python - pyenv configure: error: C compiler cannot create executables

Question

I am trying to install multiple versions of Python on my laptop running MacOS Big Sur v11.1. I initially installed xcode command line tools, homebrew, and python via instructions here

xcode-select --install

/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/master/install.sh)"

brew install python

But later on involuntarily upgraded python to 3.9, and I need to go back to 3.7.9. So I installed pyenv and attempted to install python 3.7.9 per instructions here

brew install pyenv

pyenv install 3.7.9

But I encountered the following error:

/var/folders/8n/ml0qwc091w9bhpszzxy9djl00000gn/T/python-build.20210118111111.56108 /usr/local/Cellar
/var/folders/8n/ml0qwc091w9bhpszzxy9djl00000gn/T/python-build.20210118111111.56108/Python-3.7.9 /var/folders/8n/ml0qwc091w9bhpszzxy9djl00000gn/T/python-build.20210118111111.56108 /usr/local/Cellar
checking build system type... x86_64-apple-darwin20.2.0
checking host system type... x86_64-apple-darwin20.2.0
checking for python3.7... no
checking for python3... python3
checking for --enable-universalsdk... no
checking for --with-universal-archs... no
checking MACHDEP... checking for --without-gcc... no
checking for --with-icc... no
checking for gcc... clang
checking whether the C compiler works... no
configure: error: in `/var/folders/8n/ml0qwc091w9bhpszzxy9djl00000gn/T/python-build.20210118111111.56108/Python-3.7.9':
configure: error: C compiler cannot create executables
See `config.log' for more details
make: *** No targets specified and no makefile found.  Stop.

From other answers online it seems my gcc may be out of date. I checked my gcc version with

[/usr/local/Cellar]$ gcc --version
Configured with: --prefix=/Library/Developer/CommandLineTools/usr
    --with-gxx-include-dir=/Library/Developer/CommandLineTools/SDKs/MacOSX.sdk/usr/include/c++/4.2.1 Apple clang version 11.0.3 (clang-1103.0.32.62) Target: x86_64-apple-darwin20.2.0 Thread model: posix InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Which seems to indicate I am on version 4.2.1. I have not been able to find a way to update it on my own yet, but did brew install gcc which did not resolve the problem. Any help would be much appreciated

EDIT: Seems pyenv is using clang and not gcc. Perhaps xcode-select installed both? clang --version returns:

Apple clang version 11.0.3 (clang-1103.0.32.62)
Target: x86_64-apple-darwin20.2.0
Thread model: posix
InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Answer 1

This worked for me: I removed the command line tools as root and reinstalled ...

Your Command Line Tools (CLT) does not support macOS 11. It is either outdated or was modified. Please update your Command Line Tools (CLT) or delete it if no updates are available.

Update them from Software Update in System Preferences or run:

softwareupdate --all --install --force

If that doesn't show you any updates, run:

sudo rm -rf /Library/Developer/CommandLineTools
sudo xcode-select --install

Alternatively, manually download them from:

https://developer.apple.com/download/more/.