fix string handling of DecimalElement

[heeplr]

This fixes two aspects:

1. ensure that __str__() always returns a string (this closes DecimalElement.__str__() doesn't always return string type #94)
2. provide a __float__() method so python string formatting doesn't fail with TypeError: must be real number, not DecimalElement

[JulienPalard]

Casting a Decimal to a float don't feel safe, precision can be lost.

I'd prefer forcing the type to be a Decimal in the __init__:

    self._value = None if value is None else Decimal(value)

and removing the __float__, that should work the same, without the precision loss.

Also you can just remove the __str__ as it's inheriting one from StringElement that is identical.

[heeplr]

@JulienPalard

you're absolutely right. I made the changes and it works as expected.

[JulienPalard]

In which part the __format__ is useful? Maybe in your Jinja template but not directly in drafthorse?

[heeplr]

In which part the __format__ is useful? Maybe in your Jinja template but not directly in drafthorse?

Yes, in my case it's the jinja template that directly reads from the drafthorse document. It produces

TypeError: unsupported format string passed to DecimalElement.__format__

I think what's going on is, that DecimalElement has no __format__() and doesn't inherit one. Then python uses the __format__() of the str class which doesn't support "%f".

But it's not limitied to jinja:

foo = DecimalElement("foo", "foo", value=5.0)
print(foo)           # wrongly prints ""
print(foo._value)    # correctly prints "5"

With that __format__() the print(foo) works correctly (for anything that needs formatted output of DecimalElement like jinja).

I see no other way to solve that for any application besides accessing DecimalElement._value directly.

[JulienPalard]

I see no other way to solve that for any application besides accessing DecimalElement._value directly.

OK your PR looks good to me.

But I'm not a contributor of the repo so I let the last word to those with the commit bit :)