What is the right exception for an unmet precondition?

Question

What is the appropriate exception to raise in a function to signal that a precondition was not met?

Examples:

def print_stats(name, age):
    if name is None:
        raise Exception("name cannot be None")
    if not type(name) is str:
        raise Exception("name must be a string")

    if age is None:
        raise Exception("age cannot be None")
    if age < 0:
        raise Exception("age cannot be negative")

    print("{0} is {1} years old".format(name, age))

Answer 1

You should use both TypeError and ValueError.

The first three exceptions should be TypeErrors because we are signaling that the arguments are of an incorrect type. From the docs:

exception TypeError

Raised when an operation or function is applied to an object of inappropriate type. The associated value is a string giving details about the type mismatch.

The last exception however should be a ValueError because age is the correct type but has an incorrect value (it is negative). From the docs:

exception ValueError

Raised when a built-in operation or function receives an argument that has the right type but an inappropriate value, and the situation is not described by a more precise exception such as IndexError.