How to annotate function that takes a tuple of variable length? (variadic tuple type annotation)

Question:

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.

Asked By: Montreal

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Source

Answers:

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.

Answered By: Azat Ibrakov

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
Answered By: Wolph

Python 3.9+

Use tuple:

def process_tuple(t: tuple[str, ...]):
    pass

Since Python 3.9, typing.Tuple is deprecated. 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
Answered By: BrokenBenchmark