** reference to a Variable

Question

I've that variable

    data = {
        'username' : self.username,
        'attributes' : self._get_attrs(),
        }

What does it mean when I reference it like **data ?

Answer 1

** in **data is a dictionary unpacking operator in Python. See What is the name of ** in python?

From help('CALLS'):

If the syntax "**expression" appears in the function call, "expression" must evaluate to a mapping, the contents of which are treated as additional keyword arguments. In the case of a keyword appearing in both "expression" and as an explicit keyword argument, a "TypeError" exception is raised.

See Understanding kwargs in Python.

There is also PEP: 448 -- Additional Unpacking Generalizations:

>>> {**{'a': 1, 'b': 2}, **{'a': 3, 'c': 4}}
{'b': 2, 'a': 3, 'c': 4}

Answer 2

The ** expands your dictionary. Example:

def func(username=None, attributes=None):
    print(username)

data = {
    'username'   : "Bob",
    'attributes' : {},
}

func(**data)
# results in "Bob"

It can also be used to collect keyword arguments (kwargs), as seen in this question about *args and **kwargs.