How to annotate variadic parameters in Python using typing annotations?
Question:
How to annotate parameters of the variadic function?
Example:
def foo(*args): # Each arg expected to be of type T
...
Are there any typing annotations for that?
Answers:
tl;dr
Basically args treated as a homogeneous tuple and kwds as a dictionary.
You simply annotate a type being expected for each element value.
Explanation
Explanation arrives from quote of the PEP-484:
In the body of function foo, the type of variable args is deduced as Tuple[str, ...] and the type of variable kwds is Dict[str, int].
So there is no need to annotate args as whole homogeneous typed tuple but one can reduce Tuple[T, ...] to just type T.
Same true for the keyword arguments as they deduced as Dict[str, T]
About ellipsis in the tuple annotation
In python documentation there is no much information about the usage of the ... a.k.a Ellipsis but PEP-484 does mention various usages of the ellipsis in typing annotations like for omitting some type annotations or default values but most interestingly there is a quote saying:
Tuple, used by listing the element types, for example Tuple[int, int, str]. The empty tuple can be typed as Tuple[()]. Arbitrary-length
homogeneous tuples can be expressed using one type and ellipsis, for
example Tuple[int, ...]. (The ... here are part of the syntax, a
literal ellipsis.)
So if you omit asterisk to force passing arguments as a single tuple you need keep full annotation:
def foo(args: Tuple[T, ...]):
...
About various types in a homogeneous tuple
Since homogeneous tuple means all of its elements must be of the same type then if you wish to allow several types just use a Union or even use a type alias for better readability:
MyArg = Union[int, str, bool]
def foo(*args: MyArg):
...
If each argument has a TheType type – annotate it as specified in PEP-484:
def foo(*args: TheType):
...
Do not use: def foo(*args: Tuple[TheType]):Tuple[TheType] means it’s a single-element tuple – with one TheType element, which is not what variadic args are intended to serve.
How to annotate parameters of the variadic function?
Example:
def foo(*args): # Each arg expected to be of type T
...
Are there any typing annotations for that?
tl;dr
Basically args treated as a homogeneous tuple and kwds as a dictionary.
You simply annotate a type being expected for each element value.
Explanation
Explanation arrives from quote of the PEP-484:
In the body of function foo, the type of variable args is deduced as
Tuple[str, ...]and the type of variable kwds isDict[str, int].
So there is no need to annotate args as whole homogeneous typed tuple but one can reduce Tuple[T, ...] to just type T.
Same true for the keyword arguments as they deduced as Dict[str, T]
About ellipsis in the tuple annotation
In python documentation there is no much information about the usage of the ... a.k.a Ellipsis but PEP-484 does mention various usages of the ellipsis in typing annotations like for omitting some type annotations or default values but most interestingly there is a quote saying:
Tuple, used by listing the element types, for example
Tuple[int, int, str]. The empty tuple can be typed asTuple[()]. Arbitrary-length
homogeneous tuples can be expressed using one type and ellipsis, for
exampleTuple[int, ...]. (The...here are part of the syntax, a
literal ellipsis.)
So if you omit asterisk to force passing arguments as a single tuple you need keep full annotation:
def foo(args: Tuple[T, ...]):
...
About various types in a homogeneous tuple
Since homogeneous tuple means all of its elements must be of the same type then if you wish to allow several types just use a Union or even use a type alias for better readability:
MyArg = Union[int, str, bool]
def foo(*args: MyArg):
...
If each argument has a TheType type – annotate it as specified in PEP-484:
def foo(*args: TheType):
...
Do not use: def foo(*args: Tuple[TheType]):Tuple[TheType] means it’s a single-element tuple – with one TheType element, which is not what variadic args are intended to serve.