Python 3 Object-Oriented Programming - Third Edition

Shared memory

The main problem with threads is also their primary advantage. Threads have access to all the program's memory and thus all the variables. This can too easily cause inconsistencies in the program state.

Have you ever encountered a room where a single light has two switches and two different people turn them on at the same time? Each person (thread) expects their action to turn the lamp (a variable) on, but the resulting value (the lamp) is off, which is inconsistent with those expectations. Now imagine if those two threads were transferring funds between bank accounts or managing the cruise control for a vehicle.

The solution to this problem in threaded programming is to synchronize access to any code that reads or (especially) ...