In a function call

*t means "treat the elements of this iterable as positional arguments to this function call."

def foo(x, y):    print(x, y)>>> t = (1, 2)>>> foo(*t)1 2

Since v3.5, you can also do this in a list/tuple/set literals:

>>> [1, *(2, 3), 4][1, 2, 3, 4]

**d means "treat the key-value pairs in the dictionary as additional named arguments to this function call."

def foo(x, y):    print(x, y)>>> d = {'x':1, 'y':2}>>> foo(**d)1 2

Since v3.5, you can also do this in a dictionary literals:

>>> d = {'a': 1}>>> {'b': 2, **d}{'b': 2, 'a': 1}

In a function signature

*t means "take all additional positional arguments to this function and pack them into this parameter as a tuple."

def foo(*t):    print(t)>>> foo(1, 2)(1, 2)

**d means "take all additional named arguments to this function and insert them into this parameter as dictionary entries."

def foo(**d):    print(d)>>> foo(x=1, y=2){'y': 2, 'x': 1}

In assignments and for loops

*x means "consume additional elements in the right hand side", but it doesn't have to be the last item. Note that x will always be a list:

>>> x, *xs = (1, 2, 3, 4)>>> x1>>> xs[2, 3, 4]>>> *xs, x = (1, 2, 3, 4)>>> xs[1, 2, 3]>>> x4>>> x, *xs, y = (1, 2, 3, 4)>>> x1>>> xs[2, 3]>>> y4>>> for (x, *y, z) in [ (1, 2, 3, 4) ]: print(x, y, z)...1 [2, 3] 4

Note that parameters that appear after a * are keyword-only:

def f(a, *, b): ...f(1, b=2)  # finef(1, 2)    # error: b is keyword-only

Python3.8 added positional-only parameters, meaning parameters that cannot be used as keyword arguments. They appear before a / (a pun on * preceding keyword-only args).

def f(a, /, p, *, k): ...f(  1,   2, k=3)  # finef(  1, p=2, k=3)  # finef(a=1, p=2, k=3)  # error: a is positional-only