Mailman 3 January 2008 - Python-Dev

Rounding Decimals
by Jeffrey Yasskin 14 Jan '08

14 Jan '08

On Jan 5, 2008 3:34 PM, Mark Dickinson <dickinsm(a)gmail.com> wrote: > > On Jan 5, 2008 5:54 PM, <glyph(a)divmod.com> wrote: > > > > At first I didn't realize why I'd missed this feature. While the > > rounding *modes* are well documented, though, after 20 minutes of > > reading documentation I still haven't found a method or function that > > simply rounds a decimal to a given significant digit. Is there one, > > should there be one, or is the user simply meant to use Context.quantize > > with appropriate arguments? > > > > > > > > > > quantize is about as close as it gets. Note that it's a Decimal method as > well as a Context method, so you can invoke it directly on a given decimal: > > > >>> Decimal("2.34567").quantize(Decimal("0.01")) > Decimal("2.35") > > > I've also occasionally felt a need for a simple rounding function that isn't > affected by context. Would others be interested in such a function being > added to Decimal? I guess there are two possibly useful operations: (1) > round to a particular decimal place ( e.g. nearest ten, nearest hundredth, > ..) and (2) to round to a particular number of significant digits; in both > cases, the user should be able to specify the desired rounding mode. And > for each operation, it might also be useful to specify whether the result > should be padded with zeros to the desired length or not. ( i.e. when > rounding 3.399 to 3 significant places, should it produce 3.4 or 3.40?) > > Any thoughts? I think pep 3141's round(x, ndigits) does (1). The only thing it doesn't support yet is specifying the rounding mode. Perhaps the pep should say that round() passes any extra named arguments on to the __round__() method so that users can specify a rounding mode for types that support it? The Real interface doesn't say anything about significant digits, so Decimal can do whatever we want. My first guess would be that round should remove significant digits but not add them. (i.e. round("3.199", 2) == "3.20" but round("3", 1) == "3".) As you know, I'm working on a patch to implement 3141 for Decimal at http://bugs.python.org/issue1623, which I'll update as soon as this thread comes to a conclusion. Other people who are interested in getting this right should add themselves to its nosy list so they can object before I check something dumb in. :) I currently plan to keep __round__ in 2.6's Decimal with 3.0's behavior because it no longer affects the round() builtin in 2.6. Users who want it can call it directly … or we might provide, in 2.6, a module that provides 3.0 versions of the core library functions that change behavior so that they can get the new round() explicitly. -- Namasté, Jeffrey Yasskin

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docs.python.org
by Jeroen Ruigrok van der Werven 14 Jan '08

14 Jan '08

Will docs.python.org be redirected to www.python.org/doc? I think I am not the only person on the planet that had been using docs.python.org for a long time now to get to the documentation. (And some links are now broken @ docs.python.org, so it seems all the more logical to make such a redirect. docs.python.org is the 3rd result from Google too by the way.) -- Jeroen Ruigrok van der Werven <asmodai(-at-)in-nomine.org> / asmodai イェルーンラウフロックヴァンデルウェルヴェン http://www.in-nomine.org/ | http://www.rangaku.org/ Want ik kan niet leven zonder alles of niets...

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Re: [Python-Dev] PySequence_Concat for dicts
by Raymond Hettinger 13 Jan '08

13 Jan '08

> I wasn't suggesting that the result of concatenation would > be a chained table, rather that it would perform the > equivalent of an update and return the new dict >(the same way extend works for lists) When does it come-up that you want a third summed dict while keeping the two originals around unchanged? Does it matter that the addition is non-commutative? Would a + b + c produce an intermediate a/b combo and then another new object for a/b/c so that the entries in a get copied twice and memory usage has to hold a, b, a/b, c, and a/b/c in memory all at the same time? What are the use cases? FWIW, the Py3.0 API for dicts will support some set-like operations. Perhaps, that fits the bill. Raymond

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complaints..
by Jay 13 Jan '08

