check to see whether an object exists in dict

Question

a = {1 : 4}
print(4 in a)     # case 1: False

a = {1 : 's'}
print('s' in a)   # case 2: False

a = {1 : None}
print(None in a)  # case 3: False

a = {1: True}
print(True in a)  # case 4: True

so my question is why except the forth case all returns False ?

Answer 1

Membership testing is done against dictionaries' "keys", not values. For values you can do:

a = {1: 4}
print(4 in a.values())

But remember that it does a linear search as opposed to keys which is O(1).

For last case:

bool is a subclass of int and True is equal to 1. (Their hashes are of course the same)

Basically the same effect as:

s = {1, True}
print(len(s)) # 1

and

a = {1: 10}
print(a[True]) # 10