Python Threads - Critical Section

Other people have given very nice definitions. Here's the classic example:

import threading
account_balance = 0 # The "resource" that zenazn mentions.
account_balance_lock = threading.Lock()

def change_account_balance(delta):
    global account_balance
    with account_balance_lock:
        # Critical section is within this block.
        account_balance += delta

Let's say that the += operator consists of three subcomponents:

• Read the current value
• Add the RHS to that value
• Write the accumulated value back to the LHS (technically bind it in Python terms)

If you don't have the with account_balance_lock statement and you execute two change_account_balance calls in parallel you can end up interleaving the three subcomponent operations in a hazardous manner. Let's say you simultaneously call change_account_balance(100) (AKA pos) and change_account_balance(-100) (AKA neg). This could happen:

pos = threading.Thread(target=change_account_balance, args=[100])
neg = threading.Thread(target=change_account_balance, args=[-100])
pos.start(), neg.start()

• pos: read current value -> 0
• neg: read current value -> 0
• pos: add current value to read value -> 100
• neg: add current value to read value -> -100
• pos: write current value -> account_balance = 100
• neg: write current value -> account_balance = -100

Because you didn't force the operations to happen in discrete chunks you can have three possible outcomes (-100, 0, 100).

The with [lock] statement is a single, indivisible operation that says, "Let me be the only thread executing this block of code. If something else is executing, it's cool -- I'll wait." This ensures that the updates to the account_balance are "thread-safe" (parallelism-safe).