That patch sounds nice, I don't have to edit my Python distribution! We'll
have to do with this.
On Mon, Jun 1, 2015 at 7:03 PM, <python-ideas-request@python.org> wrote:
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Today's Topics:
1. Re: Python Float Update (Nick Coghlan)
2. Re: Python Float Update (Andrew Barnert)
3. Re: Python Float Update (Andrew Barnert)
4. Re: Python Float Update (Steven D'Aprano)
----------------------------------------------------------------------
Message: 1
Date: Tue, 2 Jun 2015 10:08:37 +1000
From: Nick Coghlan <ncoghlan@gmail.com>
To: Andrew Barnert <abarnert@yahoo.com>
Cc: python-ideas <python-ideas@python.org>
Subject: Re: [Python-ideas] Python Float Update
Message-ID:
<
CADiSq7fjhS_XrKe3QfF58hXdhLSSbX6NvsFZZKjRq-+OLOQ-eQ@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
But the basic idea can be extracted out and Pythonified:
The literal 1.23 no longer gives you a float, but a FloatLiteral, which
is either a subclass of float, or an unrelated class that has a __float__
method. Doing any calculation on it gives you a float. But as long as you
leave it alone as a FloatLiteral, it has its literal characters available
for any function that wants to distinguish FloatLiteral from float, like
On 2 Jun 2015 08:44, "Andrew Barnert via Python-ideas"
<python-ideas@python.org> wrote:
the Decimal constructor.
The problem that Python faces that Swift doesn't is that Python doesn't
use static typing and implicit compile-time conversions. So in Python,
you'd be passing around these larger values and doing the slow conversions
at runtime. That may or may not be unacceptable; without actually building
it and testing some realistic programs it's pretty hard to guess.
Joonas's suggestion of storing the original text representation passed
to the float constructor is at least a novel one - it's only the idea
of actual decimal literals that was ruled out in the past.
Aside from the practical implementation question, the main concern I
have with it is that we'd be trading the status quo for a situation
where "Decimal(1.3)" and "Decimal(13/10)" gave different answers.
It seems to me that a potentially better option might be to adjust the
implicit float->Decimal conversion in the Decimal constructor to use
the same algorithm as we now use for float.__repr__ [1], where we look
for the shortest decimal representation that gives the same answer
when rendered as a float. At the moment you have to indirect through
str() or repr() to get that behaviour:
from decimal import Decimal as D
1.3
1.3
D('1.3')
Decimal('1.3')
D(1.3)
Decimal('1.3000000000000000444089209850062616169452667236328125')
D(str(1.3))
Decimal('1.3')
Cheers,
Nick.
[1] http://bugs.python.org/issue1580
------------------------------
Message: 2
Date: Mon, 1 Jun 2015 18:27:32 -0700
From: Andrew Barnert <abarnert@yahoo.com>
To: Nick Coghlan <ncoghlan@gmail.com>
Cc: python-ideas <python-ideas@python.org>
Subject: Re: [Python-ideas] Python Float Update
Message-ID: <EBB58361-19F4-4275-B6A6-E5AF2F77EF9C@yahoo.com>
Content-Type: text/plain; charset=us-ascii
On Jun 1, 2015, at 17:08, Nick Coghlan <ncoghlan@gmail.com> wrote:
On 2 Jun 2015 08:44, "Andrew Barnert via Python-ideas"
<python-ideas@python.org> wrote:
But the basic idea can be extracted out and Pythonified:
The literal 1.23 no longer gives you a float, but a FloatLiteral, which
is either a subclass of float, or an unrelated class that has a __float__
method. Doing any calculation on it gives you a float. But as long as you
leave it alone as a FloatLiteral, it has its literal characters available
for any function that wants to distinguish FloatLiteral from float, like
the Decimal constructor.
The problem that Python faces that Swift doesn't is that Python doesn't
use static typing and implicit compile-time conversions. So in Python,
you'd be passing around these larger values and doing the slow conversions
at runtime. That may or may not be unacceptable; without actually building
it and testing some realistic programs it's pretty hard to guess.
Joonas's suggestion of storing the original text representation passed
to the float constructor is at least a novel one - it's only the idea
of actual decimal literals that was ruled out in the past.
I actually built about half an implementation of something like Swift's
LiteralConvertible protocol back when I was teaching myself Swift. But I
think I have a simpler version that I could implement much more easily.
Basically, FloatLiteral is just a subclass of float whose __new__ stores
its constructor argument. Then decimal.Decimal checks for that stored
string and uses it instead of the float value if present. Then there's an
import hook that replaces every Num with a call to FloatLiteral.
