mmap in python printing binary data instead of text

Question

I am trying to read a big file of 30 MB character by character. I found an interesting article on how to read a big file. Fast Method to Stream Big files

Problem: Output printing binary data instead of actual human readable text

Code:

def getRow(filepath):
   offsets = get_offsets(filepath)
   random.shuffle(offsets)
   with gzip.open(filepath, "r+b") as f:
      i = 0
      mm = mmap.mmap(f.fileno(), 0, access = mmap.ACCESS_READ)
      for position in offsets:
          mm.seek(position)
          record = mm.readline()
          x = record.split(",")
          yield x


def get_offsets(input_filename):
   offsets = []
   with open(input_filename, 'r+b') as f:
       i = 0
       mm = mmap.mmap(f.fileno(), 0, access = mmap.ACCESS_READ)
       for record in iter(mm.readline, ''):
           loc = mm.tell()
           offsets.append(loc)
           i += 1
   return offsets

for line in getRow("hello.dat.gz"):
    print line

Output: The output is producing some weird binary data.

['w\xc1\xd9S\xabP8xy\x8f\xd8\xae\xe3\xd8b&\xb6"\xbeZ\xf3P\xdc\x19&H\\@\x8e\x83\x0b\x81?R\xb0\xf2\xb5\xc1\x88rJ\

Am I doing something terribly stupid?

EDIT:

I found the problem. It is because of gzip.open . Not sure how to get rid of this. Any ideas?

Answer 1

As per the documentation of GZipFile :

fileno(self)

 Invoke the underlying file object's `fileno()` method. 

You are mapping a view of the compressed .gz file, not a view of the compressed data.

mmap() can only operate on OS file handles, it cannot map arbitrary Python file objects.

So no, you cannot transparently map a decompressed view of a compressed file unless this is supported directly by the underlying operating system.