pip installでCythonインストール済みなのに、なぜかほかのライブラリをインストールするときにModuleNotFoundError: No module named 'Cython'となって、なかなか解決できなかったのでメモ。

環境

python3.7.3

インストール方法

$ pip3.7 install --upgrade pip
$ pip3.7 install --upgrade cython
$ pip3.7 install cython_bbox

エラー内容

cython_bboxをインストールする前にcythonのインストールは成功しているのだが、なぜかエラー。

Collecting cython_bbox
  Downloading cython_bbox-0.1.3.tar.gz (41 kB)
     ━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━━ 41.3/41.3 kB 261.9 kB/s eta 0:00:00
  Installing build dependencies: started
  Installing build dependencies: finished with status 'done'
  Getting requirements to build wheel: started
  Getting requirements to build wheel: finished with status 'error'
  error: subprocess-exited-with-error
  
  × Getting requirements to build wheel did not run successfully.
  │ exit code: 1
  ╰─> [17 lines of output]
      Traceback (most recent call last):
        File "/usr/local/lib/python3.7/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 353, in <module>
          main()
        File "/usr/local/lib/python3.7/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 335, in main
          json_out['return_val'] = hook(**hook_input['kwargs'])
        File "/usr/local/lib/python3.7/site-packages/pip/_vendor/pyproject_hooks/_in_process/_in_process.py", line 118, in get_requires_for_build_wheel
          return hook(config_settings)
        File "/tmp/pip-build-env-6p_jm7_z/overlay/lib/python3.7/site-packages/setuptools/build_meta.py", line 341, in get_requires_for_build_wheel
          return self._get_build_requires(config_settings, requirements=['wheel'])
        File "/tmp/pip-build-env-6p_jm7_z/overlay/lib/python3.7/site-packages/setuptools/build_meta.py", line 323, in _get_build_requires
          self.run_setup()
        File "/tmp/pip-build-env-6p_jm7_z/overlay/lib/python3.7/site-packages/setuptools/build_meta.py", line 488, in run_setup
          self).run_setup(setup_script=setup_script)
        File "/tmp/pip-build-env-6p_jm7_z/overlay/lib/python3.7/site-packages/setuptools/build_meta.py", line 338, in run_setup
          exec(code, locals())
        File "<string>", line 10, in <module>
      ModuleNotFoundError: No module named 'Cython'
      [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 wheel 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.
The command '/bin/sh -c pip3.7 install cython_bbox' returned a non-zero code: 1

解決方法

色々なブログでpipのupgradeが必要と書かれていたが、setuptoolsとwheelもupgradeが必要だった。
もちろん環境によっては別の原因もあると思います。

$ pip3.7 install --upgrade pip setuptools wheel
$ pip3.7 install --upgrade cython
$ pip3.7 install cython_bbox