Everything

php

Make var_dump() use serialize_precision

php.internals

Nikita Popov

6 years ago

Hi internals,

https://github.com/php/php-src/pull/5172 changes var_dump() to use
serialize_precision instead of precision to dump floating-point numbers.

To recap: serialize_precision defaults to -1, which will print exactly as
many floating-point digits as are needed to represent the number
accurately. precision defaults to 14, which will print a shorter but
potentially inaccurate float representation.

The motivation here is that var_dump() is debugging functionality and
should print values as accurately as possible. The single most common bug
report we receive is some kind of variation on:

$sum = 0.1 + 0.2;
var_dump($sum); // float(0.3)
var_dump($sum == 0.3); // bool(false) WTF???

After this change, this would instead be:

$sum = 0.1 + 0.2;
var_dump($sum); // float(0.30000000000000004)
var_dump($sum == 0.3); // bool(false) Makes sense...

I have little hope that developers will suddenly start understanding
floating-point numbers, but at least this should reduce the amount of
confusion.

Does anyone see an issue with doing this change?

Regards,
Nikita

Andreas Heigl

6 years ago

Am 18.02.20 um 12:20 schrieb Nikita Popov:

> Hi internals,
>
> https://github.com/php/php-src/pull/5172 changes var_dump() to use
> serialize_precision instead of precision to dump floating-point numbers.
>
> To recap: serialize_precision defaults to -1, which will print exactly as
> many floating-point digits as are needed to represent the number
> accurately. precision defaults to 14, which will print a shorter but
> potentially inaccurate float representation.
>
> The motivation here is that var_dump() is debugging functionality and
> should print values as accurately as possible. The single most common bug
> report we receive is some kind of variation on:
>
>     $sum = 0.1 + 0.2;
>     var_dump($sum); // float(0.3)
>     var_dump($sum == 0.3); // bool(false) WTF???
>
> After this change, this would instead be:
>
>     $sum = 0.1 + 0.2;
>     var_dump($sum); // float(0.30000000000000004)
>     var_dump($sum == 0.3); // bool(false) Makes sense...
>
> I have little hope that developers will suddenly start understanding
> floating-point numbers, but at least this should reduce the amount of
> confusion.
>
> Does anyone see an issue with doing this change?

You mean apart from people now filing bugs how var_dump() can output
such a nonsensical number from such an easy equation? And that it again
shows that PHP is not a real programming language (unlike JavaScript)
and should never be used at all?

Nope ;-)

Cheers

Andreas

PS: I'd absolutely appreciate the change!!!

--
,,,
(o o)
+---------------------------------------------------------ooO-(_)-Ooo-+
| Andreas Heigl                                                       |
| mailto:andreas@heigl.org                  N 50°22'59.5" E 08°23'58" |
| http://andreas.heigl.org                       http://hei.gl/wiFKy7 |
+---------------------------------------------------------------------+
| http://hei.gl/root-ca                                               |
+---------------------------------------------------------------------+

Andrew Faulds

6 years ago

Nikita Popov wrote:

> Hi internals,
>
> https://github.com/php/php-src/pull/5172 changes var_dump() to use
> serialize_precision instead of precision to dump floating-point numbers.

Hi Nikita,

Thank you so much for doing this! I had wanted to do this for a long
time, and I actually implemented it and began updating tests a while
ago, but I never got round to finishing it.

It looks in that PR like you also update debug_zval_dump(), maybe that
should be mentioned?

The one that gave me pause before and now is print_r(). In principle, it
is just `print` but recursive (I assume that's whence the name comes),
but unfortunately some people use it for debugging, so there might be a
case for changing it. With that said, it's already a very bad choice for
debugging due to other things ("1", 1, 1.0 and TRUE look the same, "0",
0.0, and 0 look the same, "", FALSE and NULL look the same…) that
precision is the least of the developer's problems if they choose
print_r(), and it's also not in the spirit of "print but recursive". So,
it is probably unreasonable to change print_r() here. Maybe we should
put a massive red "DO NOT USE THIS FOR DEBUGGING" warning on its
documentation page…

One more thing, does var_export() use serialize_precision already? I
think it does but haven't checked.

Thanks!
Andrea