Problems with hex-conversion functions

Arnon Yaari wiggin15 at gmail.com
Sat Sep 5 00:28:10 CEST 2009


Hello everyone.
Perhaps I'm missing something, but 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 that
"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 should always return string types and converting from hex
should always return bytes.
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 bytes
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 strings

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
option:
* 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 input)
   #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)


Any thoughts are appreciated.



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