Mailman 3 June 2015 - Python-ideas

Next steps to get type hinting become reality?
by Thomas Güttler June 7, 2015

June 7, 2015

Based on the thead "Better type hinting" here are new questions: We have: PEP 484, Python2, the standard library and a dream called "type hinting". Is it possible to get type hinting for the standard library of Python2? If not, how to get type hinting for the standard library of Python3? What can students with some spare time do, to improve the current situation? Regards, Thomas Güttler -- http://www.thomas-guettler.de/

2 3

User-defined literals
by Andrew Barnert June 6, 2015

June 6, 2015

This is a straw-man proposal for user-defined literal suffixes, similar to the design in C++. In the thread on decimals, a number of people suggested that they'd like to have decimal literals. Nick Coghlan explained why decimal.Decimal literals don't make sense in general (primarily, but not solely, because they're inherently context-sensitive), so unless we first add a fixed type like decimal64, that idea is a non-starter. However, there was some interest in either having Swift-style … [View More]convertible literals or C++-style user-defined literals. Either one would allow users who want decimal literals for a particular app where it makes sense (because there's a single fixed context, and the performance cost of Decimal('1.2') vs. a real constant is irrelevant) to add them without too much hassle or hackery. I explored the convertible literals a while ago, and I'm pretty sure that doesn't work in a duck-typed language. But the C++ design does work, as long as you're willing to have the conversion (including the lookup of the conversion function itself) done at runtime. Any number or string token followed by a name (identifier) token is currently illegal. This would change so that, if there's no whitespace between them, it's legal, and equivalent to a call to a function named `literal_{name}({number-or-string})`. For example, `1.2d` becomes `literal_d('1.2')`, `1.2_dec` becomes `literal_dec('1.2')`, `"1.2"d` also becomes `literal_d('1.2')`. Of course `0x12decimal` becomes `literal_imal('0x12dec')`, and `21jump` becomes `literal_ump('21j'), which are not at all useful, and potentially confusing, but I don't think that would be a serious problem in practice. Unlike C++, the lookup of that literal function happens at runtime, so `1.2z3` is no longer a SyntaxError, but a NameError on `literal_z3`. Also, this means `literal_d` has to be in scope in every module you want decimal literals in, which often means a `from … import` (or something worse, like monkeypatching builtins). C++ doesn't have that problem because of argument-dependent lookup, but that doesn't work for any other language. I think this is the biggest flaw in the proposal. Also unlike C++, there's no overloading on different kinds of literals; the conversion function has no way of knowing whether the user actually typed a string or a number. This could easily be changed (e.g., by using different names, or just by passing the repr of the string instead of the string itself), but I don't think it's necessary. Similarly, this idea could be extended to handle all literal types, so you can do `{'spam': 1, 'eggs': 2}_o` to create an OrderedDict literal, but I think that's ugly enough to not be worth proposing. (A prefix looks better there... but a prefix doesn't work for numbers or strings. And I'm not sure it's unambiguously parseable even for list/set/dict.) Plus, there's the problem that comprehensions and actual literals are both parsed as displays, but you wouldn't want user-defined comprehensions. I've built a quick&dirty toy implementation (at https://github.com/abarnert/userliteralhack). Unlike the real proposal, this only handles numbers, and allows whitespace between the numbers and the names, and is a terrible hack. But it's enough to play with the idea, and you don't need to patch and recompile CPython to use it. My feeling is that this would be useful, but the problems are not surmountable without much bigger changes, and there's no obvious better design that avoids them. But I'm interested to see what others think. [View Less]

15 54

Better Type Hinting
by Thomas Güttler June 5, 2015

June 5, 2015

It would be great to have better type hinting in IDEs. My usecase: logger = logging.getLogger(__name__) try: ... except FooException, exc: logger.warn('...') I remember there was a way to show the traceback via logger.warn(). I could use my favorite search engine, but a short cut via the IDE would be much easier. How can the IDE know what kind of duck "logger" is? Many IDEs parse the docstrings, but a lot of code does not provide it. How can this be improved? Regards, … [View More]

6 11

Decimal facts
by s.krah June 5, 2015

June 5, 2015

> Also, I'm not entirely sure we want to add those types to decimal in the stdlib. It would be a lot less work to implement them in terms of an existing C implementation (maybe using the native types if the exist, Intel's library if they don't). But I don't think that's necessarily desirable for the stdlib. Wrong. The specification we're following is almost IEEE-2008. Mike Cowlishaw evolved the spec while he was on the IEEE commitee. The Intel library is very slow for decimal128, and … [View More]

1 0

Re: [Python-ideas] Python Float Update
by u8y7541 The Awesome Person June 5, 2015

June 5, 2015

> But there was a big issue that we didn't anticipate. During the course of a > simple program it was quite common for calculations to slow down > dramatically, because numbers with ever-larger numerators and denominators > were being computed (and rational arithmetic quickly slows down as those > get bigger). So e.g. you might be computing your taxes with a precision of > a million digits -- only to be rounding them down to dollars for display. (Quote by Guido van Rossum) &… [View More]

1 0

Re: [Python-ideas] [Python Ideas] Python Float Update
by u8y7541 The Awesome Person June 5, 2015

June 5, 2015

On Wed, Jun 3, 2015 at 3:35 PM, Andrew Barnert <abarnert(a)yahoo.com> wrote: > On Jun 3, 2015, at 15:17, u8y7541 The Awesome Person <surya.subbarao1(a)gmail.com> wrote: > >>> I?m going to show a few examples of how Decimals violate the fundamental >>> laws of mathematics just as floats do. >> >> Decimal is also uses sign and mantissa, except it's Base 10. I think >> Decimal should use numerators and denominators, because they are more >&… [View More]

7 7

Re: [Python-ideas] Python Float Update
by u8y7541 The Awesome Person June 4, 2015

June 4, 2015

> I'm going to show a few examples of how Decimals violate the fundamental > laws of mathematics just as floats do. Decimal is also uses sign and mantissa, except it's Base 10. I think Decimal should use numerators and denominators, because they are more accurate. That's why even Decimal defies the laws of mathematics. -Surya Subbarao

5 4

Python Float Update
by u8y7541 The Awesome Person June 4, 2015

June 4, 2015

Dear Python Developers: I will be presenting a modification to the float class, which will improve its speed and accuracy (reduce floating point errors). This is applicable because Python uses a numerator and denominator rather than a sign and mantissa to represent floats. First, I propose that a float's integer ratio should be accurate. For example, (1 / 3).as_integer_ratio() should return (1, 3). Instead, it returns(6004799503160661, 18014398509481984). Second of all, even though 1 * 3 = 3 … [View More]

25 74

Re: [Python-ideas] Python-ideas Digest, Vol 103, Issue 29
by u8y7541 The Awesome Person June 4, 2015

June 4, 2015

> > Stephen J. Turnbull writes: > >> Nick Coghlan writes: >> >>> the main concern I have with [a FloatLiteral that carries the >>> original repr around] is that we'd be trading the status quo for a >>> situation where "Decimal(1.3)" and "Decimal(13/10)" gave different >>> answers. >> >> Yeah, and that kills the deal for me. Either Decimal is the default >> representation for non-integers, or this is a no-go. And that … [View More]

2 1

Re: [Python-ideas] [Python Ideas] Python Float Update
by u8y7541 The Awesome Person June 4, 2015

June 4, 2015

> I?m going to show a few examples of how Decimals violate the fundamental > laws of mathematics just as floats do. Decimal is also uses sign and mantissa, except it's Base 10. I think Decimal should use numerators and denominators, because they are more accurate. That's why even Decimal defies the laws of mathematics.

2 1