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

by

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

More Related Contents:

  1. How are tuples unpacked in for loops?
  2. how to unpack a tuple in this form (‘abc’, [20, 20])? [closed]
  3. What are “named tuples” in Python?
  4. How does swapping of members in tuples (a,b)=(b,a) work internally?
  5. Unpacking, extended unpacking and nested extended unpacking
  6. Two variables in Python have same id, but not lists or tuples
  7. Getting only element from a single-element list in Python?
  8. Type hints when unpacking a tuple?
  9. Existence of mutable named tuple in Python?
  10. lambda arguments unpack error
  11. Convert a namedtuple into a dictionary
  12. asterisk in tuple, list and set definitions, double asterisk in dict definition
  13. Sort a list of tuples depending on two elements [duplicate]
  14. How do I convert a tuple of tuples to a one-dimensional list using list comprehension? [duplicate]
  15. How does str.startswith really work?
  16. Weird MRO result when inheriting directly from typing.NamedTuple
  17. Converting string to tuple without splitting characters
  18. Star * operator on left vs right side of an assignment statement
  19. How to display position of an element in an array? [closed]
  20. Lag when win.blit() background pygame
  21. Ignore python multiple return value
  22. How do I format a string using a dictionary in python-3.x?
  23. How can I use `return` to get back multiple values from a for loop? Can I put them in a list?
  24. Print list of lists in separate lines
  25. OpenCV for Python 3.x under Windows [duplicate]
  26. Sorting by absolute value without changing to absolute value
  27. FutureWarning: The default value of regex will change from True to False in a future version
  28. Multiple keys per value
  29. Filter a list of tuples based on condition
  30. How can I convert this string to list of lists? [duplicate]

Leave a Comment