How to annotate function that takes a tuple of variable length? (variadic tuple type annotation)
PythonPython 3.xType HintingMypyPython TypingPython Problem Overview
I have a function that takes a tuple of different lengths as an argument:
from typing import Tuple
def process_tuple(t: Tuple[str]):
# Do nasty tuple stuff
process_tuple(("a",))
process_tuple(("a", "b"))
process_tuple(("a", "b", "c"))
When I annotate function like mentioned above, I get these error messages
fool.py:9: error: Argument 1 to "process_tuple" has incompatible type "Tuple[str, str]"; expected "Tuple[str]"
fool.py:10: error: Argument 1 to "process_tuple" has incompatible type "Tuple[str, str, str]"; expected "Tuple[str]"
process_tuple really works with tuples and I use them as immutable lists of variable length. I haven't found any consensus on this topic on the internet, so I wonder how should I annotate this kind of input.
Python Solutions
Solution 1 - Python
We can annotate variable-length homogeneous tuples using the ... literal (aka Ellipsis) like this:
def process_tuple(t: Tuple[str, ...]):
...
or for Python3.9+
def process_tuple(t: tuple[str, ...]):
...
After that, the errors should go away.
From the docs:
> To specify a variable-length tuple of homogeneous type, use literal
> ellipsis, e.g. Tuple[int, ...]. A plain Tuple is equivalent to
> Tuple[Any, ...], and in turn to tuple.
Solution 2 - Python
In addition to the Ellipsis answer as posted by Azat you could make it more explicit by using @typing.overload or typing.Union
from typing import Tuple
@overload
def process_tuple(t: Tuple[str]):
# Do nasty tuple stuff
@overload
def process_tuple(t: Tuple[str, str]):
...
Or with the Union:
from typing import Tuple, Union
def process_tuple(t: Union[Tuple[str], Tuple[str, str], Tuple[str, str, str]]):
# Do nasty tuple stuff
Solution 3 - Python
Python 3.9+
Use tuple:
def process_tuple(t: tuple[str, ...]):
pass
Since Python 3.9, typing.Tuple is depreciated. The documentation for typing.Tuple states:
> Deprecated since version 3.9: builtins.tuple now supports [].
Python 3.8 and earlier
If you are on Python 3.8 or earlier, you should still use typing.Tuple:
from typing import Tuple
def process_tuple(t: Tuple[str, ...]):
pass