[Jewett, Jim J]

At the C level,

PyDict_Update(old, new)

is just a shortcut for

override=1 PyDict_Merge(old, new, override)

It is today, but that's a relatively new internal ability; IIRC, Guido added it to help with the implementation of new-style classes.

C extensions can use PyDict_Merge directly with override=0, which keeps the current value in case of duplicate keys.

Yes.

Python code can only use update, which means that override is always true.

Yes.

Is there a good reason this?

Dunno -- define "good" <0.5 wink>.