[issue23602] Implement __format__ for Fraction

[Sergey B Kirpichev]

Change by Sergey B Kirpichev :


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[issue23602] Implement __format__ for Fraction

[Stefan Krah]

Stefan Krah added the comment:

 It wouldn't be too bad if Py and C version of Decimal.__format__ had
 same interface. What do you think?

Let's discuss that in a separate issue.


[DefaultContext]
 I don't understand what do you mean with this. Is this something that
 I'm doing wrong in my patch or just another (related?) issue?

Decimal.DefaultContext has global scope and currently affects
Context() creation, unless you specify all parameters.

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[issue23602] Implement __format__ for Fraction

[Stefan Krah]

Stefan Krah added the comment:

Regarding Decimal:

  1) The context precision isn't used for formatting.  If you have
 another reason for proposing the optional context argument for
 dec_format(), please open another issue.

  2) There's another problem: The mythical DefaultContext (which
 acts as a template for creating new contexts) affects not only
 new thread-local contexts, but also a new Context()!

 In my opinion this is something we should change: The mechanism
 is fine for thread-local contexts, but Context() should behave
 like a pure function.

  3) The double rounding issues are more tricky than it might seem;
 if we use Decimal for this, perhaps direct support in the module
 would be the cleanest option.

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[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

Tuomas Suutari added the comment:

On 30 March 2015 at 13:49, Stefan Krah wrote:
 Regarding Decimal:

   1) The context precision isn't used for formatting.  If you have
  another reason for proposing the optional context argument for
  dec_format(), please open another issue.

Yes, context precision isn't, but context rounding mode is. That's why
I wanted to use a known context rather than the current (thread-local)
context.

But yes, I also thought that maybe the Decimal.__format__ changes
should go to another issue, which my implementation would then depend
on though. It wouldn't be too bad if Py and C version of
Decimal.__format__ had same interface. What do you think?

   2) There's another problem: The mythical DefaultContext (which
  acts as a template for creating new contexts) affects not only
  new thread-local contexts, but also a new Context()!

  In my opinion this is something we should change: The mechanism
  is fine for thread-local contexts, but Context() should behave
  like a pure function.

I don't understand what do you mean with this. Is this something that
I'm doing wrong in my patch or just another (related?) issue?

   3) The double rounding issues are more tricky than it might seem;
  if we use Decimal for this, perhaps direct support in the module
  would be the cleanest option.

What double rounding issues you're referring to?

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[issue23602] Implement __format__ for Fraction

[Martin Panter]

Martin Panter added the comment:

I understand double rounding to mean incorrectly rounding something like 
0.14 up to 0.2. It should be rounded once to 1 decimal place (0.1). If you 
temporarily round it to a higher number of places before rounding to 1 place, 
you’re doing it wrong. So you might have to ensure that any rounding done 
before formatting step exactly matches the rounding specified in the formatting.

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[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

Tuomas Suutari added the comment:

Thanks for the comments again!

I fixed the format(F(4, 27), '.1f') - 0.2 issue
Serhiy Storchaka reported. Fix for that was as simple as adding one to the 
precision the decimals are calculated in, but while adding test cases for that 
I realized two new things:

  (a) I don't want f specifier to mean infinite precision, but instead some 
predefined value. I chose 6.
  (b) How about rounding? I don't want the current decimal context to affect 
that, since it's not logical that formatting of Fractions depends on the 
decimal context.

The rounding thing made things harder, since there was no way to pass decimal 
context for Decimal.__format__ without changing the local context -- at least 
with the C implementation; the Python implementation (_pydecimal) provided 
nicer API with optional context keyword argument.  So I decided to unify the C 
and Py API's of Decimal.__format__ and add the keyword argument support to the 
C API too. This is done in this v4 of the patch.

There's no docs for the added Decimal.__format__ kwargs, since I want some 
comments on that change first.

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Added file: http://bugs.python.org/file38728/issue23602v4.patch

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[issue23602] Implement __format__ for Fraction

[Stefan Behnel]

Stefan Behnel added the comment:

Meaning, something like this should work:

x = (nom * 10**(prec+1)) // den
if x % 10  5:
   x = x // 10
else:
   x = x // 10 + 1
print('%s.%s' % (x[:-prec], x[-prec:]))

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[issue23602] Implement __format__ for Fraction

[Stefan Behnel]

Stefan Behnel added the comment:

Or, speaking of division with remainder:

n, r = divmod(nom * 10**prec, den)
if r * 5 = den:
n += 1
x = str(n)
print('%s.%s' % (x[:-prec], x[-prec:]))

... minus the usual off-by-one that the tests would quickly find :)

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[issue23602] Implement __format__ for Fraction

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

I think that Decimal is not needed for Fraction.__format__ (and I'm not sure 
that issue23602v4.patch is correct). The correct way to format Fraction as 
fixed-precision decimal is to use Fraction.__round__() or similar algorithm. 
The implementation can look like:

f = self.__round__(prec)
i = int(f)
return '%d.%0*d' % (i, prec, abs(f - i) * 10**prec)

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[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

Tuomas Suutari added the comment:

On 29 March 2015 at 19:54, Serhiy Storchaka wrote:
 I think that Decimal is not needed for Fraction.__format__ (and I'm not sure 
 that issue23602v4.patch is correct).

