Interested in a version of parsy with type annotations?

[denisrosset]

I had a look at the code base, it shouldn't be too difficult to add type annotations. I'd be happy to make a PR in that direction.

• Is there interest in that direction?
• If typing the code requires breaking changes or bumping the minimal Python version (I don't see why, but you never know), would you prefer releasing a new major version, or that I fork the library?

[spookylukey]

Sorry I didn't see this earlier, I don't seem to be getting any notifications for this repo.

The problem is, type checking in Python currently works very badly with any dynamic tricks, and Parsy uses enough to defeat type checkers.

Of course, you can add some basic types for methods, but nothing that will really help you. I mean, a type checker could make sure that you were only calling .parse() or .parse_partial() on something that had those methods, but you typically call those very few times.

What you actually want is that in the following code, my_val should be accepted, but my_val_2 should be rejected.

import parsy

my_val: int = parsy.regex(r"\d+").map(int).parse("1234")
my_val_2: int = parsy.regex(r"\d+").parse("1234")

Similarly, parser_1 should be accepted but parser_2 should not:

from dataclasses import dataclass

@dataclass
class Foo:
    str: val

parser_1 = parsy.string("test").map(Foo)
parser_2 = parsy.regex(r"\d+").map(int).map(Foo)

For this, we need parsers that are generic in their output types. Without this, type annotations are not very useful - yes we can say that we've got some Parser instances, but we don't know what they produce or how they can be chained together, which is 90% of what you do with Parsy code.

You can see my attempt to do this in #58 I got quite far, but then it all comes unstuck for seq, which is pretty essential in many cases.

If you can do better, please let me know!

[spookylukey]

I've created an experimental fork of parsy with an attempt at this: https://github.com/python-parsy/typed-parsy

The results are interesting - it's impressive what you can get mypy/pyright to check for you, but the code is much less readable. I'll be blogging about it soon.

[spookylukey]

See my blog post:

https://lukeplant.me.uk/blog/posts/python-type-hints-parsy-case-study/

I've also added basic type hints to master in #69 but not a py.typed file, as described in the blog post.

For these reasons I'm closing the ticket, but your feedback would still be appreciated.