Many fish_prompts use display the $status (fish's equivalent to $?) somewhere in the prompt. Currently, venv's activate.fish overwrites and wraps the user's fish_prompt, the wrapping doesn't preserve this $status. This patch ensures that the $status is correctly restored before invoking the user's fish_prompt.

This is based on my similar work in virtualenv, at https://github.com/pypa/virtualenv/pull/799

Fixed an issue where fish_prompt was returning before calling _old_fish_prompt

I am a user of fish.

The use case is just execute python -mvenv demo 
source demo/bin/activate.fish

check the status and deactivate the virtualenv ?

do you confirm ?

Thank you

Not quite, no. The issue looks like this:

    user@host ~/test> python3 -mvenv env
    user@host ~/test> true
    user@host ~/test> false
    user@host ~/test [1]> source env/bin/activate.fish 
    (env) user@host ~/test> true
    (env) user@host ~/test> false
    (env) user@host ~/test> deactivate
    user@host ~/test> true
    user@host ~/test> false
    user@host ~/test [1]>

Notice that, after running the `false` command the first time, the command prompt includes a "[1]", indicating the exit status of the `false` command. However, after activating the virtual environment, the command prompt no longer shows the error code from the `false` command. After deactivating the virtualenv, the command prompt once again displays the error code from `false`.

New changeset f6125944ffb8 by Vinay Sajip in branch '3.5':
Fixes #25671: Updated prompt handling in activate.fish.
https://hg.python.org/cpython/rev/f6125944ffb8

New changeset ed34b93519ba by Vinay Sajip in branch 'default':
Closes #25671: Merged fix from 3.5.
https://hg.python.org/cpython/rev/ed34b93519ba