Hi Jakob,

parse_constant has been changed as of d95e5add3ca4 to be called only on "-Infinity, Infinity, NaN":

        ``parse_constant``, if specified, will be called with one of the
        following strings: -Infinity, Infinity, NaN.
        This can be used to raise an exception if invalid JSON numbers
        are encountered.

And indeed, if you change your example to Infinity, it gets called.

That said, neither the 2.7 nor the dev docs reflect that. So it seems like a documentation bug to me.