Exceptions

Resources

• Built-in exceptions

The RuntimeError exception is a catch-all which you can use for your own raising and subclassing errors.

Raise

raise ValueError("My message")

Output:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: My message

Catch

def foo():
    raise ValueError("Dummy error")

Bare

Not recommended.

try:
     foo()
except:
     print("Do something")

Broad

try:
     foo()
except Exception:
     print("Do something")

try:
     foo()
except Exception as e:
     print(f"Something bad happened {str(e)}")

Specific

try:
     foo()
except KeyError:
     print("Do something")

Chained

try:
     foo()
except ConnectioError:
    print("Could not connect")
except KeyError:
     print("We had a bad key")
except Exception:
     print("Default")     

Catch and re-raise

Implicit

No need to assign the exception to a variable. Just use raise after your custom logic and the caught exception will be raised. This will keep the message.

try:
    foo = "My foo"
    # Expecting bar to fail here.
    bar(foo)
except Exception:
    print(f"Foo: {foo}")
    
    raise

Explicit

Here we use the same error type but a custom message.

try:
    foo = "My foo"
    # Expecting bar to fail here.
    bar(foo)
except Exception as e:
    print(f"Foo: {foo}")
    
    raise type(e)("My custom message")

Base classes

See Base classes in the docs.

try:
    raise ValueError("My message")
except Exception as e:
    print(e)
    print(str(e))
    print(e.args)

My message
My message
('My message',)

Recommended - use str(e) when putting your error inside a string like "Got error - {str(e)).

What is avaiable as public attributes and methods:

args - The tuple of arguments given to the exception constructor.

with_traceback(tb) - This method sets tb as the new traceback for the exception and returns the exception object.

Example from the docs:

try:
    ...
except SomeException:
    tb = sys.exc_info()[2]
    raise OtherException(...).with_traceback(tb)

Custom exception

If you don’t want to use one of the built-in exception types like ValueError, or a library’s exceptions like requests.ConnectionError, you can make your own with an class.

Basic

Empty class - you can use pass or docstring or both for the contents.

class Foo(RuntimeError):
    """Description of Foo error"""

Usage:

raise Foo("Message")

Output:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
__main__.Foo: Message

Override init

Here with a custom value on the error.

class NetworkError(RuntimeError):
    def __init__(self, msg):
        self.msg = msg

try:
    # Function call that raises this...
    raise NetworkError("Message")
except NetworkError as e: 
    print(e.msg)

Output:

Message