Alright.

Let's assume you have a 3.9 virtualenv with cattrs installed.

>>> from cattr import structure

>>> structure(["1", 1.0, True], tuple[int, int, int])
(1, 1, 1)

1. We run the value `tuple[int, int, int]` through the dispatch mechanism. It matches on this function: https://github.com/Tinche/cattrs/blob/e54fa1713b39fa3c944415bbe61aabd7e8bb428a/src/cattr/_compat.py#L101, which ends up invoking this handler: https://github.com/Tinche/cattrs/blob/e54fa1713b39fa3c944415bbe61aabd7e8bb428a/src/cattr/converters.py#L433.

2. This handler fishes out the tuple type arguments, and it basically returns `tuple(cattr.structure(obj[0], int), cattr.structure(obj[1], int), cattr.structure(obj[2], int))`.

3. All three of these structure calls end up matching at https://github.com/Tinche/cattrs/blob/master/src/cattr/converters.py#L131, which points to https://github.com/Tinche/cattrs/blob/e54fa1713b39fa3c944415bbe61aabd7e8bb428a/src/cattr/converters.py#L294, which basically does int(x).

The top level call basically evaluates to `tuple(int(x[0]), int(x[1]), int(x[2]))`. This same mechanism supports other primitives, lists, dicts, sets, frozensets, optionals, enums, attrs classes, unions of attrs classes (with some restrictions) out of the box.

I feel if you'd like a more thorough walkthrough a video call would be better.

On Tue, Jan 26, 2021 at 1:52 AM Guido van Rossum <guido@python.org> wrote:

I really want to understand what you're doing to compute the results at runtime -- the PEP is already full of example signatures.

On Mon, Jan 25, 2021 at 4:10 PM Tin Tvrtković <tinchester@gmail.com> wrote:

Ok, sure. Are you looking for usage examples or me running through how the code works internally?

On Tue, Jan 26, 2021 at 12:08 AM Guido van Rossum <guido@python.org> wrote:

Could you construct some small(ish) examples and paste them here? I am still having a hard time imagining how you implement this at runtime.

--

--Guido van Rossum (python.org/~guido)

Pronouns: he/him (why is my pronoun here?)