Star and double star operators

I bet not every Python programmer knows what unary operators * and ** do.

Here's the thing: applying * to a list or a tuple extracts all the items of it and forms a function arguments sequence.

That is,

def sum3(a, b, c):
    return a + b + c

threesome = (1, 2, 3)
print "sum of", threesome, "is", sum3(*threesome)

outputs

sum of (1, 2, 3) is 6.

While ** is used for unpacking a dictionary of named function arguments:

args = { "jumper": "fox", "jumpee": "dog" }

print "quick {jumper} jumped over the lazy {jumpee}".\
    format(**args)

naturally produces

quick fox jumped over the lazy dog.

It's easy to remember, you can think of these operators as the ones literally removing parentheses of a tuple, brackets of a list or curly braces of a dictionary.