Running :Black in Neovim (installed via vim-plug) throws error: "invoking 'python_execute' on channel 3 (python3-script-host)"

[erikr]

Describe the bug

Running :Black in a fresh install of Neovim (via Homebrew), with black installed via vim-plug, results in the following error:

Error detected while processing function provider#python3#Call:
line   18:
Error invoking 'python_execute' on channel 3 (python3-script-host):
Traceback (most recent call last):
  File "<string>", line 97, in <module>
ModuleNotFoundError: No module named 'black'
Error detected while processing function black#Black[1]..provider#python3#Call
:
line   18:
Error invoking 'python_execute' on channel 3 (python3-script-host):
Traceback (most recent call last):
  File "<string>", line 1, in <module>
NameError: name 'Black' is not defined

To Reproduce

1. Install neovim via brew install neovim.
2. Install pynvim via conda install -c conda-forge pynvim.
3. Install vim-plug.
4. Add Plug 'psf/black', { 'branch': 'stable' } to ~/.config/nvim/init.vim.
5. Install plugins via nvim -c PlugInstall, or inside of neovim, running :PlugInstall.
6. Close and reopen neovim, then run Black via :Black.

Expected behavior
black formats Python code without the errors.

Environment (please complete the following information):

• Plugin manager: vim-plug
• Version of black: I have Plug 'psf/black', { 'branch': 'stable' } in my init.vim
• OS and Python version: macOS 11.4, Python 3.9.5 (in system; unclear exact version in Neovim)
• Neovim version:

  NVIM v0.5.0
  
  Build type: Release
  LuaJIT 2.1.0-beta3
  Compilation: clang -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 -DNVIM_TS_HAS_SET_MATCH_LIMIT -O2 -DNDEBUG -Wall -Wextra -pedantic -Wno-unused-parameter -Wstrict-prototypes -std=gnu99 -Wshadow -Wconversion -Wmissing-prototypes -Wimplicit-fallthrough -Wvla -fstack-protector-strong -fno-common -fdiagnostics-color=auto -DINCLUDE_GENERATED_DECLARATIONS -D_GNU_SOURCE -DNVIM_MSGPACK_HAS_FLOAT32 -DNVIM_UNIBI_HAS_VAR_FROM -DMIN_LOG_LEVEL=3 -I/tmp/neovim-20210702-14987-rep9di/neovim-0.5.0/build/config -I/tmp/neovim-20210702-14987-rep9di/neovim-0.5.0/src -I/usr/local/include -I/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include -I/usr/local/opt/gettext/include -I/tmp/neovim-20210702-14987-rep9di/neovi-0.5.0/build/src/nvim/auto -I/tmp/neovim-20210702-14987-rep9di/neovim-0.5.0/build/include
  Compiled by brew@BigSur
  
  Features: +acl +iconv +tui
  See ":help feature-compile"
  
     system vimrc file: "$VIM/sysinit.vim"
    fall-back for $VIM: "/usr/local/Cellar/neovim/0.5.0/share/nvim"
  
  Run :checkhealth for more info
  

Does this bug also happen on main?

No.

Additional context

[hauntsaninja]

Closing as a duplicate of #2547