Similar to type, isinstance checks whether a variable is of a particular data type, structure, or class. This makes it very useful for testing purposes, or if you want to check arguments for the correct type:

>>> isinstance('Hello world', str)
True

Here, we checked whether the value is a string—and indeed it is! We can also pass multiple variable types, in which case isinstance will return True if the value matches any of the given types:

>>> isinstance(1, (int, float))
True

Note the second parenthesis, surrounding two value types we pass in. This parenthesis represents a tuple, one of the data structures, which we'll discuss in the next chapter.

Finally, there is dir, which is invaluable if we need to work with obscure, badly documented code.