diff --git a/share/chpython/auto.sh b/share/chpython/auto.sh
new file mode 100644
index 0000000..5089f7b
--- /dev/null
+++ b/share/chpython/auto.sh
@@ -0,0 +1,33 @@
+unset PYTHON_AUTO_VERSION
+
+function chpython_auto() {
+	local dir="$PWD/" version
+
+	until [[ -z "$dir" ]]; do
+		dir="${dir%/*}"
+
+		if { read -r version < "$dir/.python-version"; } 2>/dev/null || [[ -n "$version" ]]; then
+			version="${version%%[[:space:]]}"
+
+			if [[ "$version" == "$PYTHON_AUTO_VERSION" ]]; then return
+			else
+				PYTHON_AUTO_VERSION="$version"
+				chpython "$version"
+				return $?
+			fi
+		fi
+	done
+
+	if [[ -n "$PYTHON_AUTO_VERSION" ]]; then
+		chpython_reset
+		unset PYTHON_AUTO_VERSION
+	fi
+}
+
+if [[ -n "$ZSH_VERSION" ]]; then
+	if [[ ! "$preexec_functions" == *chpython_auto* ]]; then
+		preexec_functions+=("chpython_auto")
+	fi
+elif [[ -n "$BASH_VERSION" ]]; then
+	trap '[[ "$BASH_COMMAND" != "$PROMPT_COMMAND" ]] && chpython_auto' DEBUG
+fi
diff --git a/share/chpython/chpython.sh b/share/chpython/chpython.sh
old mode 100644
new mode 100755
index a99ed0e..801e1c2
--- a/share/chpython/chpython.sh
+++ b/share/chpython/chpython.sh
@@ -1,94 +1,57 @@
 CHPYTHON_VERSION='0.1.0.dev'
 PYTHONS=()
 
-[ -d "${HOME}/.pythons" ] && PYTHONS+=("${HOME}/.pythons"/*)
+for dir in "$PREFIX/opt/pythons" "$HOME/.pythons"; do
+	[[ -d "$dir" && -n "$(ls -A "$dir")" ]] && PYTHONS+=("$dir"/*)
+done
+unset dir
 
-function _chpython_exec {
-  local python
-  python=$1
-  shift
-  # TODO: Clear path so invoking `chpython 3.5 python -V` when
-  # $CHPYTHON_VERSION is set to a 2.x Python gives a command not
-  # found error instead of running $CHPYTHON_VERSION/bin/python.
-  env PATH="$python/bin:$PATH" $*
-}
-
-function _chpython_reset {
+function chpython_reset() {
   [ -z "$CHPYTHON_ROOT" ] && return
-  PATH=":$PATH:"
-  PATH=${PATH//:$CHPYTHON_ROOT\/bin:/:}
-  PATH=${PATH#:}; PATH=${PATH%:}
+
+  PATH=":$PATH:"; PATH=${PATH//:$CHPYTHON_ROOT\/bin:/:}
+  PATH="${PATH#:}"; PATH="${PATH%:}"
   unset CHPYTHON_ROOT
   hash -r
-  type python || type python3
 }
 
-function _chpython_select {
-  local dir python match
-  for dir in ${PYTHONS[@]}; do
-    dir=${dir%%/}
-    python=${dir##*/}
-    case "$python" in
-      "$1")
-        # Match using 3-part version number, e.g. `chruby 2.7.10`
-        match=$dir && break
-        ;;
-      "$1"*)
-        # Match best using 2-part version number, e.g., `chpython 2.7`
-        match=$dir
-        ;;
-    esac
-  done
-  echo "$match"
-}
 
-function _chpython_use {
-  if [[ ! -x "$1/bin/python" && ! -x "$1/bin/python3" ]]; then
-    echo "chpython: neither $1/bin/python nor $1/bin/python3 are executable" >&2
+function _chpython_use() {
+  if [[ ! -x "$1/bin/python" ]]; then
+    echo "chpython: $1/bin/python is not executable" >&2
     return 1
   fi
 
-  # TODO: Alias *3 stuff to base name, so, e.g., `python` can be called
-  # instead of `python3`
+  [[ -n "$CHPYTHON_ROOT" ]] && chpython_reset
 
-  export CHPYTHON_ROOT=$1
+  export CHPYTHON_ROOT="$1"
   export PATH="${CHPYTHON_ROOT}/bin:${PATH}"
+  local python_version
+  if [[ -e "$1/bin/python3" ]]; then
+    PYTHON_VERSION="$($1/bin/python --version)"
+  else
+    PYTHON_VERSION="$($1/bin/python --version 2>&1 | awk '{print $2}')"
+  fi
   hash -r
-  type python || type python3
 }
 
-function chpython {
+
+function chpython() {
   case "$1" in
     -h|--help)
       cat <<EOS
 Usage:
   chpython [PYTHON|VERSION|system]
-  chpython exec [PYTHON|VERSION] args...
 EOS
       ;;
     -V|--version)
       echo "chpython $CHPYTHON_VERSION"
       ;;
-    exec)
-      shift
-      local match
-      match=$(_chpython_select $1)
-      if [ -z "$match" ]; then
-        echo "chpython: unknown Python: $1" >&2
-        return 1
-      fi
-
-      shift
-      _chpython_exec $match $*
-      ;;
-    system)
-      _chpython_reset
-      ;;
+    system) chpython_reset ;;
     '')
       local dir python
-      for dir in ${PYTHONS[@]}; do
-        dir="${dir%%/}"
-        python=${dir##*/}
+      for dir in "${PYTHONS[@]}"; do
+        dir="${dir%%/}"; python="${dir##*/}"
         if [[ "$dir" == "$CHPYTHON_ROOT" ]]; then
           echo " * ${python}"
         else
@@ -97,15 +60,27 @@ EOS
       done
       ;;
     *)
-      local match
-      match=$(_chpython_select $1)
+      local dir python match
+      for dir in "${PYTHONS[@]}"; do
+        dir="${dir%%/}"; python="${dir##*/}"
+        case "$python" in
+          "$1")
+            # Match using 3-part version number, e.g. `chruby 2.7.10`
+            match="$dir" && break
+            ;;
+          "$1"*)
+            # Match best using 2-part version number, e.g., `chpython 2.7`
+            match="$dir"
+            ;;
+        esac
+      done
       if [ -z "$match" ]; then
         echo "chpython: unknown Python: $1" >&2
         return 1
       fi
 
       shift
-      _chpython_use $match $*
+      _chpython_use "$match"
       ;;
   esac
 }