more references

a = 1
b = 3 - 2

a is b   # True

a and b are (coincidentally) references to the same int object with value 1.

a = a + 1
a is b    # False (b still points to the int(1))

In perl = defines or redefines. In python it associates. A variable name on the left of an = forgets everything it knew and points to the new object.

b += 1
a is b  #True
a = ['something', 'else']
b = a
print a  # ['something', 'else', 'entirely']

Ints are immutable, which makes passing ints by reference very close to passing by value.

Lists are mutable, so operations on one reference are reflected in others.