@vstinner I expected to be able to just override prefix and exec_prefix, but they don't seem to flow through properly?

Hum, I tried the following change:

diff --git a/Modules/main.c b/Modules/main.c
index 853afedd7b..471dd0803c 100644
--- a/Modules/main.c
+++ b/Modules/main.c
@@ -65,6 +65,11 @@ pymain_init(const _PyArgv *args)
         return status;
     }
 
+    status = PyConfig_SetString(&config, &config.exec_prefix, L"/victor");
+    if (_PyStatus_EXCEPTION(status)) {
+        return status;
+    }
+
     /* pass NULL as the config: config is read from command line arguments,
        environment variables, configuration files */
     if (args->use_bytes_argv) {

It works for me:

$ ./python -c 'import sys; print(sys.exec_prefix)'
/victor

I think I was expecting that updating the prefix would be enough to locate the files under the prefix, but that doesn't work - it still comes up with the wrong sys.path default. So it's probably best to set home and let it infer prefix and exec_prefix.

Thanks for your other comments!

I think I was expecting that updating the prefix would be enough to locate the files under the prefix, but that doesn't work - it still comes up with the wrong sys.path default. So it's probably best to set home and let it infer prefix and exec_prefix.

Python Path Configuration is a black box. Nobody knows how it works! :-D

I'm going to give this one more review in the morning with fresh eyes (and coffee), but it should basically match the fix that's gone in for 3.7 already. This one defines more new API rather than just using a hack though (but I still want it in for the next 3.8.0 beta).

Thanks @zooba for the PR 🌮🎉.. I'm working now to backport this PR to: 3.8.
🐍🍒⛏🤖 I'm not a witch! I'm not a witch!

Sorry, @zooba, I could not cleanly backport this to 3.8 due to a conflict.
Please backport using cherry_picker on command line.
cherry_picker 9048c49322a5229ff99610aba35913ffa295ebb7 3.8

GH-14467 is a backport of this pull request to the 3.8 branch.