Weekly - issue 39

Python Asterisk and Slash Special Function Parameters

The asterisk (*) and forward slash (/) define whether you can pass positional or keyword arguments to your functions.

Left side	Divider	Right side
Positional-only arguments	/	Positional or keyword arguments
Positional or keyword arguments	*	Keyword-only arguments

def asterisk_usage(either, *, keyword_only):
    print(either, keyword_only)

asterisk_usage(either="Frank", keyword_only="Dean")
asterisk_usage("Frank", keyword_only="Dean")

asterisk_usage("Frank", "Dean") # wrong
def print_three_members(member1, member2, *): # wrong
    pass

# Accepts only keyword arguments
def print_three_members(*, member1, member2, member3):
    print(member1, member2, member3)

def slash_usage(positional_only, /, either):
    print(positional_only, either)

slash_usage("Frank", either="Dean")
slash_usage("Frank", "Dean")

slash_usage(positional_only="Frank", either="Dean") # wrong

# Accepts only positional arguments
def print_three_members(member1, member2, member3, /):
    print(member1, member2, member3)

# Use both the asterisk and slash
# / must be ahead of *
def print_four_members(member1, member2, /, *, member3):
    print(member1, member2, member3)

Python Type Hints - *args and **kwargs

# args: Tuple[int, ...]
# kwargs: Dict[str, int]
def variable(*args: int, **kwargs: int) -> None:
    ...

* always binds to a tuple, and ** always binds to a dict with string keys.

Because of this restriction, type hints only need you to define the types of the contained arguments. The type checker automatically adds the Tuple[_, ...] and Dict[str, _] container types.

Other ways to try:

• using a type union

  def variable(*args: int, **kwargs: int | str | bool) -> None:
      ...

• using Any type

  def variable(*args: int, **kwargs: Any) -> None:
      ...

• using TypedDict (Python 3.12)

  from typing import Required, TypedDict, Unpack
  
  # all the arguments are optional, only key2 is required
  class Options(TypedDict, total = False):
      key1: int
      key2: Required[str]
      key3: bool
  
  def variable(*args: int, **kwargs: Unpack[Options]) -> None:
      ...

  But may be better to pass keyword arguments with asterisk(*) special function parameters.