Fix bug in pip command detection

modules/python/init.zsh

@@ -147,21 +147,23 @@ if (( $+VIRTUALENVWRAPPER_VIRTUALENV || $+commands[virtualenv] )) && \
 fi
 
 # Load PIP completion.
-for pip_command in pip{,2,3}; do
-  if [[ "$PYENV_ROOT" ]]; then
-    [[ "$(pyenv which "$pip_command" 2>/dev/null)" ]] && break
-  else
-    [[ "$(which "$pip_command" 2>/dev/null)" ]] && break
-  fi
-done
-if [[ "$pip_command" ]]; then
+if [[ -n "$PYENV_ROOT" ]]; then
+  for pip in pip{,2,3}; do
+    pip_command="$(pyenv which "$pip" 2>/dev/null)"
+    [[ -n "$pip_command" ]] && break
+  done
+  unset pip
+else
+  pip_command="$commands[(i)pip(|[23])]"
+fi
+if [[ -n "$pip_command" ]]; then
   cache_file="${TMPDIR:-/tmp}/prezto-pip-cache.$UID.zsh"
 
   if [[ "$pip_command" -nt "$cache_file" \
         || "${ZDOTDIR:-$HOME}/.zpreztorc" -nt "$cache_file" \
         || ! -s "$cache_file" ]]; then
     # pip is slow; cache its output. And also support 'pip2', 'pip3' variants
-    $pip_command completion --zsh \
+    "$pip_command" completion --zsh \
       | sed -e "s/\(compctl -K [-_[:alnum:]]* pip\).*/\1{,2,3}{,.{0..9}}/" \
       >! "$cache_file" \
       2> /dev/null