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PEP 818: Adding the Core of the Pyodide Foreign Function Interface to Python #4739

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PEP 818: Adding the Core of the Pyodide Foreign Function Interface to Python #4739

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@hoodmane hoodmane commented Dec 10, 2025 •
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Basic requirements (all PEP Types)

  • Read and followed PEP 1 & PEP 12
  • File created from the latest PEP template
  • PEP has next available number, & set in filename (pep-NNNN.rst), PR title (PEP 123: <Title of PEP>) and PEP header
  • Title clearly, accurately and concisely describes the content in 79 characters or less
  • Core dev/PEP editor listed as Author or Sponsor, and formally confirmed their approval
  • Author, Status (Draft), Type and Created headers filled out correctly
  • PEP-Delegate, Topic, Requires and Replaces headers completed if appropriate
  • Required sections included
    • Abstract (first section)
    • Copyright (last section; exact wording from template required)
  • Code is well-formatted (PEP 7/PEP 8) and is in code blocks, with the right lexer names if non-Python
  • PEP builds with no warnings, pre-commit checks pass and content displays as intended in the rendered HTML
  • Authors/sponsor added to .github/CODEOWNERS for the PEP

Standards Track requirements

  • PEP topic discussed in a suitable venue with general agreement that a PEP is appropriate
  • Suggested sections included (unless not applicable)
    • Motivation
    • Rationale
    • Specification
    • Backwards Compatibility
    • Security Implications
    • How to Teach This
    • Reference Implementation
    • Rejected Ideas
    • Open Issues
  • Python-Version set to valid (pre-beta) future Python version, if relevant
  • Any project stated in the PEP as supporting/endorsing/benefiting from the PEP formally confirmed such
  • Right before or after initial merging, PEP discussion thread created and linked to in Discussions-To and Post-History

📚 Documentation preview 📚: https://pep-previews--4739.org.readthedocs.build/

@hoodmane hoodmane requested a review from a team as a code owner December 10, 2025 21:31
@hoodmane hoodmane force-pushed the js-ffi branch 2 times, most recently from c55b8c8 to 7fe9188 Compare December 10, 2025 21:34
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hugovk commented Dec 10, 2025

I've added the new PEP checklist (from https://github.com/python/peps/blob/main/.github/PULL_REQUEST_TEMPLATE/Add%20a%20new%20PEP.md), please could you check off as necessary?

@ambv Please can you confirm sponsorship?

@hoodmane hoodmane force-pushed the js-ffi branch 3 times, most recently from a6472ef to 82fbc12 Compare December 10, 2025 21:47
@hoodmane hoodmane changed the title PEP TBD: Upstreaming the Pyodide js ffi PEP 817: Upstreaming the Pyodide js ffi Dec 10, 2025
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hoodmane commented Dec 10, 2025

Thanks Hugo!

@hoodmane hoodmane force-pushed the js-ffi branch 2 times, most recently from 9c773b1 to abda220 Compare December 10, 2025 21:54
@hugovk hugovk mentioned this pull request Dec 10, 2025
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@hugovk hugovk changed the title PEP 817: Upstreaming the Pyodide js ffi PEP 9999: Upstreaming the Pyodide js ffi Dec 10, 2025
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hugovk commented Dec 10, 2025

Let's renumber this to 9999 for now, we also have #4740 clashing.

@hoodmane hoodmane added the new-pep A new draft PEP submitted for initial review label Dec 10, 2025
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ambv commented Dec 11, 2025

@ambv Please can you confirm sponsorship?

Confirmed!

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hugovk commented Dec 11, 2025

Thanks!

@hoodmane Please use PEP 818.

@hugovk hugovk changed the title PEP 9999: Upstreaming the Pyodide js ffi PEP 818: Upstreaming the Pyodide js ffi Dec 11, 2025
@hoodmane hoodmane force-pushed the js-ffi branch 2 times, most recently from 68edc4b to ec236c4 Compare December 11, 2025 16:57
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hoodmane commented Dec 11, 2025 •
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Okay updated it to say pep 818 everywhere.

@AA-Turner AA-Turner changed the title PEP 818: Upstreaming the Pyodide js ffi PEP 818: Upstreaming the Pyodide JS FFI Dec 23, 2025
@hoodmane hoodmane changed the title PEP 818: Upstreaming the Pyodide JS FFI PEP 818: Adding the Core of the Pyodide Foreign Function Interface to Python Jan 6, 2026
encukou
encukou reviewed Jan 9, 2026
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peps/pep-0818.rst
``jskey``.
b. Set the ith entry of ``pykwnames`` to ``_Py_js2python(jsitem)``.

6. Let ``pyresult`` be ``_PyObject_Vectorcall(callable, pyargs, num_pos_args, pykwnames)``.
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It's PyObject_Vectorcall now; the underscored name is a backcompat alias.

Suggested change
6. Let ``pyresult`` be ``_PyObject_Vectorcall(callable, pyargs, num_pos_args, pykwnames)``.
6. Let ``pyresult`` be ``PyObject_Vectorcall(callable, pyargs, num_pos_args, pykwnames)``.

(I opened a docs PR for CPython to make this clear: python/cpython#143568)

peps/pep-0818.rst
Comment on lines +949 to +964
same thing as a Python reserved word (for example, ``Array.from``,
``Promise.then``). We access these from Python using the valid identifiers
``from_`` and ``then_``. If we want to access a JavaScript property called
``then_`` we access it from ``then__`` and so on. So if the attribute is a
Python reserved word followed by one or more underscores, we remove one
underscore from the end. The following helper function is used for this:

.. code:: python

def normalize_python_reserved_words(attr):
stripped = attr.strip("_")
if not is_python_reserved_word(stripped):
return attr
if stripped != attr:
return attr[:-1]
return attr
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They're called keywords, and there's a iskeyword function.

Suggested change
same thing as a Python reserved word (for example, ``Array.from``,
``Promise.then``). We access these from Python using the valid identifiers
``from_`` and ``then_``. If we want to access a JavaScript property called
``then_`` we access it from ``then__`` and so on. So if the attribute is a
Python reserved word followed by one or more underscores, we remove one
underscore from the end. The following helper function is used for this:
.. code:: python
def normalize_python_reserved_words(attr):
stripped = attr.strip("_")
if not is_python_reserved_word(stripped):
return attr
if stripped != attr:
return attr[:-1]
return attr
same thing as a Python keyword (for example, ``Array.from``,
``Promise.then``). We access these from Python using the valid identifiers
``from_`` and ``then_``. If we want to access a JavaScript property called
``then_`` we access it from ``then__`` and so on. So if the attribute is a
Python keyword followed by one or more underscores, we remove one
underscore from the end. The following helper function is used for this:
.. code:: python
def normalize_python_keywords(attr):
stripped = attr.strip("_")
if not keyword.iskeyword(stripped):
return attr
if stripped != attr:
return attr[:-1]
return attr

(References to reserved_words below also need updating.)

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