS
OBJECT XCOORD
OBJECT YCOORD
OBJECT DEPTH
SPACE 29768 s1_0411
1525 1525 125000.01 125000.01 5933.09
1525 1527 125164.05 125000.01 5870.35
1525 1529 125328.09 125000.01 5836.18
1525 1531 125492.13 125000.01 5805.22
1525 1533 125656.17 125000.01 5735.52
1525 1535 125820.21 125000.01 5670.15
1525 1537 125984.26 125000.01 5617.8
1525 1539 126148.30 125000.01 5574
1525 1541 126312.34 125000.01 5538
1525 1543 126476.38 125000.01 5526
1525 1545 126640.42 125000.01 5553
1525 1547 126804.47 125000.01 5574
1525 1549 126968.51 125000.01 5588.17
1525 1551 127132.55 125000.01 5559.29
1525 1553 127296.59 125000.01 5454.46
1525 1555 127460.63 125000.01 5404.4
1525 1557 127624.68 125000.01 5356.67
1525 1559 127788.72 125000.01 5337
1525 1561 127952.76 125000.01 5323.71
1525 1563 128116.80 125000.01 5338.36

Any other help to improve the code, tips, tricks are also welcome.

Upvotes: 2

Views: 673

Answers (1)

StefanS
StefanS

Reputation: 1770

I cannot reproduce your problem by copy and pasting the data file and code. When I run your code (Python 2.7.6 on Ubuntu 14.04), the output is:

18 ['XAXIS', '1525', '2', '1767']
36 ['YAXIS', '1525', '2', '2011']
47 ['NOBJECTS', '5']
60 ['OBJECT', 'YAXIS']
73 ['OBJECT', 'XAXIS']
87 ['OBJECT', 'XCOORD']
101 ['OBJECT', 'YCOORD']
114 ['OBJECT', 'DEPTH']
134
1525 
1525

Exactly as you want it to, I think.

The shebang at the top makes me thing you're on a UNIX machine, as am I. Since your pos is only 114, instead of 134, I'd guess the data.txt was created with Windows line endings, which may be your problem. Maybe try to open the file with 'rb' instead of 'r' or replace the line endings.

The documentation for file.tell mentions a problem when opening files with UNIX-style line endings on Windows, maybe there's a problem the other way around as well.

Upvotes: 1

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