Generic strings/regex patterns in strings #1202

Fidget-Spinner · 2022-05-31T14:31:47Z

Currently, we have str and LiteralStr. These don't do a good job expressing the range of values a string may have. Let's say we have the following function:

Email: TypeAlias = str

def send_email(email: Email) -> int:
    # Send an email
    ...

# passes
send_email("test@example.com")

# passes
send_email("badddd")

# passes even if unknown_var is only identified as `str`
send_email(unknown_var)

We could improve it by having something like this:

# This is a bad regex pattern, but you get the idea :)
Email: TypeAlias = str['[a-z0-9]+@[a-z0-9]+\.[a-z0-9]+']

def send_email(email: Email) -> int:
    # Send an email
    ...

# passes
send_email("test@example.com")

# fails
send_email("badddd")

# passes even if unknown_var is only identified as `str`
send_email(unknown_var)

A static type checker would be able to validate strings passed in by code. I would imagine this idea can be extended to Pattern and Match generics as well, but I haven't thought too deeply about them yet.

If we don't want to make str generic, we could add a new type to typing called StrPattern. We would need it anyways to backport to typing_extensions.

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henribru · 2022-05-31T15:00:49Z

There's kind of some prior art in Typescript: https://www.typescriptlang.org/docs/handbook/2/template-literal-types.html

It's not quite as powerful as this regex idea though.

gvanrossum · 2022-05-31T15:46:17Z

The TypeScript feature allows more than type checking though -- IIRC it allows constructing new (string literal) types from other strings.

I personally don't think a feature to let type checkers use regex matching on literals is all that useful -- I'd rather use Email = NewType(str) and leave the validation to runtime code that the type checker doesn't have to be understand. (It's likely that you already have an email validation routine in your system, and it may not be easy to replicate its exact functionality as a regex.)

Fidget-Spinner added the topic: feature Discussions about new features for Python's type annotations label May 31, 2022

Generic strings/regex patterns in strings #1202

Generic strings/regex patterns in strings #1202

Fidget-Spinner commented May 31, 2022 •

edited

henribru commented May 31, 2022

gvanrossum commented May 31, 2022

Generic strings/regex patterns in strings #1202

Generic strings/regex patterns in strings #1202

Comments

Fidget-Spinner commented May 31, 2022 • edited

henribru commented May 31, 2022

gvanrossum commented May 31, 2022

Fidget-Spinner commented May 31, 2022 •

edited