How do I raise the same Exception with a custom message in Python?

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I have this try block in my code:

try:
    do_something_that_might_raise_an_exception()
except ValueError as err:
    errmsg = 'My custom error message.'
    raise ValueError(errmsg)

Strictly speaking, I am actually raising another ValueError, not the ValueError thrown by do_something...(), which is referred to as err in this case. How do I attach a custom message to err? I try the following code but fails due to err, a ValueError instance, not being callable:

try:
    do_something_that_might_raise_an_exception()
except ValueError as err:
    errmsg = 'My custom error message.'
    raise err(errmsg)

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