in smartpy, local variable param, what's difference?

in smartpy

def aa(self) {
    value_in_func = "1234"
    msg = "abc" + value_in_func
    hex_string = msg.encode("utf-8")
    thingToSign = sp.bytes("0x" + hex_string.hex())
}

This is working well, But

value = "1234"
def aa(self, value)
{
    msg = "abc" + value
    hex_string = msg.encode("utf-8")
    thingToSign = sp.bytes("0x" + hex_string.hex())
}

is NOT working.
Error: call() missing 1 positional argument: arg

I don’t know the difference between the two.

what’s difference?

I checked sp.verify(value_in_func == value), this is true.

    c1 = YourContract(...)
    scenario += c1
    scenario += c1.aa("1234")