`IPython.utils.ipstruct:Struct.setitem`

Signature

def __setitem__(self, key: str, value: Any)

Set an item with check for allownew.

s = Struct()
s['a'] = 10
s.allow_new_attr(False)
s['a'] = 10
s['a']
try:
    s['b'] = 20
except KeyError:
    print('this is not allowed')

✓

this is not allowed