Dictionaries

Question

Please help to state simple example for mutable and immutable object and its concept in python dictionaries.
I am not clear enough by reading the lessons and comments.

Answer 1

Mutable list example:
a = [1, 2, 3]
b = [4, 5, a]
print(a) #[1, 2, 3]
print(b) #[4, 5, [1, 2, 3]]
If you change a, you also changed b[2], and if you changed b[2], you also changed a, that's because b[2] is a, not just a copy of a, Python avoids copies.
b[2].append(6)
print(a) #[1, 2, 3, 6]
print(b) #[4, 5, [1, 2, 3, 6]]
Then what if you used a as a key in a dictionary?
If dictionaries supported mutable objects as keys:
d = {[1, 2, 3]: "klm", a: "nop"}
print(d) #{[1, 2, 3]: "klm", [1, 2, 3, 6]: "nop"}
If you changed a again:
a.pop()
print(a) #[1, 2, 3]
print(d) #{[1, 2, 3]: "klm", [1, 2, 3]: "nop"}
Now there would be 2 duplicate dictionary keys.
print(d[[1, 2, 3]]) #So would this print "klm" or "nop"?