Runtime Errors (Exceptions)
Unlike syntax errors, a Runtime Error occurs while the program is actively running.
In this case, the syntax (grammar) of your code is perfectly correct, so Python successfully starts the program. However, during execution, Python encounters an operation that is impossible to perform. When this happens, Python halts the program and raises an Exception.
If these exceptions are not handled, your application will crash instantly.
Common Python Exceptions with Examples
Here are the most common runtime exceptions you will encounter in Python, what causes them, and how they look:
1. ZeroDivisionError
This happens when you attempt to divide a number by zero, which is mathematically undefined.
Example Code:
numerator = 10
denominator = 0
result = numerator / denominator
Python's Error Message:
ZeroDivisionError: division by zero
2. ValueError
This occurs when a function receives an argument of the correct data type, but the value itself is inappropriate or invalid.
Example Code:
# Converting text to integer
# "123" would work, but "hello" is not a valid number representation
number = int("hello")
Python's Error Message:
ValueError: invalid literal for int() with base 10: 'hello'
3. TypeError
This is raised when an operation or function is applied to an object of an inappropriate data type.
Example Code:
# Attempting to add an integer and a string directly
total = 10 + "5"
Python's Error Message:
TypeError: unsupported operand type(s) for +: 'int' and 'str'
4. IndexError
This happens when you try to access an index of a list or tuple that is out of its range.
Example Code:
colors = ["red", "green", "blue"] # Indexes are 0, 1, 2
print(colors[5]) # Index 5 does not exist
Python's Error Message:
IndexError: list index out of range
5. KeyError
This occurs when you attempt to access a dictionary key that does not exist.
Example Code:
student = {"name": "Sai", "course": "Python"}
# Accessing the key "age" which was not defined
print(student["age"])
Python's Error Message:
KeyError: 'age'
6. NameError
This happens when your code tries to access a variable or call a function that has not been defined or initialized.
Example Code:
# Printing a variable before declaring it
print(score)
Python's Error Message:
NameError: name 'score' is not defined
7. FileNotFoundError
This is raised when you attempt to open or work with a file that does not exist at the specified path.
Example Code:
# Trying to open a file that isn't on the disk
with open("missing_file.txt", "r") as file:
content = file.read()
Python's Error Message:
FileNotFoundError: [Errno 2] No such file or directory: 'missing_file.txt'
Why Understanding Exceptions is Crucial
Since these errors occur based on external factors (like a user typing invalid data, or a database connection drop), we cannot always prevent them simply by writing clean code.
Instead, we must write defensive code that catches these exceptions and handles them gracefully. This is where try and except blocks come in, which we will study in the next chapter.