您需要在 brew 切换之前/之后 运行 brew unlink / brew link 吗？
Do you need to run brew unlink / brew link before / after brew switch?
在brew switch之前/之后需要运行brew unlink/brew link吗？
根据这个linkhttps://docs.brew.sh/Tips-N'-Tricks#activate-a-previously-installed-version-of-a-formulabrew switch就是激活一个以前安装的版本的公式。
看起来我安装了一些'system' python（它安装在/usr/bin/python3下），不是用brew安装的，如果我安装就可以看到它brew unlink python@3.9 和 brew unlink python@3.7:
python -V
Python 3.7.3
/usr/bin/python3 -V
Python 3.7.3
which python3
/usr/bin/python3
然后我切换到python@3.9:
brew switch python@3.9 3.9.0_1
python -V
Python 3.9.0
然后我切换到python@3.7:
brew switch python@3.7 3.7.9
python -V
Python 3.9.0
但它仍然显示 Python 3.9.0，ls -l /usr/local/bin | grep python3 也显示 python3.9。
我还尝试在每次 brew 开关测试之前取消link 所有 brew python 包：
brew unlink python@3.7 && brew unlink python@3.9
brew switch python@3.7 3.7.9
python -V
Python 3.7.3
brew unlink python@3.7 && brew unlink python@3.9
brew switch python@3.9 3.9.0_1
python -V
Python 3.9.0
所以似乎出于某种原因它会在 brew 开关上自动 links python@3.9 而不要为 python@3.7 这样做，为什么？ brew switch 里面有 brew unlink / brew link 还是应该手动完成？
我可以直接 brew unlink / brew link 来切换 python 版本吗？
更新：
brew 的较新版本似乎没有 switch:
Error: Calling `brew switch` is disabled! Use `brew link` @-versioned formulae instead.
brew --version
Homebrew 2.7.7
Homebrew/homebrew-core (git revision 918f0; last commit 2021-02-04)
Homebrew/homebrew-cask (git revision 2b83c; last commit 2021-02-04)
所以现在应该是这样的：
brew unlink python@3.7 && brew unlink python@3.9
brew link python@3.7
python -V
Python 3.7.9
brew switch 不再是 brew 命令。
$ brew --version
Homebrew 3.0.0
Homebrew/homebrew-core (git revision 8d644; last commit 2021-02-10)
$ brew switch mariadb@10.3
Error: Unknown command: switch
您在问题中提到的 Tips and Tricks 页面也删除了对 brew switch 的任何提及。
Installing previous versions of formulae
The supported method of installing specific versions of some formulae
is to see if there is a versioned formula (e.g. gcc@7) available. If
the version you’re looking for isn’t available, consider using brew
extract.
或者，在 man brew 中没有提到 brew switch。
看来你的问题已经被 Homebrew 的维护者在上游解决了。
请注意，brew switch 自 Homebrew 2.6.0 以来已弃用。
==更新==
如果我们以 python@3.7 和 python@3.9 为例。我们应该首先检查 brew info python@3.7 和 brew info python@3.9 的输出。我们看到 python@3.7 是 keg-only:
python@3.7 is keg-only, which means it was not symlinked into /usr/local,
because this is an alternate version of another formula.
这不是 python@3.9 的情况，因为它现在是 python 公式的别名。这意味着自动安装 python@3.9 运行 brew link python@3.9 和安装 python@3.7 不会 运行 brew link python@3.7.
