python read from byte x to byte y

trscookie · 01-31-2015, 02:48 AM

Hello all,

I have written some code to read bytes from a file one chunk at a time, but I would like to start and end at a particular position, e.g. bytes_from_file(self, chunksize, startPos, endPos), is this possible in python, if so how can it be achieved?

Code:

def bytes_from_file(self, chunksize=8192):
  self.log.message("setting bytes_from_file to chunksize: " + str(chunksize))
  file = open(self.File, "rb")
  
  try:
    bytes_read = file.read(chunksize)
    
    while bytes_read:
      for b in bytes_read:
        yield(b) 
      bytes_read = file.read(chunksize)
  
  finally:
    self.log.message("closing file")
    file.close()

Many thanks,
trscookie

pan64 · 01-31-2015, 03:08 AM

yes, definitely possible, but the function you posted has only to parameters. looks like the first step on your road. Do you know what will that yield command do? How do you want to specify start and end position?

trscookie · 01-31-2015, 03:24 AM

Currently I am using it like so:

Code:

for b in f.bytes_from_file(1024):
  print b

I'm thinking that I will probably want to use fseek to read from byte 'y' and exit return after x amount of chunks, something like:

Code:

for b in f.bytes_from_file(1024, 0, 1024 * 10):
  print b

Code:

def bytes_from_file(self, chunksize=8192, posX, posY):
  self.log.message("setting bytes_from_file to chunksize: " + str(chunksize))
  file = open(self.File, "rb")
  
  try:
    file.seek(posX, 0)
    
    bytes_read = file.read(chunksize)
    
    while bytes_read < posY:
      for b in bytes_read:
        yield(b) 
      bytes_read = file.read(chunksize)
  
  finally:
    self.log.message("closing file")
    file.close()

pan64 · 01-31-2015, 03:38 AM

that looks much better. But I think you need to calculate the length of the result and count the bytes found (the while condition you wrote will not work)

trscookie · 01-31-2015, 04:09 AM

Hum, think I am nearly there, just not quite sure what I am missing:

Code:

def bytes_from_file(self, chunksize=8192, posX=0, count=1):
  self.log.message("file size: " + str(self.ByteSize))
  self.log.message("setting bytes_from_file to chunksize: " + str(chunksize) + " pos: " + str(posX) + " count: " + str(count * chunksize))
  file = open(self.File, "rb")
  
  try:
    file.seek(posX, 0)
    
    bytes_read = file.read(chunksize)
    
    while bytes_read < (chunksize * count):
      for b in bytes_read:
        yield(b)
      bytes_read = file.read(chunksize)
      
  finally:
    self.log.message("closing file")
    file.close()

Code:

 f = bsFile(log, myfile)
 
 if f.exists():
   for b in f.bytes_from_file(1024, 0, 1):
     print b

Log:

Code:

file size: 1631
setting bytes_from_file to chunksize: 1024 pos: 0 count: 1024
closing file

I can get the filesize by using self.bytesize but I should only be fetching the first 1024 bytes :S

Cheers again