Exception Helpers

raises[TError: Exception](action: Callable, *, exact: bool = False, expect: TError = None) → bool

Use raises() to assert that a function, method, or lambda expression raises an expected Exception.

Parameters:

• action – The Callable being tested, usually a function or method reference.

• exact – A boolean indicating that the raised Exception must be an exact type match (and not a subclass of the specified type.) Use this when you need to test for a specific exception type.

• expect – A type parameter that can be used in lieu of TError.

Exact Match Example

class MyException(Exception):
    pass

class NotMyException(Exception):
    pass

def myFunctionOne:
    raise NotMyException()

def myFunctionTwo:
    raise MyException()

# assert we can inexact-match Exception superclass
assert raises[Exception](myFunctionOne)
assert raises[Exception](myFunctionTwo)
# assert we can exact-match Exception subclass
assert raises[NotMyException](myFunctionOne, exact=True)
assert raises[MyException](myFunctionTwo, exact=True)

Syntax Example

LTS

This helper offers two syntaxes, one which relies on Python “Type Parameters” and the other which relies on an explicit expect:type argument; the latter syntax exists solely for LTS/backward-compatibility purposes.

def myFunction:
    raise MyException()

# using Python >=3.11 syntax
assert raises[MyException](myFunction)
# using Python <=3.11 syntax
assert raises(myFunction, expect=MyException)