Of course it's not needed. I'm using it to avoid re-implementing all
the various formatting variations that can be controlled with the
fill/align/sign/width/,/precision/type parameters
(https://docs.python.org/3/library/string.html#formatspec). IMHO those
should be supported as they are with floats and Decimals.

 The correct way to format Fraction as fixed-precision decimal is to use 
 Fraction.__round__() or similar algorithm. The implementation can look like:

 f = self.__round__(prec)
 i = int(f)
 return '%d.%0*d' % (i, prec, abs(f - i) * 10**prec)

Why this would be more correct than delegating the rounding (and
formatting) to Decimal.__format__? (Then we just have to make sure
that we have enough precision in the decimal context we're operating
in. That's what I got wrong in the previous round.)

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[issue23602] Implement __format__ for Fraction

[Stefan Behnel]

Stefan Behnel added the comment:

But these parameters could also be partly delegated to normal string (not 
number) formatting, right?

One of the advantages of not depending on Decimal is, well, to not depend on 
Decimal, which is a rather uncommon dependency when using Fractions in an 
application.

I think it could avoid some more calculations to first multiply the nominator 
by 10**prec, then round(), int() and str() the result, and then split the 
string at -prec.

BTW, if division with remainder wasn't (sadly) linear time, that would 
definitely be the most beautiful algorithm here. :)

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[issue23602] Implement __format__ for Fraction

[Wolfgang Maier]

Wolfgang Maier added the comment:

Initially, I also thought that this should be addressable with 
Fraction.__round__ or an optimized variation of it, but Tuomas is right that it 
gets complicated by the fact that you need to cope with the different format 
specifiers and not all of them fit the algorithm.
In particular, scientific notation poses a problem because it may require a lot 
more precision in the calculation than the one finally used for formatting. 
Consider:

 float(round(Fraction(4, 27000), 6))
0.000148

but
 format(4/27000, 'e')
'1.481481e-04'

Trying to deal with this in pure Python quickly slows your code (at least a few 
naive attempts of mine) unacceptably compared to Tuomas' patch.

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[issue23602] Implement __format__ for Fraction

[Wolfgang Maier]

Wolfgang Maier added the comment:

 Decimal formatting intentionally differs from float formatting, see #23460.

I see. Thanks for the pointer. What about the missing zero in the exponent ?

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[issue23602] Implement __format__ for Fraction

[Stefan Krah]

Stefan Krah added the comment:

The zero isn't missing. :)  We are following 
http://speleotrove.com/decimal/decarith.html, with thousands of test cases.

We could decide to do something special for g, but there are good reasons not 
to do that.

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[issue23602] Implement __format__ for Fraction

[Stefan Krah]

Stefan Krah added the comment:

Decimal formatting intentionally differs from float formatting, see #23460.

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[issue23602] Implement __format__ for Fraction

[Wolfgang Maier]

Wolfgang Maier added the comment:

actually, I'm not sure whether formatting Decimals gives correct output under 
all conditions (correctly rounded yes, but maybe not formatted correctly?).

compare:

 format(float('1.481e-6'),'.3g')
'1.48e-06'

 format(Decimal('1.481e-6'),'.3g')
'0.0148'

 format(float('1.481e-7'),'.3g')
'1.48e-07'

 format(Decimal('1.481e-7'),'.3g')
'1.48e-7'

So with the 'g' specifier the switch between floating point and scientific 
notation seems to be broken.
Also note the slightly different formatting of the exponent: the leading zero 
is missing for all specifiers, i.e. 'g', 'G', 'e', 'E'.

Are these bugs ?

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[issue23602] Implement __format__ for Fraction

[Stefan Behnel]

Stefan Behnel added the comment:

Absolutely. Fractions are all about exact calculations, much more so than 
Decimals. So the formatting output should be as accurate as requested or 
possible (well, excluding infinity).

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[issue23602] Implement __format__ for Fraction

[Mark Dickinson]

Mark Dickinson added the comment:

[Eric]

 I'm not sure it needs fixing...

Hmm.  We've gone to some lengths to make sure that we get correctly-rounded 
results for formatting of Decimal and float types, as well as to make sure that 
operations like converting a Fraction to a float are correctly rounded.  It 
would be disappointing if the result of formatting a Fraction wasn't correctly 
rounded, and I'd personally consider it a bug.

