Python Errors

An error is a problem in a program that stops it or gives a wrong result. Every programmer meets errors daily, so it is important to understand them. The process of finding and removing errors is called debugging.

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Types of errors

• Syntax errors: the code breaks the rules of the language. Python finds them before the program starts.
• Runtime errors (exceptions): the code is correct, but something goes wrong while it runs, like dividing by zero.
• Logical errors: the program runs without any error message but gives a wrong result.

Syntax error

age = 16
if age > 18:
print("you can vote")

Another common one is a missing colon: if age > 18 gives SyntaxError: expected ':'.

Runtime error (exception)

print(10 / 0)

Logical error

We want the average of two numbers, but forgot the brackets.

a = 10
b = 20
average = a + b / 2
print(average)

Python did not complain, but the correct average is 15. Because of operator precedence, only b was divided by 2. The correct line is (a + b) / 2. Logical errors are the hardest to find, so test your code with known answers.

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Common exceptions

Exception	When it happens
NameError	Using a variable that does not exist
TypeError	Wrong type, like adding a string and a number
ValueError	Right type but wrong value, like int("abc")
ZeroDivisionError	Dividing by zero
IndexError	List index is out of range
KeyError	Dictionary key does not exist
FileNotFoundError	Opening a file that is not there

When an exception happens, Python prints a message called a traceback. Read it from the bottom, because the last line tells you the exception and the reason.

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Handle exceptions with try and except

Put the risky code in try. If an exception happens, the matching except block runs and the program continues.

try:
    number = int("abc")
except ValueError:
    print("Please enter a valid number")

You can read the error message with as.

try:
    print(10 / 0)
except ZeroDivisionError as e:
    print("Error:", e)

else and finally

try:
    result = 10 / 2
except ZeroDivisionError:
    print("Cannot divide by zero")
else:
    print("Result:", result)
finally:
    print("Done")

• else runs only when there is no exception.
• finally always runs, even if there is an exception. Use it for cleanup like closing a file.

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Raise your own exception

Use raise to create an exception yourself when something is wrong.

age = -5

if age < 0:
    raise ValueError("Age cannot be negative")

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Summary

• Syntax errors break the rules, exceptions happen while running, logical errors give wrong answers.
• Handle exceptions with try and except. Catch the specific exception you expect.
• Use else and finally when needed, and raise to create your own.
• Read the interview answer How does exception handling work in Python?

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