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LESSON 99 OF 410 · LISTS I

List Length in Python

Lesson Read the full explanation

We can get the length of a list using the len() function like this:

ourList = [50, 40, 30]

# Printing the length:
print(len(ourList)) # Output: 3

# Changing the list:
ourList.pop()

# Printing the new length:
print(len(ourList)) # Output: 2

Using len()

We can use the len() function to access items from the end of a list.

Because lists are 0-indexed, the index of the last item is length - 1:

ourList = [50, 40, 30]

# Printing the last item:
print(ourList[len(ourList) - 1]) # Output: 30

# Printing the second to last item:
print(ourList[len(ourList) - 2]) # Output: 40

Assignment
Follow the Coding Tutorial and let's play with some arrays.

Hint
Look at the examples above if you get stuck.

STEP 1 OF 3
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    PYTHON · LISTS I

    Array Length in Python, explained

    Python len(): How to Get the Length of a List

    len() is one of Python's most-used built-in functions. It tells you how many items are in a list — and that number unlocks a surprising amount of power: safe indexing, loop control, size validation, progress tracking, and more. Virtually every non-trivial program that works with lists calls len() at some point.

    Basic Usage

    scores = [95, 82, 74, 88, 91]

print(len(scores))   # 5

# After removing an item, the length updates:
scores.pop()
print(len(scores))   # 4

# Empty list:
empty = []
print(len(empty))    # 0

    Using len() to Access the Last Element

    Because lists are zero-indexed, the last item is always at index len(list) - 1. (Though in modern Python, list[-1] is the simpler way to do this.)

    items = ["alpha", "beta", "gamma", "delta"]

# Both of these access the last element:
print(items[len(items) - 1])   # delta
print(items[-1])               # delta  (cleaner)

# Second to last:
print(items[len(items) - 2])   # gamma
print(items[-2])               # gamma  (cleaner)

    len() Enables Safe Indexing

    Before accessing an index, check that it actually exists — len() makes this easy:

    data = [10, 20, 30]

# Guard against IndexError:
index = 5
if index < len(data):
    print(data[index])
else:
    print(f"Index {index} is out of range (list has {len(data)} items)")
# Index 5 is out of range (list has 3 items)

    Real-World Example: Progress Tracking

    tasks = ["Design", "Build", "Test", "Deploy", "Launch"]
completed = ["Design", "Build"]

total = len(tasks)
done  = len(completed)
remaining = total - done

print(f"Progress: {done}/{total} tasks complete")   # Progress: 2/5 tasks complete
print(f"Remaining: {remaining} tasks")              # Remaining: 3 tasks
percent = (done / total) * 100
print(f"{percent:.0f}% done")                       # 40% done

    len() Works on Strings Too

    The same function works on strings, tuples, and dictionaries — not just lists:

    word = "algorithm"
print(len(word))   # 9

coords = (10, 20, 30)
print(len(coords))  # 3

info = {"name": "Alex", "age": 28}
print(len(info))    # 2  (number of key-value pairs)

    Common Mistakes to Avoid

    • Off-by-one errors: The last valid index is len(list) - 1, not len(list). Using list[len(list)] always raises IndexError.
    • Comparing to length when you should check membership: To check if an item exists, use if x in my_list, not if len(my_list) > 0.
    • Calling len() on a non-sequence: len(42) raises a TypeError. It only works on collections.
    nums = [1, 2, 3]

# Common mistake — accessing len() as the last index:
# print(nums[len(nums)])   # IndexError!

# Correct:
print(nums[len(nums) - 1])   # 3
print(nums[-1])               # 3  (simpler)

    Key Takeaways

    • len(my_list) returns the number of elements — an integer you can use like any other number.
    • Valid indices range from 0 to len(list) - 1; going outside this range crashes with IndexError.
    • Use len() to guard against index errors, control loops, and track progress.
    • len() also works on strings, tuples, and dictionaries.

    What to Learn Next

    Now that you can measure a list, the natural next step is iterating over it. Learn for loops to process every element automatically, and range(len(list)) for index-based iteration. You'll use these patterns constantly in real Python programs.

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