Reject invalid arguments
Input validation
Python · Const
raise
Python’s raise statement explicitly raises an exception, re-raises the active exception, or chains a new exception to an existing cause.
How it works
The raise statement explicitly signals an exception, can re-raise the active exception inside an exception handler, and optionally chains another exception with from. Its operand may be an exception class or instance derived from BaseException.
Syntax
raise_stmt: "raise" [expression ["from" expression]]Parameters
| Name | Type | Description |
|---|---|---|
expression | exception class or instance | Exception class or instance to raise; it must be a subclass or instance of BaseException. |
from expression | exception class or instance | Optional exception class or instance used as the explicit cause. |
Returns
Does not return normally; it raises an exception or re-raises the active exception.
Key points
- The syntax is
raise, optionally followed by an exception expression and an optionalfromexpression. - A bare statement re-raises the active exception.
- With an expression, Python evaluates it as the exception to raise.
- Supplying an exception class is shorthand for creating it with no constructor arguments.
- Raised exceptions normally receive an automatically created traceback.
- The optional
fromclause records a causal relationship between exceptions.
Common mistakes
- A bare
raiseis valid only while an exception is actively being handled; otherwise it raisesRuntimeError. - The expression after
raisemust be an exception class or instance, not an arbitrary value. - The exception after
frombecomes the explicit cause when it is an instance.
Example: Validate input and chain the cause
def parse_port(value):
try:
port = int(value)
except ValueError as exc:
raise ValueError("port must be an integer") from exc
if not 1 <= port <= 65535:
raise ValueError("port out of range")
return port
try:
parse_port("http")
except ValueError as exc:
print(exc)The function validates a port, raises a clearer exception, and preserves the original conversion error through chaining.
When to use it
Expose a domain-specific exception while retaining the cause
Error translation
Log or inspect an error, then re-raise it
Exception handling
Raise an exception with a selected traceback
Traceback control
Interview questions
QWhat forms can the expression after raise take?
raise SomeError raises an exception class, which Python instantiates without arguments when needed. raise SomeError("detail") raises an already-created exception instance; both must derive from BaseException.
Follow-up: What does bare raise do? Inside an exception handler it re-raises the active exception; outside one, it produces RuntimeError.
QWhat does a bare raise mean?
Bare raise re-raises the exception currently being handled, preserving the active exception. It is commonly used in an except block after logging or cleanup.
Follow-up: How can a handler add context while retaining the original failure? Use raise NewError(...) from original, which establishes explicit exception chaining.
QWhat is the purpose of raise ... from ...?
The optional from expression enables exception chaining. The second expression must be an exception class or instance, and an instance is recorded as the raised exception’s __cause__.
Follow-up: How can an exception be given a particular traceback? Call with_traceback() on the exception and raise the returned same instance.
Related
tryexceptBaseExceptionExceptionwith_traceback()__cause____traceback__