Numerical values of signal numbers are standardized. signal.signal(15, handler) is valid. SIGTERM is just a handy name for the constant 15.
But SIG_DFL and SIG_IGN are not integers in C (they are special pointers), and it is not declared anywhere that they should be integers in Python. The code that converts them to Python integer is implementation depending (PyLong_FromVoidPtr). Handcrafted SIG_DFL and SIG_IGN are broken by design, because the signal module uses identity comparison for SIG_DFL and SIG_IGN.