What does *tuple and **dict mean in Python?
iterable-unpacking, namedtuple, python, python-3.x, tuples
Solution
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 the starred parameter (the one marked `*`) 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
Problem
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?