python: make a reasonable guess about virtualenvwrapper python location

Fixes #1416

modules/python/README.md

@@ -8,11 +8,15 @@ Settings
 
 This module supports virtual environments from conda and virtualenvwrapper. By default, only virtualenvwrapper is enabled. To disable virtualenvwrapper, add the following to *zpreztorc*.
 
-    zstyle ':prezto:module:python' skip-virtualenvwrapper-init 'on'
+```sh
+zstyle ':prezto:module:python' skip-virtualenvwrapper-init 'on'
+```
 
 Conda support is enabled by adding the following to *zpreztorc*.
 
-    zstyle ':prezto:module:python' conda-init 'on'
+```sh
+zstyle ':prezto:module:python' conda-init 'on'
+```
 
 Caution: using conda and virtualenvwrapper at the same time may cause conflicts.
 
@@ -106,10 +110,18 @@ zstyle ':prezto:module:python:virtualenv' auto-switch 'yes'
 (`$VIRTUALENVWRAPPER_VIRTUALENV` is explicitly set or `virtualenv` is in
 `$PATH`). This can be disabled with:
 
-```
+```sh
 zstyle ':prezto:module:python:virtualenv' initialize 'no'
 ```
 
+On some systems, the `python` binary doesn't exist so in order to get
+virtualenvwrapper to work, the path needs to specified. We do our best to detect
+this, but sometimes it needs manual intervention.
+
+```sh
+zstyle ':prezto:module:python:virtualenvwrapper' python-path /opt/python3/bin/python3
+```
+
 Aliases
 -------
 

modules/python/init.zsh

@@ -114,6 +114,20 @@ if (( $+VIRTUALENVWRAPPER_VIRTUALENV || $+commands[virtualenv] )) && \
   else
     # Fallback to 'virtualenvwrapper' without 'pyenv' wrapper if available
     # in '$path' or in an alternative location on a Debian based system.
+    #
+    # If the python-path was manually specified, use that. Otherwise, fall back
+    # to python3 then python2 in that order. This is needed to fix an issue with
+    # virtualenvwrapper when used with homebrew, as it no longer shadows the
+    # system python.
+    zstyle -s ':prezto:module:python:virtualenvwrapper' python-path '_venv_python'
+    if [[ -n "$_venv_python" ]]; then
+      export VIRTUALENVWRAPPER_PYTHON=$_venv_python
+    elif (( $+commands[python3] )); then
+      export VIRTUALENVWRAPPER_PYTHON=$commands[python3]
+    elif (( $+commands[python2] )); then
+      export VIRTUALENVWRAPPER_PYTHON=$commands[python2]
+    fi
+
     virtenv_sources=(
       ${(@Ov)commands[(I)virtualenvwrapper(_lazy|).sh]}
       /usr/share/virtualenvwrapper/virtualenvwrapper(_lazy|).sh(OnN)