Can you tell me what this is?

I think this might be the fruit of the Datura - if it is, it's poisonous.

Is it an avacado?

Almost certainly the fruit of the Datura &, as Bamboo says, it's poisonous as are all parts of this plant.

Has a lovely white trumpet flower.

A prickly pear?

Datura stramonium seed pod, beautiful trumpet white flower, nutty smell and all parts poisonous.
See plant in flower on my page with comments.

I hope this helps. If it doesn't, it'll give you something to read ;-)