Try and except

Basic try-except structure

The try block contains code that might fail. The except block runs if an error happens:

try:
    # Code that might cause an error
    risky_operation()
except:
    # Code that runs if there's an error
    print("Something went wrong")

Catching specific errors

Different errors need different handling. Catch specific error types:

try:
    age = int(input("Enter your age: "))
    print(f"In 10 years, you'll be {age + 10}")
except ValueError:
    print("Please enter a number")

Multiple error types

Handle different errors differently:

try:
    # Read a number from a file
    with open('number.txt', 'r') as f:
        text = f.read()
    number = int(text)
    result = 100 / number
    print(f"Result: {result}")
except FileNotFoundError:
    print("Could not find number.txt")
except ValueError:
    print("File doesn't contain a valid number")
except ZeroDivisionError:
    print("Cannot divide by zero")

The else clause

Code in else runs only if no error happened:

try:
    with open('data.txt', 'r') as f:
        data = f.read()
except FileNotFoundError:
    print("File not found")
else:
    # This only runs if the file was opened successfully
    print(f"File has {len(data)} characters")

The finally clause

Code in finally always runs, error or not:

try:
    file = open('data.txt', 'r')
    data = file.read()
except FileNotFoundError:
    print("File not found")
finally:
    # This always runs to clean up
    if 'file' in locals() and not file.closed:
        file.close()
    print("Cleanup complete")

Don’t catch all errors with a bare except:. It hides problems and makes debugging harder. Always catch specific error types when possible.

Common mistakes

Catching too broad exceptions

# Bad - catches everything
try:
    process_data()
except:
    pass  # Silent failure!

# Good - specific error
try:
    process_data()
except ValueError:
    print("Invalid data format")

Empty except blocks

# Bad - ignores the error
try:
    risky_operation()
except ValueError:
    pass

# Good - at least log it
try:
    risky_operation()
except ValueError:
    print("Warning: Invalid value encountered")

Not re-raising critical errors

# Bad - hides all errors
try:
    critical_operation()
except Exception as e:
    print(f"Error: {e}")

# Good - log and re-raise
try:
    critical_operation()
except Exception as e:
    print(f"Critical error: {e}")
    raise  # Let the error propagate

What’s next?

Now let’s look at the most common errors you’ll encounter in Python.

Common errors

Errors you’ll see often

Functions

External tools

Practical Python

Error handling

Classes

Basic try-except structure

Catching specific errors

Multiple error types

The else clause

The finally clause

Common mistakes

What’s next?

Common errors

​Basic try-except structure

​Catching specific errors

​Multiple error types

​The else clause

​The finally clause

​Common mistakes

​What’s next?

Common errors

Basic try-except structure

Catching specific errors

Multiple error types

The else clause

The finally clause

Common mistakes

What’s next?