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[mypyc] feat: optimize f-string building from Final values #19611

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[mypyc] feat: optimize f-string building from Final values #19611

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BobTheBuidler
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@BobTheBuidler BobTheBuidler commented Aug 7, 2025 •
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We can do some extra constant folding in cases like this:

from typing import Final

BASE_URL: Final = "https://example.com"
PORT: Final = 1234

def get_url(endpoint: str) -> str:
    return f"{BASE_URL}:{PORT}/{endpoint}"

which should generate the same C code as

def get_url(endpoint: str) -> str:
    return f"https://example.com:1234/{endpoint}"

This PR makes it so.

@BobTheBuidler BobTheBuidler changed the title WIP [mypyc] optimize f-string building [mypyc] feat: optimize f-string building from Final values Aug 11, 2025
@BobTheBuidler BobTheBuidler marked this pull request as ready for review August 11, 2025 22:05
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This PR is now ready for review.

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BobTheBuidler commented Aug 11, 2025 •
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This can probably be extended later on for non-final values that type check as Literals ex:

def test():
    abcd = "abcd"
    efgh = "efgh"
    if some_check():
        ijkl = "ijkl0"
        some_numbers = 123
        return f"{abcd}{efgh}{ijkl}{some_numbers}{some_fn()}"
    else:
        ijkl = "ijkl1"
        some_numbers = 456
        return f"{abcd}{efgh}{ijkl}{some_numbers}{some_fn()}"

But I think that goes beyond the scope of this PR. Once we merge this it will be easy to modify the literal helper fn.

JukkaL
JukkaL reviewed Aug 13, 2025
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Thanks for the PR! The approach looks good, just a few minor comments.

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Thanks for your feedbacks. Tests added, comment removed.

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