Currently, the activate.fish VENV script sets fish's prompt to be prepended with a literal "__VENV_PROMPT__" rather than the contents of the $__VENV_PROMPT__ variable as intended.

The attached patch simply adds the missing "$" to the variable in the conditional test and it's usage, so it's only being set if the variable is non-zero, rather than testing if the string "__VENV_PROMPT__" is non-zero like it is doing right now.

The results in the prompt being correctly prepended by "(my_actual_venv_name)".

New changeset cfc66e37eb8e by Vinay Sajip in branch '3.5':
Fixes #26348: Corrected typos in activate.fish script.
https://hg.python.org/cpython/rev/cfc66e37eb8e

New changeset 0f1ac94d2f98 by Vinay Sajip in branch 'default':
Fixes #26348: Merged fix from 3.5.
https://hg.python.org/cpython/rev/0f1ac94d2f98

Implementing this patch has led to another issue being raised: #26664. Dan - would you care to take a look and comment? Thanks.

The original behaviour is right. __VENV_PROMPT__ is meant to be a placeholder, not a variable interpreted by shell. I think this patch should be reverted. Related issue is #26664.

I somehow missed the comment on 2016-04-01, I apologize for being late to look at this.

Xiang is right, this patch should be reverted.

It looks like the issue reported was originally brought on by an activate.fish being used from a Python after the patch in commit 7f913c6ada03 was added, but the running version of Python did not include the patch, so that the placeholder was not being swapped out. In that case, updating the script to include $ and use the variable fixed the problem, but when the patch in 7f913c6ada03 is also included it causes the placeholder to be swapped to a name wrapped in parenthesis preceded by $ at the time the venv is created causing a syntax error in fish.

Since the patch in commit 7f913c6ada03 swaps that place holder and handles doing so in the other activate scripts as well, the patch that was part of this issue (cfc66e37eb8e) is not needed and should be reverted.

Here's the commit adding that placeholder for context:

https://hg.python.org/cpython/rev/7f913c6ada03

So just to be clear, cfc66e37eb8e and 0f1ac94d2f98 should be reverted? No other changes?

And will this fix issue #26664 or affect it in any way?

Yes, reverting cfc66e37eb8e and 0f1ac94d2f98 should resolve #26664.

I'll revert the two commits then and close both this and #26664 when I have a chance.

Thanks Brett.