Python Errors

An error occurs when Python cannot successfully perform what your program asks it to do.

Errors are a normal part of programming. Even experienced developers encounter them every day.

number = 10
result = number / 0

print(result)

Python cannot divide a number by zero, so it raises an exception.

ZeroDivisionError

A small error can prevent an entire application from working correctly.

Consider an ecommerce application. If a calculation fails while processing an order, the application may not be able to generate the customer's invoice.

Understanding errors is therefore an essential programming skill, not an optional topic.

A syntax error occurs when your code does not follow Python's grammar rules.

if age >= 18
    print("Adult")

The colon after the condition is missing.

SyntaxError: expected ':'

Another Example

numbers = [1, 2, 3

Python detects the problem before the program can execute normally.

A runtime error occurs after Python has successfully understood the syntax but encounters a problem while executing the program.

numbers = [10, 20, 30]

print(numbers[5])

The syntax is valid, but index 5 does not exist in the list.

IndexError: list index out of range

Runtime errors are commonly represented by exceptions.

A logical error is different from a syntax or runtime error.

The program runs successfully, but the result is incorrect because the logic is wrong.

price = 100
discount = 20

final_price = price + discount

print(final_price)

120

If the intention was to subtract a discount, the correct logic would be:

final_price = price - discount

print(final_price)

An exception is an event that occurs during program execution and interrupts the normal flow of the program.

age = int("hello")

ValueError: invalid literal for int()

Python creates an exception object describing what went wrong.

In the next lesson, you will learn how to respond to these exceptions using try and except.

A NameError occurs when Python cannot find a variable, function or other name that your code is trying to use.

print(username)

If username was never defined, Python raises:

NameError: name 'username' is not defined

Another Common Cause

customer_name = "Ali"

print(customername)

The variable names are different. Python treats them as separate names.

A TypeError occurs when an operation is not supported for the given data types.

age = 25

print("Age: " + age)

You cannot directly concatenate a string and an integer using +.

TypeError

Possible Solution

print("Age: " + str(age))

Or, even better, use an f-string:

print(f"Age: {age}")

A ValueError occurs when a function receives a value of the correct general type, but the value itself is inappropriate.

age = int("twenty")

ValueError: invalid literal for int()

The problem is not that the input is a string. The problem is that "twenty" cannot be converted into an integer.

An IndexError occurs when you attempt to access an index that does not exist.

colors = ["red", "blue", "green"]

print(colors[5])

The valid indexes are: 0, 1 and 2.

IndexError: list index out of range

Safe Thinking

if len(colors) > 5:
    print(colors[5])

student = {
    "name": "Ali",
    "age": 20
}

print(student["email"])

The dictionary does not contain an "email" key.

KeyError: 'email'

A common alternative is to use get() when a key may not exist.

email = student.get("email")

print(email)

None

An AttributeError occurs when an object does not have the attribute or method you are trying to access.

name = "Python"

name.append("!")

Strings do not have an append() method.

AttributeError

append() belongs to lists, not strings.

names = ["Ali", "Sara"]

names.append("Ahmed")

print(names)

Python raises ZeroDivisionError when a number is divided by zero.

total = 100
students = 0

average = total / students

ZeroDivisionError: division by zero

A basic defensive check can prevent the operation:

if students != 0:
    average = total / students
else:
    print("Cannot calculate average.")

File operations can produce their own exceptions.

FileNotFoundError

with open("missing.txt", "r") as file:
    content = file.read()

If the file does not exist, Python raises FileNotFoundError.

PermissionError

Python may raise PermissionError when the operating system prevents the program from accessing a file or directory.

When an exception occurs, Python normally displays a traceback.

Traceback (most recent call last):
  File "app.py", line 8, in <module>
    result = numbers[5]
IndexError: list index out of range

A traceback contains useful information such as:

When Python displays an error, don't immediately change random lines of code.

Follow a systematic process.

Example

price = "500"
quantity = 3

total = price * quantity

print(total)

Interestingly, this code does not raise an error. Python repeats the string:

500500500

This is an example of why understanding the data type and intended logic matters. The program technically runs, but it may not be doing what the developer intended.

Your challenge is to inspect the following program and identify as many problems as possible.

students = [
    {"name": "Ali", "marks": "85"},
    {"name": "Sara", "marks": "92"},
    {"name": "Ahmed", "marks": "78"}
]

total = 0

for student in students:
    total += student["mark"]

average = total / 0

print("Average:", average)

Expected Result

Average: 85.0

In the next lesson, you will learn how to handle these exceptions gracefully using try and except.