Python Recognize IndexError Before It Happens

A user selects the first task in a list that might be empty.

What You Need to Understand

A list index is valid only within the range from -len(items) to len(items)-1. Accessing an invalid index raises IndexError; checking a list first can prevent many avoidable failures.

When to Use It

A user selects the first task in a list that might be empty.

Step-by-Step Python Example

tasks = []
if tasks:
    print(tasks[0])
else:
    print("No tasks available")
print(len(tasks))

How the Example Works

The condition checks whether any task exists before indexing. An empty list prints a helpful message instead of raising an error.

Example Output

No tasks available
0
None

A Second Example

Compare this smaller variation with the main example, then experiment with your own values.

values = [10, 20]
index = 2
print(values[index] if 0 <= index < len(values) else None)

Important Behavior and Edge Cases

Negative indexes also have limits: items[-1] fails on an empty list. Slicing out of range is generally safe and returns a shorter result.

Try It Yourself

Write a safe last-item lookup for a list that may be empty.

Common Beginner Mistakes

Negative indexes also have limits: items[-1] fails on an empty list. Slicing out of range is generally safe and returns a shorter result.
Assuming every sequence operation modifies the original data or that a tuple makes nested mutable objects immutable.

Key Takeaways

A list index is valid only within the range from -len(items) to len(items)-1.
The condition checks whether any task exists before indexing.
Negative indexes also have limits: items[-1] fails on an empty list.