Python list.index Method: Usage and Examples

A queue needs to locate the first occurrence of a task label.

What You Need to Understand

list.index(value, start=0, stop=len(list)) returns the position of the first matching element within an optional search interval.

When to Use It

A queue needs to locate the first occurrence of a task label.

Step-by-Step Python Example

queue = ["wait", "run", "wait"]
print(queue.index("wait"))
print(queue.index("wait", 1))

How the Example Works

The first match is at position 0; searching from index 1 finds position 2.

Example Output

0
2
2

A Second Example

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

colors = ["blue", "green", "blue"]
print(colors.index("blue", 1))

Important Behavior and Edge Cases

A missing value raises ValueError. Use value in items before lookup or handle the exception if missing values are expected.

Try It Yourself

Search for a repeated color only after the first position.

Common Beginner Mistakes

• A missing value raises ValueError. Use value in items before lookup or handle the exception if missing values are expected.
• Using the wrong collection method for this operation without checking whether it mutates the object, returns a result, or raises on missing data.

Key Takeaways

• list.index(value, start=0, stop=len(list)) returns the position of the first matching element within an optional search interval.
• The first match is at position 0; searching from index 1 finds position 2.
• A missing value raises ValueError.