I want to output floats in ENG format. Working with distance in micrometers, it is a bit annoying to see:
2.5e-5
.0003
instead of 
25e-6
300e-6
The solution I found was to redefine `iterencode` but that doesn't feel right.

I would like to see something similar to the "default" function that is easely modified.

Hello and thanks for the patch!  If you don’t get feedback, you could try the python-ideas mailing list to see if other people have the same need and if there’s already a solution for this.

Serhiy, would you mind refreshing my memory about json development?  I’m not sure whether it’s still supposed to follow the development of simplejson, or if it’s effectively a fork now.

May I close this? There is no discussion in python-dev/python-ideas.

I feel that this proposal is too minor. Stdlib shouldn't add option for such minor use case.

I think the less-minor issue, of which this is a small subset, is that JSONEncoder doesn't allow changing the behavior for default-serializable types at all. That means you can't choose to lose less information in round-trip serialization/deserialization, if that's what you want (e.g. there's no way to round-trip serialize a tuple with JSONEncoder, though it's trivial to do that for a set).