This design doesn't actually fix everything; in effect, 1.3 actually
compiles to FloatLiteral(str(float('1.3')) (because by the time you get to
the AST it's too late to avoid that first conversion). Which does actually
solve the problem with 1.3, but doesn't solve everything in general (e.g.,
just feed in a number that has more precision than a double can hold but
less than your current decimal context can...).
But it just lets you test whether the implementation makes sense and what
the performance effects are, and it's only an hour of work, and doesn't
require anyone to patch their interpreter to play with it. If it seems
promising, then hacking the compiler so 2.3 compiles to FloatLiteral('2.3')
may be worth doing for a test of the actual functionality.
I'll be glad to hack it up when I get a chance tonight. But personally, I
think decimal literals are a better way to go here. Decimal(1.20) magically
doing what you want still has all the same downsides as 1.20d (or implicit
decimal literals), plus it's more complex, adds performance costs, and
doesn't provide nearly as much benefit. (Yes, Decimal(1.20) is a little
nicer than Decimal('1.20'), but only a little--and nowhere near as nice as
1.20d).
Aside from the practical implementation question, the main concern I
have with it is that we'd be trading the status quo for a situation
where "Decimal(1.3)" and "Decimal(13/10)" gave different answers.
Yes, to solve that you really need Decimal(13)/Decimal(10)... Which
implies that maybe the simplification in Decimal(1.3) is more misleading
than helpful. (Notice that this problem also doesn't arise for decimal
literals--13/10d is int vs. Decimal division, which is correct out of the
box. Or, if you want prefixes, d13/10 is Decimal vs. int division.)
It seems to me that a potentially better option might be to adjust the
implicit float->Decimal conversion in the Decimal constructor to use
the same algorithm as we now use for float.__repr__ [1], where we look
for the shortest decimal representation that gives the same answer
when rendered as a float. At the moment you have to indirect through
str() or repr() to get that behaviour:
from decimal import Decimal as D
1.3
1.3
D('1.3')
Decimal('1.3')
D(1.3)
Decimal('1.3000000000000000444089209850062616169452667236328125')
D(str(1.3))
Decimal('1.3')
Cheers,
Nick.
[1] http://bugs.python.org/issue1580
------------------------------
Message: 3
Date: Mon, 1 Jun 2015 19:00:48 -0700
From: Andrew Barnert <abarnert@yahoo.com>
To: Andrew Barnert <abarnert@yahoo.com>
Cc: Nick Coghlan <ncoghlan@gmail.com>, python-ideas
<python-ideas@python.org>
Subject: Re: [Python-ideas] Python Float Update
Message-ID: <90691306-98E3-421B-ABEB-BA2DE05962C6@yahoo.com>
Content-Type: text/plain; charset=us-ascii
On Jun 1, 2015, at 18:27, Andrew Barnert via Python-ideas <
python-ideas@python.org> wrote:
On Jun 1, 2015, at 17:08, Nick Coghlan <ncoghlan@gmail.com> wrote:
On 2 Jun 2015 08:44, "Andrew Barnert via Python-ideas"
<python-ideas@python.org> wrote:
But the basic idea can be extracted out and Pythonified:
The literal 1.23 no longer gives you a float, but a FloatLiteral,
The problem that Python faces that Swift doesn't is that Python
doesn't use static typing and implicit compile-time conversions. So in
Python, you'd be passing around these larger values and doing the slow
conversions at runtime. That may or may not be unacceptable; without
actually building it and testing some realistic programs it's pretty hard
to guess.
Joonas's suggestion of storing the original text representation passed
to the float constructor is at least a novel one - it's only the idea
of actual decimal literals that was ruled out in the past.
I actually built about half an implementation of something like Swift's
LiteralConvertible protocol back when I was teaching myself Swift. But I
which is either a subclass of float, or an unrelated class that has a
__float__ method. Doing any calculation on it gives you a float. But as
long as you leave it alone as a FloatLiteral, it has its literal characters
available for any function that wants to distinguish FloatLiteral from
float, like the Decimal constructor.
think I have a simpler version that I could implement much more easily.
Basically, FloatLiteral is just a subclass of float whose __new__ stores
its constructor argument. Then decimal.Decimal checks for that stored
string and uses it instead of the float value if present. Then there's an
import hook that replaces every Num with a call to FloatLiteral.