然后要列出哪些文件将与 python@3.9 链接，您可以 运行 brew link --dry-run python@3.9
Would link:
/usr/local/bin/2to3
/usr/local/bin/2to3-3.9
/usr/local/bin/easy_install-3.9
/usr/local/bin/idle3
/usr/local/bin/idle3.9
/usr/local/bin/pip3
/usr/local/bin/pip3.9
/usr/local/bin/pydoc3
/usr/local/bin/pydoc3.9
/usr/local/bin/python3
/usr/local/bin/python3-config
/usr/local/bin/python3.9
/usr/local/bin/python3.9-config
/usr/local/bin/wheel3
/usr/local/share/man/man1/python3.1
/usr/local/share/man/man1/python3.9.1
/usr/local/lib/pkgconfig/python-3.9-embed.pc
/usr/local/lib/pkgconfig/python-3.9.pc
/usr/local/lib/pkgconfig/python3-embed.pc
/usr/local/lib/pkgconfig/python3.pc
/usr/local/Frameworks/Python.framework/Headers
/usr/local/Frameworks/Python.framework/Python
/usr/local/Frameworks/Python.framework/Resources
/usr/local/Frameworks/Python.framework/Versions/3.9
/usr/local/Frameworks/Python.framework/Versions/Current
列出哪些文件将与 python@3.7, 运行 brew link --dry-run python@3.7:
Would link:
/usr/local/bin/2to3
/usr/local/bin/2to3-3.7
/usr/local/bin/easy_install-3.7
/usr/local/bin/idle3
/usr/local/bin/idle3.7
/usr/local/bin/pip3
/usr/local/bin/pip3.7
/usr/local/bin/pydoc3
/usr/local/bin/pydoc3.7
/usr/local/bin/python3
/usr/local/bin/python3-config
/usr/local/bin/python3.7
/usr/local/bin/python3.7-config
/usr/local/bin/python3.7m
/usr/local/bin/python3.7m-config
/usr/local/bin/pyvenv
/usr/local/bin/pyvenv-3.7
/usr/local/bin/wheel3
/usr/local/share/man/man1/python3.1
/usr/local/share/man/man1/python3.7.1
/usr/local/lib/pkgconfig/python-3.7.pc
/usr/local/lib/pkgconfig/python-3.7m.pc
/usr/local/lib/pkgconfig/python3.pc
/usr/local/Frameworks/Python.framework/Headers
/usr/local/Frameworks/Python.framework/Python
/usr/local/Frameworks/Python.framework/Resources
/usr/local/Frameworks/Python.framework/Versions/3.7
/usr/local/Frameworks/Python.framework/Versions/Current
所以为了回答你的问题，在 python@3.7 和 python@3.9 之间切换的一种方法是使用 brew link 和 brew unlink 是的，这可能会中断如果您的脚本与 python@3.7 兼容而不与 python@3.9 兼容，反之亦然。
还有一种方法可以同时使用不同版本的python。这在 brew link --dry-run python@3.7:
的输出末尾进行了解释
If you need to have this software first in your PATH instead consider running:
echo 'export PATH="/usr/local/opt/python@3.7/bin:$PATH"' >> ~/.zshrc
需要python@3.7的脚本，可以设置：
export PATH="/usr/local/opt/python@3.7/bin
对于需要python@3.9的脚本，您可以设置：
export PATH="/usr/local/opt/python@3.9/bin
希望这对您有所帮助。
在brew switch之前/之后需要运行brew unlink/brew link吗？
根据这个linkhttps://docs.brew.sh/Tips-N'-Tricks#activate-a-previously-installed-version-of-a-formulabrew switch就是激活一个以前安装的版本的公式。
看起来我安装了一些'system' python（它安装在/usr/bin/python3下），不是用brew安装的，如果我安装就可以看到它brew unlink python@3.9 和 brew unlink python@3.7:
python -V
Python 3.7.3
/usr/bin/python3 -V
Python 3.7.3
which python3
/usr/bin/python3
然后我切换到python@3.9:
brew switch python@3.9 3.9.0_1
python -V
Python 3.9.0
然后我切换到python@3.7:
brew switch python@3.7 3.7.9
python -V
Python 3.9.0
但它仍然显示 Python 3.9.0，ls -l /usr/local/bin | grep python3 也显示 python3.9。
我还尝试在每次 brew 开关测试之前取消link 所有 brew python 包：
brew unlink python@3.7 && brew unlink python@3.9
brew switch python@3.7 3.7.9
python -V
Python 3.7.3
brew unlink python@3.7 && brew unlink python@3.9
brew switch python@3.9 3.9.0_1
python -V
Python 3.9.0
所以似乎出于某种原因它会在 brew 开关上自动 links python@3.9 而不要为 python@3.7 这样做，为什么？ brew switch 里面有 brew unlink / brew link 还是应该手动完成？
我可以直接 brew unlink / brew link 来切换 python 版本吗？
更新：
brew 的较新版本似乎没有 switch:
Error: Calling `brew switch` is disabled! Use `brew link` @-versioned formulae instead.
brew --version
Homebrew 2.7.7
Homebrew/homebrew-core (git revision 918f0; last commit 2021-02-04)
Homebrew/homebrew-cask (git revision 2b83c; last commit 2021-02-04)
所以现在应该是这样的：
brew unlink python@3.7 && brew unlink python@3.9
brew link python@3.7
python -V
Python 3.7.9
brew switch 不再是 brew 命令。
$ brew --version
Homebrew 3.0.0
Homebrew/homebrew-core (git revision 8d644; last commit 2021-02-10)
$ brew switch mariadb@10.3
Error: Unknown command: switch
您在问题中提到的 Tips and Tricks 页面也删除了对 brew switch 的任何提及。
Installing previous versions of formulae
The supported method of installing specific versions of some formulae is to see if there is a versioned formula (e.g. gcc@7) available. If the version you’re looking for isn’t available, consider using brew extract.
