On Thu, Mar 5, 2020 at 11:09 AM Todd <toddr...@gmail.com> wrote:
> On Thu, Mar 5, 2020 at 4:27 AM Steve Barnes <gadgetst...@live.co.uk>
> wrote:
>
>>
>> SNIP
>
>>
>> Wouldn’t it be possible to have something along the lines of:
>>
>>
>>
>> ```
>>
>> from decimal import TreatFloatsAsDecimal
>>
>> @TreatFloatsAsDecimal
>>
>> a = 0.1 # These are all now decimals
>>
>> b = 0.2
>>
>> c = 0.3
>>
>> a + b == c # This now works
>>
>> ``
>>
>>
>
> If you were going to do this you would probably need something like a
> context manager to control the scope, such as:
>
> with TreatFloatsAsDecimal:
> a = 0.1 # decimal
> b = 0.2 # decimal
> c = 0.3 # decimal
>
> d = 0.3 # float
>
> I have been interested in something like this in the context of numpy
> arrays, but I am not sure even that would even be possible under how the
> python language works, and if it was I figured it was too complicated to be
> worthwhile.
>
I like this example; I will refer to it below.
I really think that anyone proposing new syntax construct would really
benefit from being able to "play" with it, being able to write and run such
currently invalid Python syntax. As I alluded to in previous emails (and
as quite a few others did for years on this list), once you have a working
example of an import hook (or custom encoding), in some cases, it can
actually be quite straightforward to write the code needed to transform a
proposed syntax into valid Python code. So, in addition to my previous
example, I will give two more, copied verbatim from my terminal.
Note: these two examples have not been included in the ideas repository
(nor on the Pypi version). Also, I show code in a console, but it could be
in a script run via an import statement.
Example 1:
>>> from ideas.examples import decimal_math_d
>>> hook = decimal_math_d.add_hook()
>>> from ideas import console
>>> console.start()
Configuration values for the console:
source_init from ideas.examples.decimal_math_d
transform_source from ideas.examples.decimal_math_d
--------------------------------------------------
Ideas Console version 0.0.15. [Python version: 3.7.3]
~>> a = 0.1
~>> a
0.1
~>> b = 0.1 D
~>> b
Decimal('0.1')
~>> a + b
Traceback (most recent call last):
File "IdeasConsole", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'float' and
'decimal.Decimal'
~>> 1 + b
Decimal('1.1')
The actual code to do the required transformation is very short and, I
think, quite readable:
def transform_source(source, **kwargs):
"""Simple transformation: replaces any explicit float followed by
``D``
by a Decimal.
"""
tokens = token_utils.tokenize(source)
for first, second in zip(tokens, tokens[1:]):
if first.is_number() and "." in first.string and second == "D":
first.string = f"Decimal('{first.string}')"
second.string = ""
return token_utils.untokenize(tokens)
The second example (a fake context) was a bit trickier to write, but not
that difficult:
>>> from ideas.examples import decimal_math_with
>>> hook = decimal_math_with.add_hook()
>>> from ideas import console
>>> console.start()
Configuration values for the console:
source_init from ideas.examples.decimal_math_with
transform_source from ideas.examples.decimal_math_with
--------------------------------------------------
Ideas Console version 0.0.16. [Python version: 3.7.3]
~>> a = 1.0
~>> with float_as_Decimal:
... b = 0.1
... c = 0.2
... d = 0.3
...
~>> e = 0.4
~>> b + c == d
True
~>> b
Decimal('0.1')
~>> a, e
(1.0, 0.4)
~>> b, c, d
(Decimal('0.1'), Decimal('0.2'), Decimal('0.3'))
Caveat: doing source transformations prior to execution like I do is
definitely not as robust as having code parsed by a well-designed
grammar-based parser.
Cheers,
André
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