This design doesn't actually fix everything; in effect, 1.3 actually
compiles to FloatLiteral(str(float('1.3')) (because by the time you get to
the AST it's too late to avoid that first conversion). Which does actually
solve the problem with 1.3, but doesn't solve everything in general (e.g.,
just feed in a number that has more precision than a double can hold but
less than your current decimal context can...).
But it just lets you test whether the implementation makes sense and
what the performance effects are, and it's only an hour of work,
Make that 15 minutes.
https://github.com/abarnert/floatliteralhack
and doesn't require anyone to patch their interpreter to play with it.
If it seems promising, then hacking the compiler so 2.3 compiles to
FloatLiteral('2.3') may be worth doing for a test of the actual
functionality.
I'll be glad to hack it up when I get a chance tonight. But personally,
I think decimal literals are a better way to go here. Decimal(1.20)
magically doing what you want still has all the same downsides as 1.20d (or
implicit decimal literals), plus it's more complex, adds performance costs,
and doesn't provide nearly as much benefit. (Yes, Decimal(1.20) is a little
nicer than Decimal('1.20'), but only a little--and nowhere near as nice as
1.20d).
Aside from the practical implementation question, the main concern I
have with it is that we'd be trading the status quo for a situation
where "Decimal(1.3)" and "Decimal(13/10)" gave different answers.
Yes, to solve that you really need Decimal(13)/Decimal(10)... Which
implies that maybe the simplification in Decimal(1.3) is more misleading
than helpful. (Notice that this problem also doesn't arise for decimal
literals--13/10d is int vs. Decimal division, which is correct out of the
box. Or, if you want prefixes, d13/10 is Decimal vs. int division.)
It seems to me that a potentially better option might be to adjust the
implicit float->Decimal conversion in the Decimal constructor to use
the same algorithm as we now use for float.__repr__ [1], where we look
for the shortest decimal representation that gives the same answer
when rendered as a float. At the moment you have to indirect through
str() or repr() to get that behaviour:
from decimal import Decimal as D
1.3
1.3
D('1.3')
Decimal('1.3')
D(1.3)
Decimal('1.3000000000000000444089209850062616169452667236328125')
D(str(1.3))
Decimal('1.3')
Cheers,
Nick.
[1] http://bugs.python.org/issue1580
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------------------------------
Message: 4
Date: Tue, 2 Jun 2015 11:58:09 +1000
From: Steven D'Aprano <steve@pearwood.info>
To: python-ideas@python.org
Subject: Re: [Python-ideas] Python Float Update
Message-ID: <20150602015809.GE932@ando.pearwood.info>
Content-Type: text/plain; charset=us-ascii
On Tue, Jun 02, 2015 at 10:08:37AM +1000, Nick Coghlan wrote:
It seems to me that a potentially better option might be to adjust the
implicit float->Decimal conversion in the Decimal constructor to use
the same algorithm as we now use for float.__repr__ [1], where we look
for the shortest decimal representation that gives the same answer
when rendered as a float. At the moment you have to indirect through
str() or repr() to get that behaviour:
Apart from the questions of whether such a change would be allowed by
the Decimal specification, and the breaking of backwards compatibility,
I would really hate that change for another reason.
At the moment, a good, cheap way to find out what a binary float "really
is" (in some sense) is to convert it to Decimal and see what you get:
Decimal(1.3)
-> Decimal('1.3000000000000000444089209850062616169452667236328125')
If you want conversion from repr, then you can be explicit about it:
Decimal(repr(1.3))
-> Decimal('1.3')
("Explicit is better than implicit", as they say...)
Although in fairness I suppose that if this change happens, we could
keep the old behaviour in the from_float method:
# hypothetical future behaviour
Decimal(1.3)
-> Decimal('1.3')
Decimal.from_float(1.3)
-> Decimal('1.3000000000000000444089209850062616169452667236328125')
But all things considered, I don't think we're doing people any favours
by changing the behaviour of float->Decimal conversions to implicitly
use the repr() instead of being exact. I expect this strategy is like
trying to flatten a bubble under wallpaper: all you can do is push the
gotchas and surprises to somewhere else.
Oh, another thought... Decimals could gain yet another conversion
method, one which implicitly uses the float repr, but signals if it was
an inexact conversion or not. Explicitly calling repr can never signal,
since the conversion occurs outside of the Decimal constructor and
Decimal sees only the string:
Decimal(repr(1.3)) cannot signal Inexact.
But:
Decimal.from_nearest_float(1.5) # exact
Decimal.from_nearest_float(1.3) # signals Inexact
That might be useful, but probably not to beginners.
--
Steve
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