環境

Windows 10
Python 3.8.5
pip 20.1.1
venv環境

エラー内容

すでにインストールされていたnumpy 1.19.2を1.18.5にバージョンダウンしようとした時に，以下のUnicodeDecodeErrorが発生しました．
パスに日本語が含まれていたからでしょうか…？

(.venv) PS G:\研究\product> pip install numpy==1.18.5
Collecting numpy==1.18.5
  Using cached numpy-1.18.5-cp38-cp38-win_amd64.whl (12.8 MB)
Installing collected packages: numpy
  Attempting uninstall: numpy
    Found existing installation: numpy 1.19.2
ERROR: Exception:
Traceback (most recent call last):
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\cli\base_command.py", line 188, in _main
    status = self.run(options, args)
    return func(self, options, args)
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\commands\install.py", line 398, in run
    installed = install_given_reqs(
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\req\__init__.py", line 63, in install_given_reqs
    uninstalled_pathset = requirement.uninstall(
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\req\req_install.py", line 675, in uninstall
    uninstalled_pathset = UninstallPathSet.from_dist(dist)
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\req\req_uninstall.py", line 456, in from_dist
    if not dist_is_local(dist):
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\utils\misc.py", line 383, in dist_is_local
    return is_local(dist_location(dist))
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\utils\misc.py", line 529, in dist_location
    egg_link = egg_link_path(dist)
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\utils\misc.py", line 505, in egg_link_path
    if not virtualenv_no_global() and user_site:
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\utils\virtualenv.py", line 111, in virtualenv_no_global
    return _no_global_under_venv()
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\utils\virtualenv.py", line 72, in _no_global_under_venv
    cfg_lines = _get_pyvenv_cfg_lines()
  File "g:\研究\product\.venv\lib\site-packages\pip\_internal\utils\virtualenv.py", line 55, in _get_pyvenv_cfg_lines
    return f.read().splitlines()  # avoids trailing newlines
UnicodeDecodeError: 'cp932' codec can't decode byte 0x88 in position 24: illegal multibyte sequence

解決方法

エラーの一番下にある，.venv\lib\site-packages\pip\_internal\utils\virtualenv.pyの55行目を探して，以下のように変更しました．
ちなみに.venvはvenvで設定した環境名です．

変更前

with open(pyvenv_cfg_file) as f:
    return f.read().splitlines()

変更後

with open(pyvenv_cfg_file, encoding="utf-8") as f:   # encodingを追加
    return f.read().splitlines()

結果

無事バージョンダウンできました．

(.venv) PS G:\研究\product> pip install numpy==1.18.5
Collecting numpy==1.18.5
  Using cached numpy-1.18.5-cp38-cp38-win_amd64.whl (12.8 MB)
Installing collected packages: numpy
  Attempting uninstall: numpy
    Found existing installation: numpy 1.19.2
    Uninstalling numpy-1.19.2:
      Successfully uninstalled numpy-1.19.2
Successfully installed numpy-1.18.5