5 Python errors every beginner meets, and how to read them

A red error message can feel like failure, but it's really the most helpful thing Python does. It tells you what went wrong and where.

Read it from the bottom up. The last line gives the type of error and the reason. The lines just above it show where it happened.

Each program below fails on purpose. Press Run to see the error, then Edit to fix it.

1. NameError: a name Python doesn't know

total_marks = 87
print(total_mark)

What it means: you used a name that hasn't been created. It's usually a typing slip, and newer versions of Python may even suggest the right name.

The fix: make the two spellings match. Names are case-sensitive as well, so Total and total are different.

2. TypeError: the wrong kind of value

age = "17"          # this is what input() gives you: text, even if someone types a number
print("Next year you will be", age + 1)

What it means: Python won't add text to a number. You'll meet this one when you use input(), which always gives you text.

The fix: convert the text first, with age = int("17"), or age = int(input("Your age: ")) in a real program.

3. IndexError: past the end of a list

marks = [72, 85, 90]
print(marks[3])

What it means: counting starts at 0, so a list of 3 items has positions 0, 1 and 2.

The fix: use marks[2] for the last item, or marks[-1], which works for any length.

4. KeyError: a dictionary key that isn't there

student = {"name": "Asha", "cgpa": 8.4}
print(student["CGPA"])

What it means: the key doesn't exist. Keys must match exactly, including capital letters.

The fix: use student["cgpa"]. If a key might be missing, use student.get("cgpa"), which gives None instead of an error.

5. IndentationError: spaces in the wrong place

marks = 75
if marks >= 40:
print("Passed")

What it means: Python uses indentation to see what belongs inside the if. Here, it expected the next line to be pushed in.

The fix: put 4 spaces before print.

A habit worth building

When you see an error:

1. Read the last line slowly.
2. Go to the line number that it names.
3. Print the values you're unsure of, using print(type(x), x).
4. Change one thing, and run again.

Then try deliberately breaking a program that works, and see which error appears. A few minutes of that makes these messages far less scary.

Ready for more? Every topic in Advanced Algorithms and Data Structures and Learning ML through Project has programs you can run and change, right on the page.