Python-ideas October 2012

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127 participants
70 discussions

Re: [Python-ideas] checking for identity before comparing built-in objects
by Guido van Rossum 08 Oct '12

08 Oct '12

On Mon, Oct 8, 2012 at 12:03 PM, Rob Cliffe <rob.cliffe(a)btinternet.com> wrote: > > On 08/10/2012 19:39, Guido van Rossum wrote: >> >> Does this mean that the following behaviour of lists is a bug? >>>>>>> >>>>>>> x=float('NAN') >>>>>>> [x]==[x], [x]<=[x], [x]>=[x] >>>> >>>> (True, True, True) >>> >>> No. That's a special case in the comparisons for sequences. >> >> [Now that I'm back at a real keyboard I can elaborate...] >> >> This applies to all container comparisons: without the rule that if >> two contained items reference the same object they are to be >> considered equal without calling their __eq__, containers couldn't >> take the shortcut that a container is always equal to itself (i.e. c1 >> is c2 => c1 == c2). Without this shortcut, container comparisons would >> be much more expensive: any time a large container was compared to >> itself, it would be forced to recursively compare all the contained >> items. You might say that it has to do this anyway when comparing to a >> container that is not itself, but if the anser is "unequal" the >> comparison can stop as soon as two unequal items are found, whereas if >> the answer is "equal" you end up comparing all items. For two >> different containers there is no possible shortcut, but comparing a >> container to itself is quite common and really does deserve the >> shortcut. We discussed this in the past and always came to the same >> conclusion: despite the rules for NaN, the shortcut for containers is >> required. A similar shortcut exists for 'x in [x]' BTW. >> > Thank you for elaborating, I was going to ask what the justification for the > special case was. > You have explained why > >>>> x=float('NAN'); A=[x]; A==A > True > > but not as far as I can see why > >>>> x=float('NAN'); A=[x]; B=[x]; A==B, [x]=[x] > (True, True) > > where neither of the results is comparing a container to itself. It's so that when the container is iterating over pairs of elements it can check for item identity (a simple pointer comparison) first, which makes a pretty big difference in speed. -- --Guido van Rossum (python.org/~guido)

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Re: [Python-ideas] PEP 428 - object-oriented filesystem paths
by massimo.dipierro＠gmail.com 08 Oct '12

08 Oct '12

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Re: [Python-ideas] checking for identity before comparing built-in objects
by Guido van Rossum 08 Oct '12

08 Oct '12

On Mon, Oct 8, 2012 at 10:36 AM, Guido van Rossum <guido(a)python.org> wrote: > >>> It's not about equality. If you ask whether two NaNs are *unequal* the >>> answer is *also* False. >> >> Does this mean that the following behaviour of lists is a bug? >> >>> x=float('NAN') >> >>> [x]==[x], [x]<=[x], [x]>=[x] >> (True, True, True) > > No. That's a special case in the comparisons for sequences. [Now that I'm back at a real keyboard I can elaborate...] This applies to all container comparisons: without the rule that if two contained items reference the same object they are to be considered equal without calling their __eq__, containers couldn't take the shortcut that a container is always equal to itself (i.e. c1 is c2 => c1 == c2). Without this shortcut, container comparisons would be much more expensive: any time a large container was compared to itself, it would be forced to recursively compare all the contained items. You might say that it has to do this anyway when comparing to a container that is not itself, but if the anser is "unequal" the comparison can stop as soon as two unequal items are found, whereas if the answer is "equal" you end up comparing all items. For two different containers there is no possible shortcut, but comparing a container to itself is quite common and really does deserve the shortcut. We discussed this in the past and always came to the same conclusion: despite the rules for NaN, the shortcut for containers is required. A similar shortcut exists for 'x in [x]' BTW. -- --Guido van Rossum (python.org/~guido)

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Re: [Python-ideas] asyncore: included batteries don't fit
by Guido van Rossum 08 Oct '12

08 Oct '12

On Sun, Oct 7, 2012 at 7:16 PM, Duncan McGreggor <duncan.mcgreggor(a)gmail.com> wrote: > > > On Sun, Oct 7, 2012 at 5:52 PM, Guido van Rossum <guido(a)python.org> wrote: >> >> On Sat, Oct 6, 2012 at 9:09 PM, Duncan M. McGreggor >> <duncan.mcgreggor(a)gmail.com> wrote: >> > We're here ;-) >> > >> > I'm forwarding this to the rest of the Twisted cabal... >> >> Quick question. I'd like to see how Twisted typically implements a >> protocol parser. Where would be a good place to start reading example >> code? > > > I'm not exactly sure what you're looking for (e.g., I'm not sure what your > exact definition of a protocol parser is), but this might be getting close > to what you want: > > * https://github.com/twisted/twisted/blob/master/twisted/mail/pop3.py > * https://github.com/twisted/twisted/blob/master/twisted/protocols/basic.py > > The POP3 protocol implementation in Twisted is a pretty good example of how > one should create a protocol. It's a subclass of the > twisted.protocol.basic.LineOnlyReceiver, and I'm guessing when you said > "parsing" you're wanting to look at what's in the dataReceived method of > that class. > > Hopefully that's what you were after... Yes, those are perfect. The term I used came from one of Josiah's previous messages in this thread, but I think he really meant protocol handler. My current goal is to see if it would be possible to come up with an abstraction that makes it possible to write protocol handlers that are independent from the rest of the infrastructure (e.g. transport, reactor). I honestly have no idea if this is a sane idea but I'm going to look into it anyway; if it works it would be cool to be able to reuse the same POP3 logic in different environments (e.g. synchronous thread-based, Twisted) without having to pul in all of Twisted. I.e. Twisted could contribute the code to the stdlib and the stdlib could make it work with SocketServer but Twisted could still use it (assuming Twisted ever gets ported to Py3k :-). -- --Guido van Rossum (python.org/~guido)