13 Jan '08

I'm sure you've heard most/all of this before but..it..just..seems..so..true... Finally this week I've "written" (ported from sh) a bunch of Perl, 2000 sparse lines. While it sure beats Perl, it has some glaring flaws, more glaring due to its overall goodness. I feel compelled to voice my opinion, as if we don't live in a benevolent dictatorship :), and as if the weight of existing code was zero. Much of what I dislike cannot be changed without massive breakage. Below is what i get from: jbook:/dev2/cm3/scripts/python/flaws jay$ ls -l total 72 -rw-r--r-- 1 jay admin 834 Dec 29 16:02 1_not_lexically_scoped.py -rw-r--r-- 1 jay admin 238 Dec 29 16:02 2_reads_scoped_writes_not.py -rw-r--r-- 1 jay admin 593 Dec 29 16:02 3_lambda_is_neutered.py -rw-r--r-- 1 jay admin 377 Dec 29 16:03 4_assignment_is_not_expression.py -rw-r--r-- 1 jay admin 760 Dec 29 16:02 5_for_loop_off_by_one.py -rw-r--r-- 1 jay admin 412 Dec 29 16:01 6_docs_good_but_a_complaint.txt -rw-r--r-- 1 jay admin 254 Dec 29 16:02 7_print_is_wierd.py -rw-r--r-- 1 jay admin 286 Dec 29 16:06 8_eval_vs_exec.py -rw-r--r-- 1 jay admin 824 Dec 29 16:14 9_typo_on_read_error_but_write_ok.py jbook:/dev2/cm3/scripts/python/flaws jay$ cat * > all.txt jbook:/dev2/cm3/scripts/python/flaws jay$ edit all.txt Each "---" seperates a file and they are executable Python. - Jay #---------------------------------------------------------- # flaw #1 # not lexically scopied # Functions have their own locals, but other blocks do not. # This is true both for "normal" variables and lexically nested functions. # # # This is probably largely an outcome of not declaring variables? # A = "A1:global" def F1(): A = "A1:in first F1" print "F1:global" if True: A = "A1:in if" def F1(): A = "A1:in if.F1" # This does the right thing. print "F1:in if" F1() # This does the right thing. # This should go to "global" but it goes to "in if" F1() def F2(): A = "A1:in F2" def F1(): A = "A1:in F2.F1" print "F1:in F2" # Given how if behaved, you'd expect this to go to F2.F1 but it does not. F1() # This should be "global" but is "in if". print("A is " + A) #---------------------------------------------------------- # flaw #2 # # In functions, reads are scoped. Writes are not. # A = "1" def F1(): A = "2" # not an error def F2(): #B = A # error A = "3" F1() F2() print(A) #---------------------------------------------------------- # flaw #3: # lambda is neutered # It allows just one expression, and no statements # # This should work: import os #os.path.walk( # "..", # lambda(a, b, c): # print(b) # print(b) # None) # Instead I must say: def Callback(a, b, c): print(b) print(b) os.path.walk( "..", Callback, None) # # Moving callbacks away from their point of use hurts readability. # This is probably mitigated by lexical scoping of functions, but # notice that other blocks are not lexically scoped. # #---------------------------------------------------------- # flaw #4: # assignment is not an expression # # This should work (seen in Perl code, nice idiom): #A = [1,2] #while (B = A.pop()): # print(B) # instead I must say: A = [1,2] while (A): B = A.pop() print(B) # Even if you reject popping an empty list, ok # there are PLENTY of applications of this. #---------------------------------------------------------- # flaw #5 # # for loops suck # # It should be easy to iterate from 0 to n, not 0 to n - 1, # thereby knowing why the loop terminated # #This should work: # print the first even number, if there are any A = [1, 3] for i in range(0, len(A)): if ((A[i] % 2) == 0): print("even number found") break; if (i == len(A)): print("no even numbers found") # instead I must write: i = 0 while (i != len(A)): if ((A[i] % 2) == 0): print("even number found") break; i += 1 if (i == len(A)): print("no even numbers found") # with the attendent problem of not being able to "continue" ever, since # the end of the loop body must be run in order to proceed Flaw #6 The docs are very good. However the reference doesn't give much in the way of semantic description. It is surprising that an experienced programmer MUST depend on the tutorial or any examples or semantics. Light on examples, ok for reference. The language reference is little more than the grammar in parts. There needs to be links from the reference back to the tutorial. Perhaps merge the indices for Tutorial, Language Reference, Library Reference. Or maybe that's what search is for. On the other hand, way better than Perl. #---------------------------------------------------------- # Flaw #7 # # This is a compatibility issue. # print is both a statement and a function, or something. # print() # should print just a newline, but prints two parens # workaround: print("") #---------------------------------------------------------- # flaw #8 # # Having to eval expressions but exec statements feels wrong. # There should just be eval. # # eval("print(1)") # error exec("print(1)") # not an error exec("1 + 2") # not an error? eval("1 + 2") # not an error #---------------------------------------------------------- # flaw #9 # # Python protects me, via early checking, from typos # when I read, but not when I write. It should do both. # That is, I should declare variables. # Correct = 1 # proposed typo is Corect #A = Corect # error, good Corect = 1 # no error, bad # For classes, by default, same thing: class Foo(object): def __init__(self): self.Correct =1 F = Foo() # print(F.Corect) # error, good F.Corect = 1 # no error, bad # __slots__ fixes this, but is not the default class Bar(object): __slots__ = ["Correct"] def __init__(self): self.Correct =1 B = Bar() # print(B.Corect) # error, good #B.Corect = 1 # error, good # __slots__ should be the default and then some other syntax # for "expandable" types, like __slots__ = [ "*" ] _________________________________________________________________ Get the power of Windows + Web with the new Windows Live. http://www.windowslive.com?ocid=TXT_TAGHM_Wave2_powerofwindows_122007

