This should do the trick:

pw_bytes.decode("utf-8")

You can use bytes.decode function if you really need to "get rid of b": http://docs.python.org/3.3/library/stdtypes.html#bytes.decode

But it seems from your code that you do not really need to do this, you really need to work with bytes.

Use slice operation to remove the first two characters.

In [1]: x = 17

In [2]: y = bin(x)[2:]

In [3]: y
Out[3]: '10001'

The b symbol indicates that the output of check_process is a bytes rather than a str. The best way to remove it is to convert the output to string before you do any further work on it:

byte_data=subprocess.check_output(["df -k | awk '{print $6}'"],shell=True)
str_data = byte_data.decode('utf-8')
data_arr=str_data.split()
...

The decode method will take care of any unicode you may have in the string. If your default encoding (or the one used by awk I suppose) is not UTF-8, substitute the correct one in the example above.

Possibly an even better way to get around this issue is to tell check_output to open stdout as a text stream. The easiest way is to add a universal_newlines=True argument, which will use the default encoding for your current locale:

str_data = subprocess.check_output(["df -k | awk '{print $6}'"], shell=True, universal_newlines=True)

Alternatively, you can specify an explicit encoding:

str_data = subprocess.check_output(["df -k | awk '{print $6}'"], shell=True, encoding='utf-8')

In both of these cases, you do not need to decode because the output will already be str rather than bytes.

Using zfill():

Return the numeric string left filled with zeros in a string of length width. A sign prefix is handled correctly. The original string is returned if width is less than len(s).

>>> bin(30)[2:].zfill(8)
'00011110'
>>>