[Python-ideas] Python Float Update

[Andrew Barnert]

Sorry, I accidentally sent that before it was done...

Sent from my iPhone

> On Jun 1, 2015, at 15:30, Andrew Barnert via Python-ideas <python-ideas at python.org> wrote:
> 
>> On Jun 1, 2015, at 08:12, Joonas Liik <liik.joonas at gmail.com> wrote:
>> 
>> I'm sorry..
>> 
>> what i meant was not a literal that results in a Decimal, what i meant was a special literal proxy object that usualyl acts like a float except you can ask for its original string form.
> 
> This is essentially what I was saying with new "literal constant" types. Swift is probably the most prominent language with this feature. http://nshipster.com/swift-literal-convertible/ is a good description of how it works. Many of the reasons Swift needed this don't apply in Python. For example, in Swift, it's how you can build a Set at compile time from an ArrayLiteral instead of building an Array and converting it to Set at compile time. Or how you can use 0 as a default value for a non-integer type without getting a TypeError or a runtime conversion. Or how you can build an Optional that acts like a real ADT but assign it nil instead of a special enumeration value. Or how you can decode UTF-8 source text to store in UTF-16 or UTF-32 or grapheme-cluster at compile time. And so on.

Anyway, my point was that the Swift feature is complicated, and has some controversial downsides (e.g., see the example at the end of silently using a string literal as if it were a URL by accessing an attribute of the NSURL class--which works given the Smalltalk-derived style of OO, but many people still find it confusing). 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.

The advantage of C++-style user-defined literal suffixes is that the absence of a suffix is something the compiler can see, so 1.23d might still require a runtime call, but 1.23 just is compiled as a float constant the same as it's been since Python 1.x.