[Python-Dev] Problems with hex-conversion functions

Ender Wiggin wiggin15 at gmail.com
Sat Sep 5 23:26:14 CEST 2009

Hello everyone.

I see several problems with the two hex-conversion function pairs that
Python offers:
1. binascii.hexlify and binascii.unhexlify
2. bytes.fromhex and bytes.hex

Problem #1:
bytes.hex is not implemented, although it was specified in PEP 358.
This means there is no symmetrical function to accompany bytes.fromhex.

Problem #2:
Both pairs perform the same function, although The Zen Of Python suggests
"There should be one-- and preferably only one --obvious way to do it."
I do not understand why PEP 358 specified the bytes function pair although
it mentioned the binascii pair...

Problem #3:
bytes.fromhex may receive spaces in the input string, although
binascii.unhexlify may not.
I see no good reason for these two functions to have different features.

Problem #4:
binascii.unhexlify may receive both input types: strings or bytes, whereas
bytes.fromhex raises an exception when given a bytes parameter.
Again there is no reason for these functions to be different.

Problem #5:
binascii.hexlify returns a bytes type - although ideally, converting to hex
always return string types and converting from hex should always return
IMO there is no meaning of bytes as an output of hexlify, since the output
is a
representation of other bytes.
This is also the suggested behavior of bytes.hex in PEP 358

Problems #4 and #5 call for a decision about the input and output of the
functions being discussed:

Option A : Strict input and output
unhexlify (and bytes.fromhex) may only receives string and may only return
hexlify (and bytes.hex) may only receives bytes and may only return strings

Option B : Robust input and strict output
unhexlify (and bytes.fromhex) may receive bytes and strings and may only
return bytes
hexlify (and bytes.hex) may receive bytes or strings and may only return

Of course we may also consider a third option, which will allow the return
type of
all functions to be robust (perhaps specified in a keyword argument), but as
I wrote in
the description of problem #5, I see no sense in that.

Note that PEP 3137 describes: "... the more strict definitions of encoding
and decoding in
Python 3000: encoding always takes a Unicode string and returns a bytes
sequence, and decoding
always takes a bytes sequence and returns a Unicode string." - suggesting
option A.

To repeat problems #4 and #5, the current behavior does not match any
* The return type of binascii.hexlify should be string, and this is not the
current behavior.
As for the input:
* Option A is not the current behavior because binascii.unhexlify may
receive both input types.
* Option B is not the current behavior because bytes.fromhex does not allow
bytes as input.

To fix these issues, three changes should be applied:
1. Deprecate bytes.fromhex. This fixes the following problems:
   #4 (go with option B and remove the function that does not allow bytes
   #2 (the binascii functions will be the only way to "do it")
   #1 (bytes.hex should not be implemented)
2. In order to keep the functionality that bytes.fromhex has over unhexlify,
   the latter function should be able to handle spaces in its input (fix #3)
3. binascii.hexlify should return string as its return type (fix #5)
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