It is not possible to provide a dict as the input of a dict.update() method, for example:

d1 = {1:'a'}
d2 = {2:'b'}

d1.update(d2)

The exact error will vary depending on the type of the key. In this case, the error "'int' object is not iterable" is thrown. If the key was a string, then the error "ValueError: dictionary update sequence has the wrong length" is thrown. Both together make it clear that it tries to iterate the key object, looking for a list of pairs. This code does work correctly in Python 3.

A common way around this issue is to use the Python 2 style:

d3 = (d1, **d2)

This does not work in Python 3 for the given example, but it silently fails in micropython. Instead of throwing an error, the result of d3 will simply be d1. It would work in Python 3 if the keys were strings. However, again, in that case in micropython the result is simply that d3 will be the same as d1.

Only other solution I can think of for the moment is to just iterate over all items of the second dictionary and add them to the first dictionary.