We could understand missing: when a key is not found in a dictionary, if it provides the dunder missing special method, then that method will be called and whatever is returned, will be the result of key access:
#!python
class A(dict):
def __missing__(self, key): return 42
a = A()
print(a[10])
Originally reported by: Claudiu Popa (BitBucket: PCManticore, GitHub: PCManticore)
We could understand missing: when a key is not found in a dictionary, if it provides the dunder missing special method, then that method will be called and whatever is returned, will be the result of key access: