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```import math
import string

class size( long ):
""" define a size class to allow custom formatting
format specifiers supported :
em : formats the size as bits in IEC format i.e. 1024 bits (128 bytes) = 1Kib
eM : formats the size as Bytes in IEC format i.e. 1024 bytes = 1KiB
sm : formats the size as bits in SI format i.e. 1000 bits = 1kb
sM : formats the size as bytes in SI format i.e. 1000 bytes = 1KB
cm : format the size as bit in the common format i.e. 1024 bits (128 bytes) = 1Kb
cM : format the size as bytes in the common format i.e. 1024 bytes = 1KB
"""
def __format__(self, fmt):
# is it an empty format or not a special format for the size class
if fmt == "" or fmt[-2:].lower() not in ["em","sm","cm"]:
if fmt[-1].lower() in ['b','c','d','o','x','n','e','f','g','%']:
# Numeric format.
return long(self).__format__(fmt)
else:
return str(self).__format__(fmt)

# work out the scale, suffix and base
factor, suffix = (8, "b") if fmt[-1] in string.lowercase else (1,"B")
base = 1024 if fmt[-2] in ["e","c"] else 1000

# Add the i for the IEC format
suffix = "i"+ suffix if fmt[-2] == "e" else suffix

mult = ["","K","M","G","T","P"]

val = float(self) * factor
i = 0 if val < 1 else int(math.log(val, base))+1
v = val / math.pow(base,i)
v,i = (v,i) if v > 0.5 else (v*base,i-1)

# Identify if there is a width and extract it
width = "" if fmt.find(".") == -1 else fmt[:fmt.index(".")]
precis = fmt[:-2] if width == "" else fmt[fmt.index("."):-2]

# do the precision bit first, so width/alignment works with the suffix
t = ("{0:{1}f}"+mult[i]+suffix).format(v, precis)

return "{0:{1}}".format(t,width) if width != "" else t

if __name__ == "__main__":
# Example usages

# You can use normal format spcifiers as expected - just use the correct the presentation type (instead of f, e, G etc)
# and cast the integer byte count to type size.

# Example format specifications
print "{0:.1f}".format(4386) # output - 4386.0
print "{0:.1f}".format(size(4386)) # output 4386.0 - default numeric presentations respected.
print "{0:.2eM}".format(size(86247)) # output 84.23KiB - base 1024
print "{0:.2sM}".format(size(86247)) # output 86.25KB - base 1000
print "{0:.2cM}".format(size(86247)) # output 84.23KB - base 1024
print "{0:.2cm}".format(size(86247)) # output 0.66Mb - base 1024 in bits.
```