Emscripten build fails with Python 3.14

Bug report

Bug description:

I tried to use emsdk 4.10.0 and ran ./Tools/wasm/wasm_build.py emscripten-browser build repl against the v3.14.0rc1 branch, but the build failed:

configure: error: You must get working getaddrinfo() function or pass the "--disable-ipv6" option to configure.
emconfigure: error: '../../configure -C --host=wasm32-unknown-emscripten --build=x86_64-unknown-linux-gnu --with-emscripten-target=browser --enable-wasm-dynamic-linking --with-build-python=/tmp/wasm/cpython/builddir/build/python CONFIG_SITE=/tmp/wasm/cpython/Tools/wasm/config.site-wasm32-emscripten' failed (returned 1)
Traceback (most recent call last):
  File "/tmp/wasm/cpython/./Tools/wasm/wasm_build.py", line 932, in <module>
    main()
    ~~~~^^
  File "/tmp/wasm/cpython/./Tools/wasm/wasm_build.py", line 902, in main
    builder.run_build(*cm_args)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/tmp/wasm/cpython/./Tools/wasm/wasm_build.py", line 571, in run_build
    self.run_configure(*args)
    ~~~~~~~~~~~~~~~~~~^^^^^^^
  File "/tmp/wasm/cpython/./Tools/wasm/wasm_build.py", line 577, in run_configure
    return self._run_cmd(self.configure_cmd, args)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/wasm/cpython/./Tools/wasm/wasm_build.py", line 557, in _run_cmd
    return subprocess.check_call(
           ~~~~~~~~~~~~~~~~~~~~~^
        cmd,
        ^^^^
        cwd=os.fspath(cwd),
        ^^^^^^^^^^^^^^^^^^^
        env=self.getenv(),
        ^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/trey/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/subprocess.py", line 419, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['/opt/emsdk/upstream/emscripten/emconfigure', '../../configure', '-C', '--host=wasm32-unknown-emscripten', '--build=x86_64-unknown-linux-gnu', '--with-emscripten-target=browser', '--enable-wasm-dynamic-linking', '--with-build-python=/tmp/wasm/cpython/builddir/build/python', 'CONFIG_SITE=/tmp/wasm/cpython/Tools/wasm/config.site-wasm32-emscripten']' returned non-zero exit status 1.

It seems like the config_site settings are pointing to the wrong subdirectories.

I fixed that for Emscripten like this (in this branch):

 EMSCRIPTEN = Platform(
     "emscripten",
     pythonexe="python.js",
-    config_site=WASMTOOLS / "config.site-wasm32-emscripten",
+    config_site=WASMTOOLS / "emscripten"/ "config.site-wasm32-emscripten",

The build still fails after this:

emcc: warning: EXPORTED_FUNCTIONS is not valid with LINKABLE set (normally due to SIDE_MODULE=1/MAIN_MODULE=1) since all functions are exported this mode.  To export only a subset use SIDE_MODULE=2/MAIN_MODULE=2 [-Wunused-command-line-argument]
emcc: error: undefined exported symbol: "__PyEM_EMSCRIPTEN_COUNT_ARGS_OFFSET" [-Wundefined] [-Werror]
make: *** [Makefile:1115: python.mjs] Error 1
emmake: error: 'make all' failed (returned 2)
Traceback (most recent call last):
  File "/tmp/wasm/cpython/./Tools/wasm/wasm_build.py", line 932, in <module>
    main()
    ~~~~^^
  File "/tmp/wasm/cpython/./Tools/wasm/wasm_build.py", line 902, in main
    builder.run_build(*cm_args)
    ~~~~~~~~~~~~~~~~~^^^^^^^^^^
  File "/tmp/wasm/cpython/./Tools/wasm/wasm_build.py", line 572, in run_build
    self.run_make("all", *args)
    ~~~~~~~~~~~~~^^^^^^^^^^^^^^
  File "/tmp/wasm/cpython/./Tools/wasm/wasm_build.py", line 581, in run_make
    return self._run_cmd(self.make_cmd, args)
           ~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^
  File "/tmp/wasm/cpython/./Tools/wasm/wasm_build.py", line 557, in _run_cmd
    return subprocess.check_call(
           ~~~~~~~~~~~~~~~~~~~~~^
        cmd,
        ^^^^
        cwd=os.fspath(cwd),
        ^^^^^^^^^^^^^^^^^^^
        env=self.getenv(),
        ^^^^^^^^^^^^^^^^^^
    )
    ^
  File "/home/trey/.local/share/uv/python/cpython-3.13.1-linux-x86_64-gnu/lib/python3.13/subprocess.py", line 419, in check_call
    raise CalledProcessError(retcode, cmd)
subprocess.CalledProcessError: Command '['/opt/emsdk/upstream/emscripten/emmake', 'make', 'all']' returned non-zero exit status 1.

Using emsdk 4.10.0 and running ./Tools/wasm/wasm_build.py emscripten-browser build repl against the v3.13.5 branch works.

