[pycrypto] Python 3.x vs. Python 2.1 - prep the axe

Dwayne C. Litzenberger dlitz at dlitz.net
Wed Dec 29 00:39:12 CST 2010

To do integer division a/b across Python 2.1 through 2.7 (including support for floatdiv), PyCrypto already uses divmod(a,b)[0] IIRC.

"Thorsten Behrens" <sbehrens at gmx.li> wrote:

>On 12/23/2010 11:51 PM, Thorsten Behrens wrote:
>> On 12/23/2010 9:18 PM, Dwayne C. Litzenberger wrote:
>>> Also, down the road, I could be convinced to drop Python 2.1
>support, if I
>>> had some concrete examples showing that the result would be
>>> less error-prone, easier to maintain, etc.
>> So far, it doesn't look like that's needed. dict.has_key() cannot be
>> replaced with "in" for 2.1, but 2to3 seems to handle it fine.
>I've run into a bit of a snag. The / operator in 2.x returns an int,
>in 3.x it can return a float. This causes an infinite loop in 
>numbers.py. I can solve it with //, which is supported from 2.2 on, but
>not in 2.1.
>I've pasted the offending code snippet from numbers.py below, in the 
>form that doesn't cause an infinite loop in 3.x.
>I'm at a bit of a loss here. int(math.floor(a/b)) is not an option due 
>to the size of the operands - it actually fails on 2.1, and gives 
>incorrect results on 3.x. That means I am stuck with //.
>I don't know how to "import something" and have "something" show up in 
>the namespace above. I don't think it works that way, unlike a C 
>#include. That means I can't just bring the right function in depending
>on version. And even if I could, 2.1 doesn't have the "as" keyword, so 
>it would never show up with the right numbers.getStrongPrime name 
>anyway, even _if_ such nested namespace manipulations were supported. 
>Which I don't think they are.
>I can't just "if sys.version" the offending code snippet, either: 2.1 
>will still complain that // is a syntax error.
>We could duplicate the code and have setup.py bring in a special-cased 
>numbers.py for 2.1. I'm not sure how to do that with setup.py, but 
>there's got to be a way, even if it's just renaming files as needed.
>But that means duplicating an entire module, which is ugly. And I can't
>guarantee that this is the only occurrence of / that causes issues. In 
>fact, I'd wager some beer that it likely isn't.
>If you can think of a reasonably clean way of handling the "/" vs. "//"
>issue - or if anyone else can - please share.
>Barring that, I think my message is: If Python 3.x is to be supported 
>without code duplication, Python 2.1 support may have to go.
>         # if e is given make sure that e and X-1 are coprime
>     # this is not necessarily a strong prime criterion but useful when
>     # creating them for RSA where the p-1 and q-1 should be coprime to
>         # the public exponent e
>         if e and is_possible_prime:
>             if e & 1:
>                 if GCD (e, X-1) != 1:
>                     is_possible_prime = 0
>             else:
>                # Python 2.1 does not understand //, and 3.x returns a 
>float on /
>                 # Infinite loop, wheee!!!
>                 if GCD (e, (X-1)//2) != 1:
>                     is_possible_prime = 0
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