Re: How do I convert numpy NaN objects to SQL nulls?

On Thu, Aug 20, 2015 at 8:34 PM, Gregory Arenius <gregory@arenius.com> wrote:
> So, I'm trying to make a custom adapter to convert np.NaN to SQL null but
> everything I've tried results in the same NaN strings being inserted in the
> database.
>
> The code I'm currently trying is:
>
> def adapt_nans(null):
>     a = adapt(None).getquoted()
>     return AsIs(a)
>
> register_adapter(np.NaN, adapt_nans)
>
> I've tried a number of variations along this theme but haven't had any luck.

register_adapter takes a type as argument. np.NaN is a value of type
float instead:

In [3]: type(np.NaN)
Out[3]: float

so you should really customize the float adapter to do what you want.

    def nan_to_null(f):
        if f is np.NaN:
            return psycopg2.extensions.AsIs('NULL')
        else:
            return psycopg2.extensions.Float(f)

    psycopg2.extensions.register_adapter(float, nan_to_null)

    >>> print cur.mogrify("select %s, %s", [10.0, np.NaN])
    select 10.0, NULL

Note: the above is for clarity. For sake of
premature-micro-optimization I would actually write the adapter as:

    def nan_to_null(f,
            _NULL=psycopg2.extensions.AsIs('NULL'),
            _NaN=np.NaN,
            _Float=psycopg2.extensions.Float):
        if f is not _NaN:
            return _Float(f)
        return _NULL

-- Daniele