How might I trigger "Instance as master" from the API?

Scripting
6

I can make a new master with a weightValue with:

f = Glyphs.font
newMaster = f.masters[0].copy()
f.masters.append(newMaster)

…then separately, I can copy some of the glyphs with:

f = Glyphs.font

import copy

for glyph in f.glyphs:
    glyph.layers[-1] = glyph.layers[0].copy()
    if len(glyph.layers[0].components) > 0:
        glyph.layers[-1].components = copy.copy(glyph.layers[0].components)
    glyph.layers[-1].reinterpolate()

But somehow, these don’t work the same when run in the same script. Also, glyphs stop copying at l, don’t copy components, and I get this error:

Traceback (most recent call last):
  File "<string>", line 6, in <module>
  File "GlyphsApp/GlyphsApp/__init__.py", line 2002, in __setitem__
AttributeError: 'NoneType' object has no attribute 'id'

I may come back to this later. Thanks for any pointers in the meantime, if you see how I can make this work better!