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Thread: [patch] PL/Python is too lossy with floats

[patch] PL/Python is too lossy with floats

From
Marko Kreen
Date:
PL/Python uses str(v) to convert float data, but is lossy
by design.  Only repr(v) is guaranteed to have enough
precision to make floats roundtrip properly:

  https://docs.python.org/2/library/functions.html#func-repr
  https://docs.python.org/2/library/functions.html#str

Example:

  $ python
  >>> repr(100100100.654321)
  '100100100.654321'
  >>> str(100100100.654321)
  '100100100.654'

Attached patch uses PyObject_Repr() for float data.

As it's annoying-to-debug problem and the patch is simple,
perhaps it's worth backporting?

--
marko
Attachment

Re: [patch] PL/Python is too lossy with floats

From
Peter Eisentraut
Date:
On 3/3/15 9:32 AM, Marko Kreen wrote:
> PL/Python uses str(v) to convert float data, but is lossy
> by design.  Only repr(v) is guaranteed to have enough
> precision to make floats roundtrip properly:

committed

Re: [patch] PL/Python is too lossy with floats

From
Marko Kreen
Date:
On Wed, Mar 11, 2015 at 9:49 PM, Peter Eisentraut <peter_e@gmx.net> wrote:
> On 3/3/15 9:32 AM, Marko Kreen wrote:
>> PL/Python uses str(v) to convert float data, but is lossy
>> by design.  Only repr(v) is guaranteed to have enough
>> precision to make floats roundtrip properly:
>
> committed

In 9.3 and before, numeric is converted to float in PL/Python.

Please reconsider backporting.

-- 
marko

Re: [patch] PL/Python is too lossy with floats

From
Peter Eisentraut
Date:
On 6/1/15 5:34 AM, Marko Kreen wrote:
> On Wed, Mar 11, 2015 at 9:49 PM, Peter Eisentraut <peter_e@gmx.net> wrote:
>> On 3/3/15 9:32 AM, Marko Kreen wrote:
>>> PL/Python uses str(v) to convert float data, but is lossy
>>> by design.  Only repr(v) is guaranteed to have enough
>>> precision to make floats roundtrip properly:
>>
>> committed
> 
> In 9.3 and before, numeric is converted to float in PL/Python.
> 
> Please reconsider backporting.

It's been like this forever, so I don't think it's appropriate to
backpatch this.

Re: [patch] PL/Python is too lossy with floats

From
Tom Lane
Date:
Peter Eisentraut <peter_e@gmx.net> writes:
> On 6/1/15 5:34 AM, Marko Kreen wrote:
>> Please reconsider backporting.

> It's been like this forever, so I don't think it's appropriate to
> backpatch this.

http://www.postgresql.org/docs/devel/static/plpython-data.html
states in so many words that floats are converted with str().
Presumably this needs an update in HEAD, but I would say that
the current behavior is per specification in all back branches.
        regards, tom lane