[3.15] gh-154399: Fix venv activate.csh in a non-interactive shell (GH-154400) (GH-154408)

activate.csh saved the prompt unconditionally, but the prompt variable
only exists in interactive shells, and csh fails on an undefined
variable.
(cherry picked from commit 4448705)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

Author: 3 people

Lib/venv/scripts/posix/activate.csh

@@ -16,10 +16,12 @@ setenv PATH "$VIRTUAL_ENV/"__VENV_BIN_NAME__":$PATH"
 setenv VIRTUAL_ENV_PROMPT __VENV_PROMPT__
 
 
-set _OLD_VIRTUAL_PROMPT="$prompt"
+if ($?prompt) then
+    set _OLD_VIRTUAL_PROMPT="$prompt"
 
-if (! "$?VIRTUAL_ENV_DISABLE_PROMPT") then
-    set prompt = "("__VENV_PROMPT__") $prompt:q"
+    if (! "$?VIRTUAL_ENV_DISABLE_PROMPT") then
+        set prompt = "("__VENV_PROMPT__") $prompt:q"
+    endif
 endif
 
 alias pydoc python -m pydoc

Misc/NEWS.d/next/Library/2026-07-21-21-36-59.gh-issue-154399.VSa6oH.rst

@@ -0,0 +1,3 @@
+Fix :mod:`venv` activation in a non-interactive csh:
+``activate.csh`` no longer fails
+when the ``prompt`` variable is not set.