在pyproject.toml中添加system_platform限定符报错，如何解决？

我在空目录中有一个基础的pyproject.toml文件，内容如下：

[build-system]
requires = ["setuptools"]
build-backend = "setuptools.build_meta"

[project]
name = "test-virtualenvwrapper"
version = "0.1.0"

dependencies = [
    'virtualenvwrapper      ; system_platform == "linux"',
    'virtualenvwrapper-win  ; system_platform == "win32"',
]

运行pip install -e .时出现如下错误：

(dev38) go|c:\srv\tmp\pyprojplat> pip install -e .
Looking in indexes: https://pypi.org/simple, http://pypi.norsktest.xyz
Obtaining file:///C:/srv/tmp/pyprojplat
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
  Getting requirements to build editable ... error
  error: subprocess-exited-with-error

  × Getting requirements to build editable did not run successfully.
  │ exit code: 1
  ╰─> [48 lines of output]
      configuration error: `project.dependencies[{data__dependencies_x}]` must be pep508
      DESCRIPTION:
          Project dependency specification according to PEP 508

      GIVEN VALUE:
          "virtualenvwrapper      ; system_platform == \"linux\""

      OFFENDING RULE: 'format'

      DEFINITION:
          {
              "$id": "#/definitions/dependency",
              "title": "Dependency",
              "type": "string",
              "format": "pep508"
          }
      Traceback (most recent call last):
        File "C:\srv\venv\dev38\lib\site-packages\pip\_vendor\pyproject_hooks\_in_process\_in_process.py", line 353, in <module>
          main()
        File "C:\srv\venv\dev38\lib\site-packages\pip\_vendor\pyproject_hooks\_in_process\_in_process.py", line 335, in main
          json_out['return_val'] = hook(**hook_input['kwargs'])
        File "C:\srv\venv\dev38\lib\site-packages\pip\_vendor\pyproject_hooks\_in_process\_in_process.py", line 132, in get_requires_for_build_editable
          return hook(config_settings)
        File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\build_meta.py", line 447, in get_requires_for_build_editable
          return self.get_requires_for_build_wheel(config_settings)
        File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\build_meta.py", line 338, in get_requires_for_build_wheel
          return self._get_build_requires(config_settings, requirements=['wheel'])
        File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\build_meta.py", line 320, in _get_build_requires
          self.run_setup()
        File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\build_meta.py", line 335, in run_setup
          exec(code, locals())
        File "<string>", line 1, in <module>
        File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\__init__.py", line 108, in setup
          return distutils.core.setup(**attrs)
        File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\_distutils\core.py", line 159, in setup
          dist.parse_config_files()
        File "C:\srv\venv\dev38\lib\site-packages\_virtualenv.py", line 21, in parse_config_files
          result = old_parse_config_files(self, *args, **kwargs)
        File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\dist.py", line 885, in parse_config_files
          pyprojecttoml.apply_configuration(self, filename, ignore_option_errors)
        File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\config\pyprojecttoml.py", line 62, in apply_configuration
          config = read_configuration(filepath, True, ignore_option_errors, dist)
        File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\config\pyprojecttoml.py", line 126, in read_configuration
          validate(subset, filepath)
        File "C:\Users\me\AppData\Local\Temp\pip-build-env-2lncmkj6\overlay\Lib\site-packages\setuptools\config\pyprojecttoml.py", line 51, in validate
          raise ValueError(f"{error}\n{summary}") from None
      ValueError: invalid pyproject.toml config: `project.dependencies[{data__dependencies_x}]`.
      configuration error: `project.dependencies[{data__dependencies_x}]` must be pep508
      [end of output]

  note: This error originates from a subprocess, and is likely not a problem with pip.
error: subprocess-exited-with-error

× Getting requirements to build editable did not run successfully.
│ exit code: 1
╰─> See above for output.

note: This error originates from a subprocess, and is likely not a problem with pip.

(dev38) go|c:\srv\tmp\pyprojplat>

我认为自己的system_platform限定符写法符合PEP508规范，请问问题出在哪里？

问题出在依赖声明中分号前的空格。PEP508规范要求分号和前面的包名之间不能有多余空格，必须紧挨着写。

修改后的pyproject.toml依赖部分应该是这样：

dependencies = [
    'virtualenvwrapper; system_platform == "linux"',
    'virtualenvwrapper-win; system_platform == "win32"',
]

去掉分号前的所有空格后，setuptools的PEP508验证就能通过了。

内容的提问来源于stack exchange，提问作者thebjorn