What does *tuple and **dict mean in Python? [duplicate]

Question

As mentioned in PythonCookbook, * can be added before a tuple. What does * mean here?

Chapter 1.18. Mapping Names to Sequence Elements:

from collections import namedtuple Stock = namedtuple('Stock', ['name', 'shares', 'price']) s = Stock(*rec)  # here rec is an ordinary tuple, for example: rec = ('ACME', 100, 123.45) 

In the same section, **dict presents:

from collections import namedtuple Stock = namedtuple('Stock', ['name', 'shares', 'price', 'date', 'time']) # Create a prototype instance stock_prototype = Stock('', 0, 0.0, None, None) # Function to convert a dictionary to a Stock def dict_to_stock(s):     return stock_prototype._replace(**s) 

What is **'s function here?

Answer 1

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) >>> x 1 >>> xs [2, 3, 4]  >>> *xs, x = (1, 2, 3, 4) >>> xs [1, 2, 3] >>> x 4  >>> x, *xs, y = (1, 2, 3, 4) >>> x 1 >>> xs [2, 3] >>> y 4  >>> 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)  # fine f(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)  # fine f(  1, p=2, k=3)  # fine f(a=1, p=2, k=3)  # error: a is positional-only