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+ <H1>[pycrypto] Python 3.x vs. Python 2.1 - prep the axe</H1>

+ Dwayne C. Litzenberger

+ <A HREF="mailto:pycrypto%40lists.dlitz.net?Subject=%5Bpycrypto%5D%20Python%203.x%20vs.%20Python%202.1%20-%20prep%20the%20axe&In-Reply-To=4D1972A2.4080305%40gmx.li"

+ TITLE="[pycrypto] Python 3.x vs. Python 2.1 - prep the axe">dlitz at dlitz.net

+ </A>

+ Wed Dec 29 00:39:12 CST 2010

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+<PRE>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" <<A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">sbehrens at gmx.li</A>> 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

+>substantially

+>>> 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,

+>and

+>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.

+>

+>Yours

+>

+>Thorsten

+>

+> # 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

+>_______________________________________________

+>pycrypto mailing list

+><A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">pycrypto at lists.dlitz.net</A>

+><A HREF="http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto">http://lists.dlitz.net/cgi-bin/mailman/listinfo/pycrypto</A>

+

+--

+Sent from my Android phone with K-9 Mail. Please excuse my brevity.

+</PRE>

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