Python raise Statement

Raising an Exception Manually

Use raise followed by an exception type (and usually a message) to stop execution and signal a problem, exactly the way Python's built-in errors do.

def set_age(age):
    if age < 0:
        raise ValueError("Age cannot be negative")
    return age

set_age(-5)

Traceback (most recent call last):
  ...
ValueError: Age cannot be negative

When Should You Raise?

Raise an exception whenever your function receives bad input or reaches a state it genuinely cannot continue from — invalid arguments, a missing required file, an impossible calculation. This stops bad data from silently spreading through the rest of your program and produces a clear, immediate error message instead.

Re-raising Inside except

Sometimes you want to log or partially handle an error, but still let it propagate up so the calling code knows something failed. A bare raise inside an except block re-raises the exact same exception that was caught.

try:
    result = 10 / 0
except ZeroDivisionError:
    print("Logging the error before passing it on...")
    raise

raise ... from ...

When you catch one exception and raise a different, more meaningful one, use raise NewError(...) from original_error. This keeps the original traceback attached, so anyone debugging can see the true root cause instead of a confusing, disconnected new error.

try:
    int("abc")
except ValueError as e:
    raise RuntimeError("Could not process user input") from e

• raise ExceptionType("message") triggers an exception on purpose.
• A bare raise inside except re-raises the same exception unchanged.
• raise NewError(...) from original keeps both tracebacks linked for easier debugging.
• Prefer raising specific, descriptive exceptions over generic ones.