try — Advanced Examples

Starts a block of code that will be monitored for exceptions

EasyIntermediateAdvancedExpert

Exception chaining and context

Using 'raise from' and understanding __cause__.

python
class DatabaseError(Exception):
    pass

class ConnectionError(DatabaseError):
    pass

def connect(host):
    try:
        if host == "bad":
            raise OSError(f"Cannot reach {host}")
    except OSError as e:
        raise ConnectionError(f"Database connection failed") from e

try:
    connect("bad")
except ConnectionError as e:
    print(f"Caught: {e}")
    print(f"Caused by: {e.__cause__}")
    print(f"Context: {e.__context__}")

'raise X from Y' sets __cause__ for explicit chaining. __context__ is set automatically when an exception occurs while handling another.

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