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[issue23602] Implement __format__ for Fraction

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 from fractions import Fraction as F
 format(F(4, 27), 'f')
'0.1481481'
 format(F(4, 27), '.1f')
'0.2'

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[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

Tuomas Suutari added the comment:

Martin Panter wrote:
 Regarding sharing fractions._parse_format_specifier(), perhaps have a look at 
 _pydecimal._parse_format_specifier()

I did find that, but since it was a private function in private
module, I was unsure if I can use it here. The _pydecimal one parser
also does more stuff that I need.

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[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

Tuomas Suutari added the comment:

Version 3 of the patch. Changes to v2:
 * Use raw-strings for the regexps.
 * Make the specifier regexp more robust with \A, \Z and re.DOTALL.
 * Add more test cases; especially g and e formats and negative fractions.

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[issue23602] Implement __format__ for Fraction

[Serhiy Storchaka]

Changes by Serhiy Storchaka storch...@gmail.com:


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assignee:  - serhiy.storchaka
priority: normal - low

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[issue23602] Implement __format__ for Fraction

[Eric V. Smith]

Eric V. Smith added the comment:

I'm not sure it needs fixing: it follows from the definition of using 
Decimal(num) / Decimal(denom). Plus, it's controllable with a decimal context:

 from decimal import localcontext
 with localcontext() as ctx:
...   ctx.prec = 100
...   format(F(1, 3), '.30f')
...
'0.33'


For all of the tests, I suggest using format(value, str) instead of 
''.format(value). It more directly tests Fraction.__format__.

In general I think adding Fraction.__format__ is a good idea, and I think 
converting to Decimal is reasonable for the specified codes. My only question 
is what to do when natively formatting Fractions themselves. We might want to 
support field widths, padding, etc.

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[issue23602] Implement __format__ for Fraction

[Martin Panter]

Martin Panter added the comment:

I’ve never actually used the Fraction class, but I doubt its behaviour should 
depend on whatever settings are in the current decimal context. Maybe you can 
extract the precision out of the format string, and base the internal decimal 
object on that.

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[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

Tuomas Suutari added the comment:

Serhiy Storchaka wrote:
 from fractions import Fraction as F
 format(F(1, 3), '.30f')
 '0.00'

Good catch! I'll try to fix this and add some more test cases.

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[issue23602] Implement __format__ for Fraction

[Martin Panter]

Martin Panter added the comment:

Regarding sharing fractions._parse_format_specifier(), perhaps have a look at 
_pydecimal._parse_format_specifier()

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[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

Tuomas Suutari added the comment:

Eric V. Smith wrote:
 I'm not sure it needs fixing: it follows from the definition of using 
 Decimal(num) / Decimal(denom). Plus, it's controllable with a decimal context:

Hmm... Even though it's tempting to agree with you and just ignore the
precision bug, but to be honest I have to agree with Martin Panter
here. Depending on the current decimal context is not the way of
Least Surprise when formatting Fractions.

 For all of the tests, I suggest using format(value, str) instead of 
 ''.format(value). It more directly tests Fraction.__format__.

I agree. Will change those.

 In general I think adding Fraction.__format__ is a good idea, and I think 
 converting to Decimal is reasonable for the specified codes. My only question 
 is what to do when natively formatting Fractions themselves. We might want 
 to support field widths, padding, etc.

Thanks! Actually I already tried to support field widths, padding and
such. (See the test cases.) Or what do you mean?

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[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

Tuomas Suutari added the comment:

Here's the next round of the patch.

For formatting fractions with any given precision I had to parse the precision 
from format specifier and at this point it seemed easier to just create a 
general parser for the Format Specification Mini-Language.  In this patch it is 
implemented in fractions._parse_format_specifier function, but maybe this kind 
of general function should be moved to better place and be documented and 
exported. What do you think?

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[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

New submission from Tuomas Suutari:

Since Decimal supports __format__, it would be nice that Fraction did too.

--
components: Library (Lib)
messages: 237460
nosy: tuomas.suutari
priority: normal
severity: normal
status: open
title: Implement __format__ for Fraction
versions: Python 3.5

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[issue23602] Implement __format__ for Fraction

[Ezio Melotti]

Changes by Ezio Melotti ezio.melo...@gmail.com:


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nosy: +eric.smith, ezio.melotti, mark.dickinson, rhettinger, skrah
stage:  - patch review
type:  - enhancement

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[issue23602] Implement __format__ for Fraction

[Tuomas Suutari]

Tuomas Suutari added the comment:

Here's a patch that adds Fraction.__format__ implementation, test cases and 
documentation.

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Added file: http://bugs.python.org/file38378/issue23602.patch

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[issue23602] Implement __format__ for Fraction

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

 from fractions import Fraction as F
 format(F(1, 3), '.30f')
'0.00'

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[issue23602] Implement __format__ for Fraction

[Martin Panter]

Changes by Martin Panter vadmium...@gmail.com:


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