[Tutor] Python .decode issue

[Peter Otten]

james campbell wrote:

> I have been currently trying to get a small piece of code to work, but
> keep getting an error:
> 
> header_bin = header_hex.decode('hex')
> AttributeError: 'str' object has no attribute 'decode'
> 
> 
> The source of this code is from:
> https://en.bitcoin.it/wiki/Block_hashing_algorithm Here is the the code:
> >>> import hashlib >>> header_hex = ("01000000" +
> "81cd02ab7e569e8bcd9317e2fe99f2de44d49ab2b8851ba4a308000000000000" +
> "e320b6c2fffc8d750423db8b1eb942ae710e951ed797f7affc8892b0f1fc122b" +
> "c7f5d74d" + "f2b9441a" + "42a14695") >>> header_bin =
> header_hex.decode('hex') >>> hash =
> hashlib.sha256(hashlib.sha256(header_bin).digest()).digest() >>>
> hash.encode('hex_codec')
> '1dbd981fe6985776b644b173a4d0385ddc1aa2a829688d1e0000000000000000' >>>
> hash[::-1].encode('hex_codec')
> '00000000000000001e8d6829a8a21adc5d38d0a473b144b6765798e61f98bd1d'
> 
> I get the above error when I input this line by line into Win7 IDLE 3.3.3.
> Hopefully somebody can point me in the right direction. Thanks.

With the move from Python 2 to Python 3 string handling was changed

py2     --> py3
-----------------
str     --> bytes
unicode --> str

Also the encode decode methods have changed. 
bytes.decode() always gives you a str and str.encode() always returns bytes; 
there are no bytes.encode or str.decode methods.

Unfortunately the bytes --> bytes conversion codecs in Python 2 have no 
convenient analog in Python 3 yet. 

This will change in Python 3.4, where you can use

>>> import codecs
>>> codecs.decode(b"ff10", "hex")
b'\xff\x10'
>>> codecs.encode(b"\xff\x10", "hex")
b'ff10'

But for now you are stuck with binascii.b2a_hex() etc. 
Converting the 2x code of your example to 3x gives

>>> import hashlib
>>> import binascii
>>> header_hex = (
... b"01000000"
... b"81cd02ab7e569e8bcd9317e2fe99f2de44d49ab2b8851ba4a308000000000000"
... b"e320b6c2fffc8d750423db8b1eb942ae710e951ed797f7affc8892b0f1fc122b"
... b"c7f5d74d"
... b"f2b9441a"
... b"42a14695")
>>> header_bin = binascii.a2b_hex(header_hex)
>>> hash = hashlib.sha256(hashlib.sha256(header_bin).digest()).digest()
>>> binascii.b2a_hex(hash)
b'1dbd981fe6985776b644b173a4d0385ddc1aa2a829688d1e0000000000000000'
>>> binascii.b2a_hex(hash[::-1])
b'00000000000000001e8d6829a8a21adc5d38d0a473b144b6765798e61f98bd1d'