The only thing that is a bit strange is the behavior of something like the following. I understand why this happens, but this may not be the most intuitive behavior for some users.

font.math.MathLeadingDeviceTable[10] = 1
a = font.math.MathLeadingDeviceTable
print(font.math.MathLeadingDeviceTable[10]) # 1
print(a[10]) # 1
print('')

a[10] = 2
print(font.math.MathLeadingDeviceTable[10]) # 2
print(a[10]) # 2
print('')

font.math.SpaceAfterScriptDeviceTable = a
a[10] = 3
print(font.math.MathLeadingDeviceTable[10]) # 3
print(a[10]) # 3
print(font.math.SpaceAfterScriptDeviceTable[10]) # 2

(a = font.math.*DeviceTable by reference and font.math.*DeviceTable = a by value)

I'm not sure if this can or should be changed. However, it's fine for me to merge it with this behavior if there is no easy alternative.

I think we can resolve this ambiguity by forcing user into explicit copy:

a = font.math.MathLeadingDeviceTable
a[10] = 3
font.math.SpaceAfterScriptDeviceTable = a # Error: direct assignment to another device table not allowed
font.math.SpaceAfterScriptDeviceTable = a.copy() # OK: data explicitly copied from a into another device table

Alternatively, we can just forbid copying into another device table altogether.

@skef, what do you think about the desired API?

I think we can resolve this ambiguity by forcing user into explicit copy:

What would be the disadvantage of always copying on assignment ourselves in this case? Would that lead to too much confusion about what was by value and what was by reference?

font.math.SpaceAfterScriptDeviceTable = a # Error: direct assignment to another device table not allowed
font.math.SpaceAfterScriptDeviceTable = a.copy() # OK: data explicitly copied from a into another device table

Is this even possible to differentiate at the C-API level (genuine question)? I would have thought both would just look the same internally.

Copy on assignment is inherently non-Pythonic, so the following would be confusing:

# User unknowingly creating a detached copy of device table
a = font.math.MathLeadingDeviceTable

# Assigning element inside a
a[10] = 1

# Expecting font.math.MathLeadingDeviceTable to change
print(font.math.MathLeadingDeviceTable[10]) # Not "1" here

For my copy()-based proposal the differentiation would technically be as follows:

# a is a FontForge-specific object PyFF_MathDeviceTable, which has no data,
# but only reference to font and specific device table inside it.
a = font.math.MathLeadingDeviceTable

# Assignment of PyFF_MathDeviceTable object to other device table fails
font.math.SpaceAfterScriptDeviceTable = a # Error: direct assignment to another device table not allowed

# a.copy() copies data into a plain dictionary like {10: 1, 12: -1},
# disconnected from the specific font and the specific device table
#
# Assignment of plain dictionary to other device table succeeds.
font.math.SpaceAfterScriptDeviceTable = a.copy() # OK: assignment of plain data to device table allowed

@mf2vec-dev, please review