How do you choose the right exception and deal with the exception hierarchy?

Short answer

Raise an exception that precisely expresses the cause of the failure (such as ValueError, KeyError or a domain-specific exception), and catch the narrowest type you can actually handle instead of except Exception everywhere.

Explanation

Regular exceptions inherit from Exception, whereas KeyboardInterrupt and SystemExit inherit directly from BaseException; that is why a bare except: or except BaseException swallows shutdown signals and prevents the program from stopping. Catch the exception in the layer that owns the decision to recover or to translate it into a meaningful error, and let the rest propagate. A broad except Exception is acceptable at the top-level boundary (such as a request handler) for logging and returning a 500.

Code example

def parse_age(raw: str) -> int:
    try:
        age = int(raw)
    except ValueError:
        raise ValueError(f"invalid age: {raw!r}") from None
    if age < 0:
        raise ValueError("age must be positive")
    return age

Common mistakes

• Writing except: pass, so every error disappears, including Ctrl+C.
• Catching Exception in a layer that does not know how to recover and then continuing in an invalid state.

Follow-up questions

• When do you need ExceptionGroup and except*?
• When should you create a custom exception?

Sources

• Python documentation — Built-in exceptions: Exception hierarchy