diff --git a/share/completions/python.fish b/share/completions/python.fish
index d36aa9236..91565921d 100644
--- a/share/completions/python.fish
+++ b/share/completions/python.fish
@@ -17,14 +17,13 @@ complete -c python -s x -d 'Skip first line of source, allowing use of non-Unix
 complete -c python -a "(__fish_complete_suffix .py)"
 complete -c python -a '-' -d 'Read program from stdin'
 
-switch (python -V 2>&1 | string replace -r '^.*\s([23])..*' '$1')[1]
-    case 2
-        complete -c python -s 3 -d 'Warn about Python 3.x incompatibilities that 2to3 cannot trivially fix'
-        complete -c python -s t --description "Warn on mixed tabs and spaces"
-        complete -c python -s Q -x -a "old new warn warnall" --description "Division control"
-    case 3
-        complete -c python -s q --description 'Don\'t print version and copyright messages on interactive startup'
-        complete -c python -s X -x -d 'Set implementation-specific option'
-        complete -c python -s b  -d 'Issue warnings about str(bytes_instance), str(bytearray_instance) and comparing bytes/bytearray with str'
-        complete -c python -o bb -d 'Issue errors'
-end
+# Version-specific completions
+# We have to detect this at runtime because pyenv etc can change
+# what `python` refers to.
+complete -c python -n 'python -V 2>&1 | string match -rq "^.*\s2"' -s 3 -d 'Warn about Python 3.x incompatibilities that 2to3 cannot trivially fix'
+complete -c python -n 'python -V 2>&1 | string match -rq "^.*\s2"' -s t --description "Warn on mixed tabs and spaces"
+complete -c python -n 'python -V 2>&1 | string match -rq "^.*\s2"' -s Q -x -a "old new warn warnall" --description "Division control"
+complete -c python -n 'python -V 2>&1 | string match -rq "^.*\s3"' -s q --description 'Don\'t print version and copyright messages on interactive startup'
+complete -c python -n 'python -V 2>&1 | string match -rq "^.*\s3"' -s X -x -d 'Set implementation-specific option'
+complete -c python -n 'python -V 2>&1 | string match -rq "^.*\s3"' -s b  -d 'Issue warnings about str(bytes_instance), str(bytearray_instance) and comparing bytes/bytearray with str'
+complete -c python -n 'python -V 2>&1 | string match -rq "^.*\s3"' -o bb -d 'Issue errors'