@@ -247,6 +247,21 @@ A clean install every time

         RECLONE=yes

+Upgrade packages installed by pip

+---------------------------------

+

+    | *Default: ``PIP_UPGRADE=""``*

+    |  By default ``stack.sh`` only installs Python packages if no version

+       is currently installed or the current version does not match a specified

+       requirement. If ``PIP_UPGRADE`` is set to ``True`` then existing required

+       Python packages will be upgraded to the most recent version that

+       matches requirements.

+    |

+

+    ::

+

+        PIP_UPGRADE=True

+

 Swift

 -----

@@ -54,17 +54,23 @@ function get_python_exec_prefix {

 # Wrapper for ``pip install`` to set cache and proxy environment variables

 # Uses globals ``INSTALL_TESTONLY_PACKAGES``, ``OFFLINE``, ``PIP_VIRTUAL_ENV``,

-# ``TRACK_DEPENDS``, ``*_proxy``

+# ``PIP_UPGRADE``, ``TRACK_DEPENDS``, ``*_proxy``

 # pip_install package [package ...]

 function pip_install {

     local xtrace=$(set +o | grep xtrace)

     set +o xtrace

+    local upgrade=""

     local offline=${OFFLINE:-False}

     if [[ "$offline" == "True" || -z "$@" ]]; then

         $xtrace

         return

     fi

+    PIP_UPGRADE=$(trueorfalse False PIP_UPGRADE)

+    if [[ "$PIP_UPGRADE" = "True" ]] ; then

+        upgrade="--upgrade"

+    fi

+

     if [[ -z "$os_PACKAGE" ]]; then

         GetOSVersion

     fi

@@ -98,7 +104,7 @@ function pip_install {

         https_proxy="${https_proxy:-}" \

         no_proxy="${no_proxy:-}" \

         PIP_FIND_LINKS=$PIP_FIND_LINKS \

-        $cmd_pip install \

+        $cmd_pip install $upgrade \

         $@

     # Also install test requirements

@@ -110,7 +116,7 @@ function pip_install {

             https_proxy=${https_proxy:-} \

             no_proxy=${no_proxy:-} \

             PIP_FIND_LINKS=$PIP_FIND_LINKS \

-            $cmd_pip install \

+            $cmd_pip install $upgrade \

             -r $test_req

     fi

 }