Demonstration of need for garbage collection in Python

gc.py

# Copyright (c) 2018 Bart Massey

# GC demo.

# Create a circular structure. The reference
# count of x and y would be 2: one for the
# variable and 1 for the internal reference.
x = [1]
y = [2]
y[0] = x
x[0] = y
# Python has special-case code for printing this.
print(x)

# Drop a reference. The reference count
# of y would go from 2 to 1.
y = None
print(x)

# Drop the other reference. The reference count of
# x would go from 2 to 1. The structure is now
# unreachable, but nothing will be freed until
# the garbage collector runs.
x = None