Modifying sometype.__dict__

You can't directly set an attribute of a builtin type

int.real = 69

TypeError: cannot set 'real' attribute of immutable type 'int'

You also can't modify __dict__

int.__dict__["real"] = 69

TypeError: 'mappingproxy' object does not support item assignment

Mappingproxy provides a read-only proxy for mapping, which prevents modification. However, you can now bypass that using the bypassmappingproxy module

(it is written in pure python)

from bypassmappingproxy as bmp

assert (1).real == 1
bmp.type_set(int, "real", 69)
assert (1).real == 69

reference