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Some tardy mailman checking
by skip＠pobox.com 13 Jan '08

13 Jan '08

The previous four posts were dredged out of the holding pen in Mailman. Sorry for the delay. Skip

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OSCON 2008 Call for Proposals
by Aahz 13 Jan '08

13 Jan '08

The O'Reilly Open Source Convention (OSCON) is accepting proposals for tutorials and presentations. The submission period ends Feb 4. OSCON 2008 will be in Portland, Oregon July 21-25. For more information and to submit a proposal, see http://conferences.oreilly.com/oscon/ -- Aahz (aahz(a)pythoncraft.com) <*> http://www.pythoncraft.com/ Weinberg's Second Law: If builders built buildings the way programmers wrote programs, then the first woodpecker that came along would destroy civilization.

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How to change path at compile time?
by Jason Garber 12 Jan '08

12 Jan '08

Hello, Is there any reasonable way to change the default sys.path at compile time? (ie. add a directory). (I am aware of $PYTHONPATH for runtime) -- Best Regards, Jason Garber Senior Systems Engineer IonZoft, Inc. (814) 941-2390 jgarber(a)ionzoft.com <mailto:jgarber@ionzoft.com>

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Bug day preparations: tagging bugs?
by A.M. Kuchling 12 Jan '08

12 Jan '08

No one pushed back on the 19th as a bug day, so I'm going to go ahead and send out announcements this evening. We should mark issues in the tracker that are good candidates for new developers. How do we want to do this? We could add a new keyword ('beginner', 'bugday', ???) or add a prefix to the subject line. Does Roundup search also look at the text of the change notes added to a bug? If yes, then just adding a comment with the magic keyword would be sufficient. --amk

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New Developer
by Mark Dickinson 12 Jan '08

12 Jan '08

Hello all, I've recently been granted commit privileges; so, following the usual protocol, here's a quick introduction. I'm a mathematician by day; my degree is in number theory, but five summers of Fortran 77 programming and two semesters of teaching numerical analysis have given me a taste for numerics as well. I discovered Python around twelve years ago and found that it fit my brain nicely (even more so after nested namespaces were introduced) and now use it almost daily for a wide variety of tasks. I've been lurking on python-dev for longer than I care to admit to. I also dabble in Haskell and O'Caml. I'd like to say a huge thank you to Facundo Batista: I've been helping him update Decimal to the most recent specification over the last few months, and without his support and encouragement I certainly wouldn't be writing this now---he's been a fantastic guide and mentor. Please don't hesitate to let me know if there's anything I can help out with; I'm primarily interested in mathematical, numerical, and algorithmic aspects of Python at the moment, but am looking to broaden my knowledge of the internals. I've got so much out of Python over the years, and I hope I can now give something back. Mark

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Re: [Python-Dev] PySequence_Concat for dicts
by Raymond Hettinger 12 Jan '08

12 Jan '08

[Jared] > I think it would be convenient and pythonic if dict > objects implemented the PySequence_Concat method. IMO, the chainmap() recipe on ASPN is a much better solution since it doesn't create a third dictionary with the all the attendant allocation and copying effort. It isn't a common use case to need to sum two dictionaries while keeping both of the inputs unaltered. http://aspn.activestate.com/ASPN/Cookbook/Python/Recipe/305268. Raymond

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