或者，在 man brew 中没有提到 brew switch。
看来你的问题已经被 Homebrew 的维护者在上游解决了。
请注意，brew switch 自 Homebrew 2.6.0 以来已弃用。
==更新==
如果我们以 python@3.7 和 python@3.9 为例。我们应该首先检查 brew info python@3.7 和 brew info python@3.9 的输出。我们看到 python@3.7 是 keg-only:
python@3.7 is keg-only, which means it was not symlinked into /usr/local,
because this is an alternate version of another formula.
这不是 python@3.9 的情况，因为它现在是 python 公式的别名。这意味着自动安装 python@3.9 运行 brew link python@3.9 和安装 python@3.7 不会 运行 brew link python@3.7.
然后要列出哪些文件将与 python@3.9 链接，您可以 运行 brew link --dry-run python@3.9
Would link:
/usr/local/bin/2to3
/usr/local/bin/2to3-3.9
/usr/local/bin/easy_install-3.9
/usr/local/bin/idle3
/usr/local/bin/idle3.9
/usr/local/bin/pip3
/usr/local/bin/pip3.9
/usr/local/bin/pydoc3
/usr/local/bin/pydoc3.9
/usr/local/bin/python3
/usr/local/bin/python3-config
/usr/local/bin/python3.9
/usr/local/bin/python3.9-config
/usr/local/bin/wheel3
/usr/local/share/man/man1/python3.1
/usr/local/share/man/man1/python3.9.1
/usr/local/lib/pkgconfig/python-3.9-embed.pc
/usr/local/lib/pkgconfig/python-3.9.pc
/usr/local/lib/pkgconfig/python3-embed.pc
/usr/local/lib/pkgconfig/python3.pc
/usr/local/Frameworks/Python.framework/Headers
/usr/local/Frameworks/Python.framework/Python
/usr/local/Frameworks/Python.framework/Resources
/usr/local/Frameworks/Python.framework/Versions/3.9
/usr/local/Frameworks/Python.framework/Versions/Current
列出哪些文件将与 python@3.7, 运行 brew link --dry-run python@3.7:
Would link:
/usr/local/bin/2to3
/usr/local/bin/2to3-3.7
/usr/local/bin/easy_install-3.7
/usr/local/bin/idle3
/usr/local/bin/idle3.7
/usr/local/bin/pip3
/usr/local/bin/pip3.7
/usr/local/bin/pydoc3
/usr/local/bin/pydoc3.7
/usr/local/bin/python3
/usr/local/bin/python3-config
/usr/local/bin/python3.7
/usr/local/bin/python3.7-config
/usr/local/bin/python3.7m
/usr/local/bin/python3.7m-config
/usr/local/bin/pyvenv
/usr/local/bin/pyvenv-3.7
/usr/local/bin/wheel3
/usr/local/share/man/man1/python3.1
/usr/local/share/man/man1/python3.7.1
/usr/local/lib/pkgconfig/python-3.7.pc
/usr/local/lib/pkgconfig/python-3.7m.pc
/usr/local/lib/pkgconfig/python3.pc
/usr/local/Frameworks/Python.framework/Headers
/usr/local/Frameworks/Python.framework/Python
/usr/local/Frameworks/Python.framework/Resources
/usr/local/Frameworks/Python.framework/Versions/3.7
/usr/local/Frameworks/Python.framework/Versions/Current
所以为了回答你的问题，在 python@3.7 和 python@3.9 之间切换的一种方法是使用 brew link 和 brew unlink 是的，这可能会中断如果您的脚本与 python@3.7 兼容而不与 python@3.9 兼容，反之亦然。
还有一种方法可以同时使用不同版本的python。这在 brew link --dry-run python@3.7:
If you need to have this software first in your PATH instead consider running:
echo 'export PATH="/usr/local/opt/python@3.7/bin:$PATH"' >> ~/.zshrc
需要python@3.7的脚本，可以设置：
export PATH="/usr/local/opt/python@3.7/bin
对于需要python@3.9的脚本，您可以设置：
export PATH="/usr/local/opt/python@3.9/bin
希望这对您有所帮助。