Multithreading (Python)

Documentation

• threading — Thread-based parallelism

What’s a Thread?

• Mechanism for parallel code execution

• ⟶ more than one flow of control in the same address space

• Issues

  • Startup/shutdown

  • Sharing data: concurrent access, and protecting against

  • Communication between threads

Threads: The Pythonic Way

code/thread-pythonic.py

import threading
import time

def increment_background():
    global background_integer
    while True:
        print('background: value is', background_integer)
        time.sleep(0.5)
        background_integer += 1

background_integer = 0
t = threading.Thread(target=increment_background)
t.start()

foreground_integer = 0
while True:
    print('foreground: value is', foreground_integer)
    time.sleep(0.5)
    foreground_integer -= 1

$ python code/thread-pythonic.py
background: value is 0
foreground: value is 0
background: value is 1
foreground: value is -1
background: value is 2
foreground: value is -2
background: value is 3
foreground: value is -3
background: value is 4
...

Two threads exist, runnning in parallel

• Main thread (not explicitly created)

• Background integer incrementer (explicitly created)

Threads: The OO Way - Inheritance

• Inherit from threading.Thread

• Inheritance is a mechanism for late binding, brought to the world by languages that have no other mechanism

• In Python there is duck typing

• This method of starting a thread is equivalent to the pythonic method, but should not be used (trainer’s opinion only, though)

Anyway:

code/thread-inheritance.py

import threading
import time

class BackgroundIncrementer(threading.Thread):
    def run(self):
        global background_integer
        while True:
            print('background: value is', background_integer)
            time.sleep(0.5)
            background_integer += 1

background_integer = 0
t = BackgroundIncrementer()
t.start()

foreground_integer = 0
while True:
    print('foreground: value is', foreground_integer)
    time.sleep(0.5)
    foreground_integer -= 1

• Uncool: inheritance is not strictly Pythonic (at least not for such kinds of relationship)

• Uncool: people might not even know what inheritance is

And Program Termination?

Problem: threads continue running when main thread terminates (spoiler: daemon threads don’t)

code/program-termination-fails.py

import threading
import time

def increment_background():
    global background_integer
    while True:
        print('background: value is', background_integer)
        time.sleep(0.5)
        background_integer += 1

background_integer = 0
t = threading.Thread(target=increment_background)
t.start()

foreground_integer = 0
for _ in range(3):
    print('foreground: value is', foreground_integer)
    time.sleep(0.5)
    foreground_integer -= 1

$ python code/thread-program-termination.py
background: value is 0
foreground: value is 0
background: value is 1
foreground: value is -1
background: value is 2
foreground: value is -2  # <--- last notice of main thread
background: value is 3
background: value is 4
background: value is 5
background: value is 6
background: value is 7
background: value is 8
...   # only background thread running

Daemon Threads

Definition

• A program exits when only daemon threads are left

• The main thread is not a daemon thread

Attention

This definition is Python specific!

Corollary: if I only create daemon threads, then program exit is

fine.

code/program-termination-ok.py

import threading
import time

def increment_background():
    global background_integer
    while True:
        print('background: value is', background_integer)
        time.sleep(0.5)
        background_integer += 1

background_integer = 0
t = threading.Thread(target=increment_background,
                     daemon=True)    # <----- mark as daemon
t.start()

foreground_integer = 0
for _ in range(3):
    print('foreground: value is', foreground_integer)
    time.sleep(0.5)
    foreground_integer -= 1

Dependencies