HTML5 number input type not working for DecimalField with big decimal_places

[aleksey.kladov@…]

DecimalFiled's widget is a NumberInput with step attribute equal to 10^{-decimal_placecese}. ​https://github.com/django/django/blob/stable/1.6.x/django/forms/fields.py#L374).

If decimal_places is too big(the exact value for big should be found here ​http://www.w3.org/TR/2012/WD-html5-20121025/common-microsyntaxes.html#rules-for-parsing-floating-point-number-values) the step value parses to 0, which is not valid (​http://www.w3.org/TR/2012/WD-html5-20121025/common-input-element-attributes.html#attr-input-step). It is not good in itself, but it also causes surprising and unwanted behaviour.

For example in Google Chrome input with type="number" and step="0.0000000000000000000000000000001" accepts only integer values!

A possible fix is to add optional step argument to DecimalField and, and if it is not provided just use "any".

[Simon Charette]

We shouldn't add a step argument to DecimalField since it only concern the widget.

Do you know the exact value for big?

We could attempt defaulting step to the current behavior when decimal_place < big and fallback to any just how ​`FloatField` does.

[anonymous]

Do you know the exact value for big?

No, I'm not an IEEE 754 expert =(

We could attempt defaulting step to the current behavior when decimal_place < big and fallback to any just how ​FloatField does.

I think that step > 0.001 is anyway useless in general. May be just .setdefault("step", "any") exactly as in FloatField? In the face of ambiguity, refuse the temptation to guess.

[Simon Charette]

It ​seems to work until 1^-18.

IMHO ​the float parsing algorithm is quite unambiguous and the default step attribute is still overridable. I think we should be safe assuming our step generation mechanism works for decimal_places <= 18.

[Aleksey Kladov]

the default step attribute is still overridable

Not really, look again at ​https://github.com/django/django/blob/stable/1.6.x/django/forms/fields.py#L374

Even if I do

answer = fields.DecimalField(max_digits=50,
                             decimal_places=25,
                             widget=NumberInput(attrs={"step": "any"}))

the step gets overriden.

[Simon Charette]

I guess we should use setdefault here instead of direct assignment.

[Aleksey Kladov]

setdefult defenetelly should be there.

However I think that it's still worth to check that decimal_places <= 18 and use any as a default otherwise(or just always use any).

[Claude Paroz]

What's definitely a bug is that step is not overridable.

As of the number of decimal places, it's browser-dependant, and I'm less enthousiast to do something about it.

[Simon Charette]

I'll write a patch and fix tests for the setdefault behavior.

[Aleksey Kladov]

I would be completely happy if I could override step. But it's because I know that I have to override it. Let me try to convince you for the last time that guessing step is not a good idea. There will be three points.

1. Suppose I know nothing about this issue and just wrote in one of my forms number = fields.DecimalField(max_digits=50, decimal_places=25). I use firefox and the form is working OK in it. Moreover, tests are working too. However, very soon I get a bug report that it's impossible to use non-integer numbers in Chrome! So I have to look into the source of the page, notice suspicious step="0.0000000000000000000000000001", and find out that it is the reason of strange behaviour. Next, I have to look inside django's source, to find out, from where this step emerges. And only then I realize that I need to override it.

2. Current default can produce invalid html.

The step attribute, if specified, must either have a value that is a valid floating-point number that parses to a number that is greater than zero, or must have a value that is an ASCII case-insensitive match for the string "any".

3. If I close my eyes, forget about everything in the universe and ask myself, "what is the most reasonable default for step?" I hear a voice whispering "any".

Also, this issue can show more often than it seems. 10^-18 is quit a little number, and it looks like most users won't use such a high precision. However, it is the DecimalField. And if someone is using a Decimal instead of Float, it means that he cares about high precision. So it's rather likely that he uses many decimal places. Also, python's default prec for decimals is 28 places!

[Simon Charette]

Added a patch that still sets step as default for max_digits <= 18 and 'any' in the other case.

[Aleksey Kladov]

This looks good. I can see only one very minor caveat. If decimal_places=0 then step is equal to any, while 1 seems more reasonable.

[Simon Charette]

I think I've found a middleground. Parsing works flawlessly in ​exponential form. Submitting an updated patch.

[Aleksey Kladov]

Looks perfect!

As a bonus, exponential form improves html readability.

[Claude Paroz]

RFC, but please either add a comment about the rationale or refer to this ticket. Thanks!

[Simon Charette <charette.s@…>]

In 415a36947cb4b8b61230698e13cca2df9400e245:

Fixed #20765 -- Set small values of step using exponential notation.

Browsers parse small factors of 10 as 0 under decimal notation.

Thanks to Trac alias matklad for the report and Claude Paroz for the review.

[Simon Charette <charette.s@…>]

In 9d3f7a21a337aa82d5c9ac5ef9a6a3888bff62a8:

[1.6.x] Fixed #20765 -- Set small values of step using exponential notation.

Browsers parse small factors of 10 as 0 under decimal notation.

Thanks to Trac alias matklad for the report and Claude Paroz for the review.

Backport of 415a36947c from master.