Python Decision Making

Python Decision Making

Decision making involves anticipating conditions that might occur during program execution and specifying actions based on whether those conditions are met.

Decision structures evaluate expressions, which result in a Boolean value: True or False. Based on this outcome, your program decides which block of code to execute.

The general flow of decision making often looks like this: Check a condition. If it’s true, perform Action A. If it’s false, perhaps check another condition or perform Action B.

Understanding Truthiness in Python

In Python, values are evaluated in a Boolean context (like in an if statement). The following are considered False:

• The boolean value False itself.
• The special object None.
• Numeric types equal to zero (e.g., 0, 0.0, 0j).
• Empty sequences (e.g., '', (), []).
• Empty mappings (e.g., {}).
• Objects from user-defined classes that implement a __bool__() or __len__() method returning False or 0.

All other values are generally considered True. This concept is often called “truthiness”.

Python provides the following types of decision-making statements:

Statement	Description
if statements	Executes a block of code only if a specific condition is True.
if...else statements	Executes one block of code if a condition is True and another block if it is False.
if...elif...else statements	Checks multiple conditions in sequence and executes the block corresponding to the first True condition. An optional else block runs if none of the conditions are True.
Nested if statements	You can place if, elif, or else statements inside the blocks of other decision-making statements.

Let’s explore these structures.

Basic if Statement Example

Here’s a simple example demonstrating an if statement:

#!/usr/bin/env python3

temperature = 35

# Use modern f-string formatting
if temperature > 30:
    print(f"Temperature is {temperature}. It's hot outside!")

print("Execution finished.")

When the above code is executed, it produces:

Temperature is 35. It's hot outside!
Execution finished.

if...elif...else Example

Using elif (else if) allows checking multiple conditions:

#!/usr/bin/env python3

score = 75

if score >= 90:
    grade = 'A'
elif score >= 80:
    grade = 'B'
elif score >= 70:
    grade = 'C'
elif score >= 60:
    grade = 'D'
else:
    grade = 'F'

print(f"Score: {score}, Grade: {grade}")

Output:

Score: 75, Grade: C

Single Statement Suites (One-Liners)

If the code block for an if, elif, or else clause consists only of a single statement, you can place it on the same line as the header. However, for readability (following PEP 8 guidelines), it’s generally recommended to use separate lines and indentation, especially for more complex logic.

Example of a one-line if clause:

#!/usr/bin/env python3

var = 100

# Note: idiomatic Python avoids parentheses around the condition
if var == 100: print("Value of expression is 100")

print("Goodbye!")

When the above code is executed, it produces:

Value of expression is 100
Goodbye!

Further Resources

For more details on control flow, check the official Python documentation: https://docs.python.org/3/tutorial/controlflow.html#if-statements