[wnoise]

Shouldn't it be expanded by the shell when it is first set?

FOO=~/bin/vim export FOO env | grep FOO shows it as expanded.

[junkblocker]

You are running that in shell by hand. The python program running here is not shell. If you do a print(os.environ["GIT_EDITOR"]) in the python3 program you'll still see ~/bin/vim .