Human friendly format without using math libraries

humanSize.py

"""
Write a function in Python called ‘humanSize’ that takes a non-negative number
of bytes and returns a string with the equivalent number of ‘kB’, ‘MB’, ‘GB’,
‘TB’, ‘PB’, ‘EB’, ‘ZB’, or ‘YB’,between [0, 1000), with at most 1 digit of 
precision after the decimal. 
If the number of bytes is >= 1000 YB, return this number of YB, for example 5120 YB.
"""
from collections import OrderedDict

# using ordered dict to make sure it evaluates first the biggest one.
SIZE_BYTES = OrderedDict({            
    "YB": 1e24,
    "ZB": 1e21,
    "EB": 1e18,
    "PB": 1e15,
    "TB": 1e12,
    "GB": 1e9,
    "MB": 1e6,
    "KB": 1e3,
})

def humanSize(total_bytes):
    """Returns the total number of bytes in human friendly format."""
    if total_bytes < 0 :
        raise ValueError("Number should be positive")

    if total_bytes >= SIZE_BYTES["YB"]:
        total = int(total_bytes // SIZE_BYTES["YB"])
        return f"{total} YB"

    for size, value in SIZE_BYTES.items():
        if total_bytes // value > 0:
            result = _format_number(total_bytes/value)
            return f"{result} {size}"

    return f"{total_bytes} bytes"

def _format_number(n):
    """Returns the decimal number truncated to 1 position.

    Can't use format(".1f") because it will round it up.
    """
    num = str(n).split(".")

    if len(num) == 1:
        return n

    integer, decimal = num
    return ".".join((integer, decimal[:1]))


if __name__ == "__main__":

    # assert base cases
    assert humanSize(10) == "10 bytes"
    assert humanSize(1e3) == "1.0 KB"
    assert humanSize(1e6) == "1.0 MB"
    assert humanSize(1e9) == "1.0 GB"
    assert humanSize(1e12) == "1.0 TB"
    assert humanSize(1e15) == "1.0 PB"
    assert humanSize(1e18) == "1.0 EB"
    assert humanSize(1e21) == "1.0 ZB"
    assert humanSize(1e24) == "1 YB"

    # assert not round
    assert humanSize(9990) == "9.9 KB"
    assert humanSize(9999990) == "9.9 MB"
    assert humanSize(9.9e18) == "9.9 EB"
    assert humanSize(9.9e21) == "9.9 ZB"
    

    # some  big numbers
    assert humanSize(21367329234) == "21.3 GB"
    assert humanSize(2136732923453884) == "2.1 PB"
    assert humanSize(657892349896458698) == "657.8 PB"
    assert humanSize(6578923498964586980) == "6.5 EB"
    assert humanSize(657892349896458649646849359) == "657 YB"
    assert humanSize(578923498964586496468499) == "578.9 ZB"