There is hurry.filesize that will take the size in bytes and make a nice string out if it.

>>> from hurry.filesize import size
>>> size(11000)
'10K'
>>> size(198283722)
'189M'

Or if you want 1K == 1000 (which is what most users assume):

>>> from hurry.filesize import size, si
>>> size(11000, system=si)
'11K'
>>> size(198283722, system=si)
'198M'

It has IEC support as well (but that wasn't documented):

>>> from hurry.filesize import size, iec
>>> size(11000, system=iec)
'10Ki'
>>> size(198283722, system=iec)
'189Mi'

Because it's written by the Awesome Martijn Faassen, the code is small, clear and extensible. Writing your own systems is dead easy.

Here is one:

mysystem = [
    (1024 ** 5, ' Megamanys'),
    (1024 ** 4, ' Lotses'),
    (1024 ** 3, ' Tons'), 
    (1024 ** 2, ' Heaps'), 
    (1024 ** 1, ' Bunches'),
    (1024 ** 0, ' Thingies'),
    ]

Used like so:

>>> from hurry.filesize import size
>>> size(11000, system=mysystem)
'10 Bunches'
>>> size(198283722, system=mysystem)
'189 Heaps'

def humanbytes(B):
    """Return the given bytes as a human friendly KB, MB, GB, or TB string."""
    B = float(B)
    KB = float(1024)
    MB = float(KB ** 2) # 1,048,576
    GB = float(KB ** 3) # 1,073,741,824
    TB = float(KB ** 4) # 1,099,511,627,776

    if B < KB:
        return '{0} {1}'.format(B,'Bytes' if 0 == B > 1 else 'Byte')
    elif KB <= B < MB:
        return '{0:.2f} KB'.format(B / KB)
    elif MB <= B < GB:
        return '{0:.2f} MB'.format(B / MB)
    elif GB <= B < TB:
        return '{0:.2f} GB'.format(B / GB)
    elif TB <= B:
        return '{0:.2f} TB'.format(B / TB)


tests = [1, 1024, 500000, 1048576, 50000000, 1073741824, 5000000000, 1099511627776, 5000000000000]

for t in tests: print("{0} == {1}".format(t,humanbytes(t)))

Output:

1 == 1.0 Byte
1024 == 1.00 KB
500000 == 488.28 KB
1048576 == 1.00 MB
50000000 == 47.68 MB
1073741824 == 1.00 GB
5000000000 == 4.66 GB
1099511627776 == 1.00 TB
5000000000000 == 4.55 TB

and for future me here it is in Perl too:

sub humanbytes {
   my $B = shift;
   my $KB = 1024;
   my KB ** 2; # 1,048,576
   my KB ** 3; # 1,073,741,824
   my KB ** 4; # 1,099,511,627,776

   if (KB) {
      return "$B " . ((B > 1) ? 'Bytes' : 'Byte');
   } elsif (KB && MB) {
      return sprintf('%0.02f',KB) . ' KB';
   } elsif (MB && GB) {
      return sprintf('%0.02f',MB) . ' MB';
   } elsif (GB && TB) {
      return sprintf('%0.02f',GB) . ' GB';
   } elsif (TB) {
      return sprintf('%0.02f',TB) . ' TB';
   }
}