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Re: [Python-ideas] asyncore: included batteries don't fit
by Guido van Rossum 07 Oct '12

07 Oct '12

On Sat, Oct 6, 2012 at 9:09 PM, Duncan M. McGreggor <duncan.mcgreggor(a)gmail.com> wrote: > We're here ;-) > > I'm forwarding this to the rest of the Twisted cabal... Quick question. I'd like to see how Twisted typically implements a protocol parser. Where would be a good place to start reading example code? -- --Guido van Rossum (python.org/~guido)

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Re: [Python-ideas] PEP 428 - object-oriented filesystem paths
by massimo.dipierro＠gmail.com 05 Oct '12

05 Oct '12

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Visually confusable unicode characters in identifiers
by Oscar Benjamin 04 Oct '12

04 Oct '12

Having just discovered that PEP 3131 [1] enables me to use greek letters to represent variables in equations, it was pointed out to me that it also allows visually confusable characters in identifiers [2]. When I previously read the PEP I thought that the normalisation process resolved these issues but now I see that the PEP leaves it as an open problem. I also previously thought that the PEP would be irrelevant if I was using ascii-only code but now I can see that if a GREEK CAPITAL LETTER ALPHA can sneak into my code (just like those pesky tab characters) I could still have a visually undetectable bug. An example to show how an issue could arise: """ #!/usr/bin/env python3 code = ''' {0} = 123 {1} = 456 print('"{0}" == "{1}":', "{0}" == "{1}") print('{0} == {1}:', {0} == {1}) ''' def test_identifier(identifier1, identifier2): exec(code.format(identifier1, identifier2)) test_identifier('\u212b', '\u00c5') # Different Angstrom code points test_identifier('A', '\u0391') # LATIN/GREEK CAPITAL A/ALPHA """ When I run this I get: $ ./test.py "Å" == "Å": False Å == Å: True "A" == "Α": False A == Α: False Is the proposal mentioned in the PEP (to use something based on Unicode Technical Standard #39 [3]) something that might be implemented at any point? Oscar References: [1] http://www.python.org/dev/peps/pep-3131/#open-issues [2] http://article.gmane.org/gmane.comp.python.tutor/78116 [3] http://unicode.org/reports/tr39/#Confusable_Detection

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Deprecate the round builtin
by Mike Graham 03 Oct '12

03 Oct '12

The builtin round function is completely useless. I've never seen anyone use it constructively. Usually people using it are new programmers who are not comfortable with or aware of string formatting. Sometimes people use it to poorly replicate functionality that's implemented correctly in the decimal module. Mike

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use multiprocessing in included compileall script
by Daniel Holth 01 Oct '12

01 Oct '12

As an option, compileall should use a multiprocessing Pool() to speed up its work.

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Re: [Python-ideas] make decimal the default non-integer instead of float?
by Gregory P. Smith 01 Oct '12

01 Oct '12

-cc: python-dev +cc: python-ideas On Sat, Sep 29, 2012 at 11:39 AM, Chris Angelico <rosuav(a)gmail.com> wrote: > On Sun, Sep 30, 2012 at 4:26 AM, Brett Cannon <brett(a)python.org> wrote: > > Does this mean we want to re-open the discussion about decimal constants? > > Last time this came up I think we decided that we wanted to wait for > > cdecimal (which is obviously here) and work out how to handle contexts, > the > > syntax, etc. > > Just to throw a crazy idea out: How bad a change would it be to make > decimal actually the default? > > (Caveat: I've not worked with decimal/cdecimal to any real extent and > don't know its limitations etc.) > Painful for existing code, unittests and extension modules. Definitely python-ideas territory (thread moved there with an appropriate subject). I'm not surprised at all that a decimal type can be "fast" in an interpreted language due to the already dominant interpreter overhead. I wish all spreadsheets had used decimals from day one rather than binary floating point (blame Lotus?). Think of the trouble that would have saved the world. -gps

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Python-ideas October 2012