According to the C-API documentation¹ for `Py_BuildValue`:

> The characters space, tab, colon and comma are ignored in format strings (but not within format units such as s#). This can be used to make long format strings a tad more readable.

However format strings such as `"(d )"` cause the error:

> Unmatched paren in format

By contrast `"( d)"` and `"(d d)"` cause no problems. I therefore assume that tuples are not considered “format units” in the sense of the above quote and this is a bug. Alternatively, the documentation needs clarification.

I could reproduce this problem with Python 3.7 and 3.8. I did not try other versions.

Appended is a minimal C extension exhibiting the problem. I compile and run with:

gcc -fPIC -I/usr/include/python3.8 -c foo.c -o foo.o
gcc -shared foo.o -o foo.so
python3.8 -c "import foo; foo.bar()"

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¹ https://docs.python.org/3/c-api/arg.html