> It's worth explaining why mutable defaults are bad
They can also be good. Here's an example from the Reddit discussion, showing how a mutable default can be used to very neatly and cleanly add memorization to a function:
def fib(n, m={}):
if n not in m:
m[n] = 1 if n < 2 else fib(n-1) + fib(n-2)
return m[n]
They can also be good. Here's an example from the Reddit discussion, showing how a mutable default can be used to very neatly and cleanly add memorization to a function: