try & except in Python

Learn how to handle exceptions using Python's try and except blocks.

What are try and except?

The try and except blocks are used to handle exceptions in Python.

Code that may cause an exception is placed inside the try block.

If an exception occurs, Python moves to the matching except block.

try & except Flow

Basic Syntax

try:

    # code that may cause an exception

except:

    # code to handle the exception

The try block must be followed by an except block.

Simple Example

try:

    number = 10 / 0

    print(number)

except:

    print("An error occurred")

Output:

An error occurred

The division causes an exception, so Python executes the except block.

Handling a Specific Exception

It is better to catch the specific exception that we expect.

try:

    number = 10 / 0

except ZeroDivisionError:

    print("Cannot divide by zero")

Output:

Cannot divide by zero

Here we specifically handle ZeroDivisionError.

Handling ValueError

A ValueError can occur when a value is not valid for an operation.

try:

    age = int("abc")

    print(age)

except ValueError:

    print("Please enter a valid number")

Output:

Please enter a valid number

Handling TypeError

try:

    result = "10" + 5

    print(result)

except TypeError:

    print("Invalid data types")

Output:

Invalid data types

Handling IndexError

numbers = [10, 20, 30]

try:

    print(numbers[5])

except IndexError:

    print("Invalid index")

Output:

Invalid index

Handling KeyError

student = {
    "name": "Rahul",
    "course": "Python"
}

try:

    print(student["fees"])

except KeyError:

    print("Key not found")

Output:

Key not found

Handling FileNotFoundError

try:

    with open(
        "students.txt",
        "r"
    ) as file:

        data = file.read()

        print(data)

except FileNotFoundError:

    print("File not found")

This is useful when the file may not exist.

Using as with except

We can store the exception object in a variable using the as keyword.

try:

    number = int("abc")

except ValueError as error:

    print("Error:", error)

The variable error contains information about the exception.

try & except with User Input

User input may contain invalid data, so exception handling is useful in input programs.

try:

    marks = int(
        input("Enter marks: ")
    )

    print("Marks:", marks)

except ValueError:

    print("Please enter numbers only")

If the user enters text instead of a number, the except block handles the problem.

When No Exception Occurs

If the code inside try does not raise an exception, the except block is skipped.

try:

    number = 10 / 2

    print(number)

except ZeroDivisionError:

    print("Cannot divide by zero")

print("Program continues")

Output:

5.0
Program continues

Step-by-Step Execution

Why Catch Specific Exceptions?

Catching a specific exception makes the program easier to understand and helps avoid hiding unrelated programming errors.

Less Specific

try:

    number = int("abc")

except:

    print("Something went wrong")

More Specific

try:

    number = int("abc")

except ValueError:

    print("Invalid number")

The second example clearly tells us which problem is being handled.

Using the General Exception Class

We can use Exception to catch many standard exceptions.

try:

    result = 10 / 0

except Exception as error:

    print("Error:", error)

Use broad exception handling carefully so that important programming errors are not hidden.

Nested try Blocks

A try block can also be placed inside another try block when the program specifically requires nested error handling.

try:

    number = int(
        input("Enter a number: ")
    )

    try:

        result = 100 / number

        print(result)

    except ZeroDivisionError:

        print("Cannot divide by zero")

except ValueError:

    print("Invalid number")

Example: Student Marks 👀📩

try:

    marks = int(
        input("Enter student marks: ")
    )

    if marks < 0 or marks > 100:

        print("Marks must be between 0 and 100")

    else:

        print("Valid marks:", marks)

except ValueError:

    print("Please enter a valid number")

This example combines input validation with exception handling.

Best Practices

Common Mistakes

• Forgetting the except block.
• Catching the wrong exception type.
• Using a completely generic except without understanding the error.
• Putting too much code inside the try block.
• Hiding useful error information.

Practice Programs

1. Handle division by zero using ZeroDivisionError.
2. Convert user input into an integer and handle ValueError.
3. Handle an invalid list index.
4. Handle a missing dictionary key.
5. Handle a missing text file.
6. Print the exception message using as error.