Python Basic Errors

Why Errors Are Useful

Errors are not just failures. They are messages that help you find what Python could not understand or could not run. When you learn to read errors, debugging becomes much less stressful.

Python usually shows a traceback. A traceback points to the file, line number, and error type.

NameError: name 'score' is not defined

This means Python tried to use score, but no value had been assigned to that name.

SyntaxError

A SyntaxError happens when Python cannot understand the structure of the code.

if score >= 50
    print("Pass")

The colon is missing after the condition. The fixed version is:

if score >= 50:
    print("Pass")

Syntax errors must be fixed before the program can run.

NameError

A NameError happens when a name is used before Python knows it.

print(total)

If total has not been assigned, Python raises a NameError.

Check spelling and make sure the variable is assigned before use.

TypeError

A TypeError happens when an operation is used with an unsuitable type.

age = "16"
print(age + 1)

Python cannot add a string and an integer. Convert first:

age = int(age)
print(age + 1)

ValueError

A ValueError happens when the type is correct, but the value is not valid for the operation.

age = int("sixteen")

The string type is allowed as input to int(), but "sixteen" cannot be converted into a number.

IndexError And KeyError

An IndexError happens when a list or string index is outside the valid range.

items = ["pen", "book"]
print(items[2])

The valid indexes are 0 and 1.

A KeyError happens when a dictionary key does not exist.

student = {"name": "Asha"}
print(student["marks"])

Use get() if the key might be missing.

Reading A Traceback

When you see a traceback, read from the bottom first to identify the error type. Then look at the line number near the top of the message. Check that line and the line just before it, especially for missing colons, quotes, or parentheses.

Common Mistakes

A common mistake is changing several lines before reading the exact message. Slow down and identify the error type first.

Another mistake is assuming the highlighted line is always the real cause. With syntax errors, the mistake is sometimes on the previous line.

Quick Summary

• Error messages help you debug.
• SyntaxError means Python could not understand the code structure.
• NameError means a name is not defined.
• TypeError means an operation used the wrong type.
• ValueError means a value is invalid for an operation.
• IndexError and KeyError happen during invalid lookups.
• Read the error type and line number before changing code.

Practice Quiz

Now practice this topic with MCQs and explanations:

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