Lesson 7 — Logical Operators

Estimated time: 60 minutes

Learning Objectives

By the end of this lesson you will be able to:

  1. Use and, or, not to combine multiple conditions
  2. Read and use truth tables
  3. Combine two buttons for richer control
  4. Build complex, real-world conditions

Concepts

Logical Operators

Logical operators combine multiple conditions into one:

  • and — True only if BOTH sides are True
    • “I’ll go to the park if it’s sunny AND it’s not raining.”
  • or — True if EITHER side is True (or both)
    • “I’ll stay home if I’m tired OR it’s raining.”
  • not — Flips True to False and False to True
    • “If NOT raining, I’ll go outside.”

Truth Tables

A truth table shows all possible inputs and outputs for a logical operator:

and:

A B A and B
True True True
True False False
False True False
False False False

or:

A B A or B
True True True
True False True
False True True
False False False

not:

A not A
True False
False True

Precedence of Logical Operators

not is applied first, then and, then or. Use brackets to be explicit:

# Without brackets (can be confusing):
if a or b and c:    # Means: a or (b and c)

# With brackets (clear):
if a or (b and c):  # Same, but readable
if (a or b) and c:  # Different meaning!

When in doubt, add brackets. It doesn’t hurt and makes your intent clear.

Common Mistake: if x == 1 or 2

This is a classic mistake:

# WRONG:
if x == 1 or 2:     # Python reads: (x == 1) or (2)
                    # (2) is always True! This condition is always True.

# CORRECT:
if x == 1 or x == 2:  # Both comparisons are explicit

Combining Buttons

With two buttons (A on GPIO 0, B on GPIO 14), you can detect four states:

a_pressed = button_a.value() == 0
b_pressed = button_b.value() == 0

if a_pressed and b_pressed:
    print("Both pressed!")
elif a_pressed and not b_pressed:
    print("Only A")
elif b_pressed and not a_pressed:
    print("Only B")
else:
    print("Neither")

Hardware Setup

Add a second button:

Button B Leg Connect To
Leg 1 GPIO 14
Leg 2 GND

Guided Walkthrough

Step 1: and, or, not in the REPL 

>>> True and True
True
>>> True and False
False
>>> False or True
True
>>> False or False
False
>>> not True
False
>>> not False
True
>>> x = 10
>>> x > 5 and x < 20
True
>>> x > 5 and x < 8
False
>>> x < 5 or x > 8
True

Step 2: Combining Conditions for the LED 

import machine, neopixel

pin = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin, 1)

temp = int(input("Temperature (°C): "))
humidity = int(input("Humidity (%): "))

# Comfortable: 18–25°C AND humidity 40–60%
if 18 <= temp <= 25 and 40 <= humidity <= 60:
    np[0] = (0, 255, 0)     # Green — comfortable
    print("Comfortable conditions!")
elif temp > 30 or humidity > 80:
    np[0] = (255, 0, 0)     # Red — uncomfortable
    print("Uncomfortable — too hot or too humid!")
else:
    np[0] = (255, 200, 0)   # Yellow — acceptable but not ideal
    print("Acceptable, but not ideal.")

np.write()

Notice the chained comparison: 18 <= temp <= 25. Python allows this! It means 18 <= temp AND temp <= 25.

Step 3: Two-Button LED Control

Wire both buttons (A = GPIO 0, B = GPIO 14), then run:

import machine, neopixel, time
from machine import Pin

pin48 = machine.Pin(48, machine.Pin.OUT)
np = neopixel.NeoPixel(pin48, 1)
button_a = Pin(0, Pin.IN, Pin.PULL_UP)
button_b = Pin(14, Pin.IN, Pin.PULL_UP)

print("Button controls:")
print("  A only = Red")
print("  B only = Blue")
print("  A + B  = Purple")
print("  None   = Off")
print("Ctrl+C to stop.")

while True:
    a = button_a.value() == 0   # True if pressed
    b = button_b.value() == 0

    if a and b:
        np[0] = (128, 0, 255)   # Purple
    elif a and not b:
        np[0] = (255, 0, 0)     # Red
    elif b and not a:
        np[0] = (0, 0, 255)     # Blue
    else:
        np[0] = (0, 0, 0)       # Off

    np.write()
    time.sleep(0.05)

Try all four combinations: neither button, only A, only B, both together.

Step 4: Complex Real-World Conditions 

# Simulate a security door access system
name = input("Enter name: ")
pin_code = input("Enter PIN: ")
is_vip = False   # Could be True for VIP users

has_name = name.lower() in ["alice", "bob", "charlie"]
has_correct_pin = pin_code == "1234"
time_is_okay = True   # Imagine this checks business hours

# Access rule:
# - VIP OR (correct name AND correct pin) AND it's business hours
if (is_vip or (has_name and has_correct_pin)) and time_is_okay:
    print("Access GRANTED")
else:
    print("Access DENIED")
    if not has_name:
        print("  — Name not on list")
    if not has_correct_pin:
        print("  — Wrong PIN")

Did you know? The and, or, not operators you’re learning here are the same operations used in electronic circuit design. A computer’s CPU is built from billions of tiny circuits called logic gates that each perform AND, OR, and NOT operations.

Challenges

⭐ Core

Write a “safe driving” alert system. Ask for current speed (as an int) and whether it’s raining (enter “yes” or “no”). Print a warning if: speed > 120 OR (speed > 80 and it's raining). Otherwise print “Driving safely”.

⭐⭐ Extension

Create a password system that requires both a correct numeric PIN (4 digits) and a correct keyword. If either is wrong, print which one failed. Only turn the LED green if both are correct.

⭐⭐⭐ Stretch

Using two buttons, implement a “combination lock”:

  • The lock opens ONLY if button A is pressed, fully released, then button B is pressed
  • Any other sequence (both at once, B first, etc.) keeps it locked
  • Show green when unlocked, red when locked
  • After unlocking, it automatically re-locks after 3 seconds

Common Mistakes & Debugging

if a == 1 or 2: is always True Python evaluates this as (a == 1) or (2). The value 2 is truthy — it’s always True. Write if a == 1 or a == 2:.

not applies to one value by default not a and b means (not a) and b. If you want not (a and b), use brackets.

Confusing and and or Use the English translation as a check: “a and b” means “both A AND B must be true”. If your sentence uses “or”, use or.

Short-circuit evaluation Python doesn’t always evaluate the right side of and/or. With a and b, if a is False, Python skips b entirely. This matters if b has a side effect or could error.

Key Vocabulary

Term Definition
logical operator and, or, not — combines boolean conditions
truth table A table showing all possible inputs and the resulting output of a logical operation
and True only if both conditions are True
or True if at least one condition is True
not Flips True to False and False to True
chained comparison 18 <= x <= 25 — Python’s shorthand for x >= 18 and x <= 25
short-circuit evaluation Python stops evaluating and if the left side is False; stops evaluating or if the left side is True

Copyright © Paul Baumgarten.