CPython versions tested on:

3.14

Operating systems tested on:

Linux

Linked PRs

@hoodmane

cc @hoodmane

@brettcannon

The new way to build Emscripten is with:

python Tools/wasm/emscripten build

I don't think anyone uses wasm_build.py anymore so we should probably remove it (cc @brettcannon )

Maybe it would be good to add a news entry about this change.

I agree a news entry is a good idea. I would also update Tools/wasm/README.md to no longer recommend wasm_build.py:

cpython/Tools/wasm/README.md

Lines 12 to 14 in aafb143

    
           Users and developers are encouraged to use the script 
        
           `Tools/wasm/wasm_build.py`. The tool automates the build process and provides 
        
           assistance with installation of SDKs, running tests, etc.

Right, the new instructions are in the dev guide here:
https://devguide.python.org/contrib/workflows/compile/#emscripten

I can update the readme to link there.

Further down in the readme it does say to use python Tools/wasm/emscripten:

cpython/Tools/wasm/README.md

Lines 58 to 68 in aafb143

    
           ```shell 
        
           python Tools/wasm/emscripten build 
        
           ``` 
        
           or you can break it out into four separate steps: 
        
           ```shell 
        
           python Tools/wasm/emscripten configure-build-python 
        
           python Tools/wasm/emscripten make-build-python 
        
           python Tools/wasm/emscripten make-libffi 
        
           python Tools/wasm/emscripten configure-host 
        
           python Tools/wasm/emscripten make-host 
        
           ```

The new way to build Emscripten is with:
python Tools/wasm/emscripten build
I don't think anyone uses wasm_build.py anymore so we should probably remove it (cc @brettcannon )

I should have read further down in the README. Thanks!

Using python Tools/wasm/emscripten build seemed to work, but the demo didn't seem to work.

I had to manually copy Tools/wasm/emscripten/web_example/python.worker.mjs to cross-build/wasm32-emscripten/build/python/web_example. Then running python server.py from cross-build/wasm32-emscripten/build/python/web_example worked as expected.

Separately, I'm not sure how to get the JSPI demo working. I noticed there was not a server.py file in the new web_example_pyrepl_jspi/ directory and tried python ../web_example/server.py but that didn't seem to result in a working demo. The demo at http://127.0.0.1:8000 showed a dark background (the xterm.js terminal background color I assume) and didn't load anything nor show any errors in the console or network inspector tabs. The browser tab also froze. This same same black window and freezing happened in 2 versions of Chrome and in Firefox.

The JSPI demo can be served with http.server. It should work with Chrome version >= 137. (Firefox requires an experimental flag.)

Which versions of Chrome did you try? Could you try opening the dev console before visiting the page with Ctrl+Shift+J and then navigate to the page and see if you see anything in the console?

I had to manually copy Tools/wasm/emscripten/web_example/python.worker.mjs to cross-build/wasm32-emscripten/build/python/web_example.

Oops, I messed that up, sorry. I'll make a PR to fix it tomorrow.

The JSPI demo can be served with http.server. It should work with Chrome version >= 137. (Firefox requires an experimental flag.)

Which versions of Chrome did you try? Could you try opening the dev console before visiting the page with Ctrl+Shift+J and then navigate to the page and see if you see anything in the console?

Ah I was apparently still on Chrome 136. I just upgraded and it works now!

I'm very excited to ship an in-browser pyrepl in the future. Thanks to all who made this possible.

gh-137025: Include ``python.worker.mjs`` in the Emscripten Web Example (

pythongh-137025: Include ``python.worker.mjs`` in the Emscripten Web …

[3.14] gh-137025: Include ``python.worker.mjs`` in the Emscripten Web…

gh-137025: Update Emscripten Build Docs (#137312)

I'm fairly certain this is resolved now; closing. If anyone is still having problems, let us know.

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Emscripten build fails with Python 3.14 #137025

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Bug description:

CPython versions tested on:

Operating systems tested on:

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	Users and developers are encouraged to use the script
	`Tools/wasm/wasm_build.py`. The tool automates the build process and provides
	assistance with installation of SDKs, running tests, etc.

	```shell
	python Tools/wasm/emscripten build
	```
	or you can break it out into four separate steps:
	```shell
	python Tools/wasm/emscripten configure-build-python
	python Tools/wasm/emscripten make-build-python
	python Tools/wasm/emscripten make-libffi
	python Tools/wasm/emscripten configure-host
	python Tools/wasm/emscripten make-host
	```

Emscripten build fails with Python 3.14 #137025

Description

Bug report

Bug description:

CPython versions tested on:

Operating systems tested on:

Linked PRs

Activity

StanFromIreland commented on Jul 22, 2025

hoodmane commented on Jul 22, 2025

hoodmane commented on Jul 22, 2025

emmatyping commented on Jul 22, 2025

hoodmane commented on Jul 22, 2025

hoodmane commented on Jul 22, 2025

treyhunner commented on Jul 22, 2025

hoodmane commented on Jul 22, 2025

hoodmane commented on Jul 22, 2025

treyhunner commented on Jul 23, 2025

freakboy3742 commented on Oct 9, 2025

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