Code Smell 230 - Schrödinger Code

Your code is dead and alive

Maximiliano Contieri

Nov 04, 2023

TL;DR: Look carefully for race conditions

Problems

Principle of Least Surprise violation
Race Conditions

Solutions

Avoid race conditions
Avoid global variables
Use proper synchronization

Context

Schrödinger code is code that can be in two different states at the same time, but the state of the code is not determined until it is executed.

This can happen when the code contains a race condition, or when the code depends on the state of a global variable that can be changed by other threads or processes.

Sample Code

Wrong

import threading

cats_alive = 0

def thread_1():
  cats_alive += 1

def thread_2():
  cats_alive -= 1

if cats_alive == 0:
  feedThem()

# The value of cats_alive is indeterminate, 
# so the code can be in either of the two states:
#
# 1. cats_alive > 0 and feedThem() is called.
# 2. cats_alive <= 0 and feedThem() is not called.

Right

import threading

lock = threading.Lock()
cats_alive = 0

def thread_1():
  with lock:
    cats_alive += 1

def thread_2():
  with lock:
    cats_alive -= 1

if cats_alive > 0:
  feedThem()

# With the lock, the two threads cannot access 
# the `cats_alive` variable at the same time.
# This means that the value of `cats_alive` is always determined, 
# and the program will not exhibit Schrödinger code behavior.

Detection

[X] Manual

Make code reviews on concurrent code

Conclusion

To avoid Schrödinger code, avoid race conditions and avoid depending on the state of global variables that can be changed by other threads or processes.

If you need to use a global variable in your code, ensure it is correctly synchronized.