extension _yaml is not built

[dfroger]

Trying to build the extension _yaml for pyyaml 4.1 on debian 8:

python setup.py build_ext

Fails with:

/home/dfroger/.pyenv/versions/3.6.2/lib/python3.6/distutils/dist.py:261: UserWarning: Unknown distribution option: 'python_requires'
  warnings.warn(msg)
running build_ext
building '_yaml' extension
creating build
creating build/temp.linux-x86_64-3.6
creating build/temp.linux-x86_64-3.6/ext
gcc -pthread -Wno-unused-result -Wsign-compare -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -fPIC -I/home/dfroger/.pyenv/versions/3.6.2/include/python3.6m -c ext/_yaml.c -o build/temp.linux-x86_64-3.6/ext/_yaml.o
gcc: error: ext/_yaml.c: No such file or directory
gcc: fatal error: no input files
compilation terminated.
Error compiling module, falling back to pure Python

Note that directory build/temp.linux-x86_64-3.6/ext is empty.

[dfroger]

When cython is installed, building the extension works.

[dfroger]

Actually, I still have a question:

python -m venv venv3.12
./venv3.12/bin/pip install pyyaml==3.12

python -m venv venv4.1
./venv4.1/bin/pip install pyyaml==4.1

find . -name '_yaml*'
./venv3.12/lib/python3.6/site-packages/_yaml.cpython-36m-x86_64-linux-gnu.so

why with 3.12, the extension was built, but not in 4.1?

Edit: In both cases, I didn't had cython installed.

[dfroger]

The problem is that in https://pypi.org/simple/pyyaml/ PyYAML-3.12.tar.gz contains ext/_yaml.c, but PyYAML-4.1.tar.gz do not, so cython is required for 4.1 to build the extension. (.pyx -> .c)

@sigmavirus24 could you make a 4.1 tarball that contains ext/_yaml.c, or would you accept a PR on that?

[ingydotnet]

I'll look into this as soon as our issues around building wheels are resolved.

[ingydotnet]

FYI PyYAML release 4.1 was retracted but this has already been fixed in 4.2b1 by applying #188