failure to build cython package

[christian-rauch]

I am trying to build a cython package with version 0.2.7. This fails with invalid syntax:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/colcon_core/package_identification/__init__.py", line 142, in _identify
    retval = extension.identify(_reused_descriptor_instance)
  File "/usr/lib/python3/dist-packages/colcon_python_setup_py/package_identification/python_setup_py.py", line 48, in identify
    config = get_setup_information(setup_py)
  File "/usr/lib/python3/dist-packages/colcon_python_setup_py/package_identification/python_setup_py.py", line 241, in get_setup_information
    _setup_information_cache[hashable_env] = _get_setup_information(
  File "/usr/lib/python3/dist-packages/colcon_python_setup_py/package_identification/python_setup_py.py", line 286, in _get_setup_information
    return ast.literal_eval(output)
  File "/usr/lib/python3.8/ast.py", line 59, in literal_eval
    node_or_string = parse(node_or_string, mode='eval')
  File "/usr/lib/python3.8/ast.py", line 47, in parse
    return compile(source, filename, mode, flags,
  File "<unknown>", line 1
    Compiling <PACKAGE> because it changed.
              ^
SyntaxError: invalid syntax

I can trace this down to stdout output in Cython https://github.com/cython/cython/blob/3424926e9c8f03061b55516d2516a9f98999399e/Cython/Build/Dependencies.py#L1074.

colcon-python-setup-py takes this stdout output and tries to evaluate it:

colcon-python-setup-py/colcon_python_setup_py/package_identification/python_setup_py.py

Lines 285 to 293 in ed426b5

	# invoke distutils.core.run_setup() in a separate interpreter
	cmd = [
	sys.executable, '-c', ';'.join(line.lstrip() for line in code_lines)]
	result = subprocess.run(
	cmd, stdout=subprocess.PIPE,
	cwd=os.path.abspath(str(setup_py.parent)), check=True, env=env)
	output = result.stdout.decode('utf-8')
	return ast.literal_eval(output)

[christian-rauch]

A workaround for this is to check for the --dry-run flag in the args and set the quiet flag in cythonize to suppress stdout and stderr.