[python] Improve and document `virtualenvwrapper` initialization flow

Changes:
* Simplify zstyle name `skip-virtualenvwrapper-init` to `initialize`
  avoiding double negation in name
* Always perform `eval (pyenv virtualenv-init -)` at initialization
* Prefer `virtualenvwrapper_lazy` over `virtualenvwrapper` when available
* Honor `VIRTUALENVWRAPPER_VIRTUALENV` if it is defined.
* Document about `VIRTUALENVWRAPPER_PYTHON` and `VIRTUALENVWRAPPER_VIRTUALENV`
  (this would be particularly important in macOS after recent homebrew update)
* Add additional documentation for `initialize` in _README.md_ and _zpreztorc_
* Add aliases `py2`, `py3` as shortcut for `python2`, `python3` respectively

modules/python/README.md

@@ -56,6 +56,20 @@ is used. Replace *Developer* with your projects directory.
 export PROJECT_HOME="$HOME/Developer"
 ```
 
+The variable `VIRTUALENVWRAPPER_PYTHON` tells virtualenvwrapper to use the
+specified full path of `python` interpreter overriding the `$PATH` search.
+
+```sh
+export VIRTUALENVWRAPPER_PYTHON=/usr/local/bin/python
+```
+
+The variable `VIRTUALENVWRAPPER_VIRTUALENV` tells virtualenvwrapper to use the
+specified full path of `virtualenv` binary overriding the `$PATH` search.
+
+```sh
+export VIRTUALENVWRAPPER_VIRTUALENV=/usr/local/bin/virtualenv
+```
+
 The variable `$VIRTUALENVWRAPPER_VIRTUALENV_ARGS` tells virtualenvwrapper what
 arguments to pass to `virtualenv`. For example, set the value to
 *--system-site-packages* to ensure that all new environments have access to the
@@ -76,10 +90,20 @@ This can be enabled with:
 zstyle ':prezto:module:python:virtualenv' auto-switch 'yes'
 ```
 
+virtualenvwrapper is automatically initialized if pre-requisites are met
+(`$VIRTUALENVWRAPPER_VIRTUALENV` is explicitly set or `virtualenv` is in
+`$PATH`). This can be disabled with:
+
+```
+zstyle ':prezto:module:python:virtualenv' initialize 'no'
+```
+
 Aliases
 -------
 
   - `py` is short for `python`.
+  - `py2` is short for `python2`.
+  - `py3` is short for `python3`.
 
 Functions
 ---------

modules/python/init.zsh

@@ -82,26 +82,32 @@ if zstyle -t ':prezto:module:python:virtualenv' auto-switch 'yes'; then
   add-zsh-hook chpwd _python-workon-cwd
 fi
 
-# Load virtualenvwrapper into the shell session, unless requested not to
-if zstyle -T ':prezto:module:python' skip-virtualenvwrapper-init; then
+# Load virtualenvwrapper into the shell session, if pre-requisites are met
+# and unless explicitly requested not to
+if (( $+VIRTUALENVWRAPPER_VIRTUALENV || $+commands[virtualenv] )) && \
+  zstyle -T ':prezto:module:python:virtualenv' initialize ; then
   # Set the directory where virtual environments are stored.
   export WORKON_HOME="${WORKON_HOME:-$HOME/.virtualenvs}"
 
   # Disable the virtualenv prompt.
   VIRTUAL_ENV_DISABLE_PROMPT=1
 
-  if (( $+commands[pyenv-virtualenvwrapper] )); then
-    pyenv virtualenvwrapper
-  elif (( $+commands[pyenv-virtualenv-init] )); then
+  if (( $+commands[pyenv] )); then
+    if (( $+commands[pyenv-virtualenv-init] )); then
       eval "$(pyenv virtualenv-init -)"
-  elif (( $+commands[virtualenvwrapper_lazy.sh] )); then
-    source "$commands[virtualenvwrapper_lazy.sh]"
-  elif (( $+commands[virtualenvwrapper.sh] )); then
-    source "$commands[virtualenvwrapper.sh]"
-  elif  [[ -f /usr/share/virtualenvwrapper/virtualenvwrapper_lazy.sh ]]; then
-    source /usr/share/virtualenvwrapper/virtualenvwrapper_lazy.sh
-  elif  [[ -f /usr/share/virtualenvwrapper/virtualenvwrapper.sh ]]; then
-    source /usr/share/virtualenvwrapper/virtualenvwrapper.sh
+    fi
+    if (( $#commands[(i)pyenv-virtualenvwrapper(_lazy|)] )); then
+      pyenv "${${(@O)commands[(I)pyenv-virtualenvwrapper(_lazy|)]}[1]#pyenv-}"
+    fi
+  else
+    # Try 'virtualenvwrapper' without 'pyenv' wrapper in '$path' and other
+    # known locations on a Debian based system.
+    virtenv_sources=(
+      ${(@Ov)commands[(I)virtualenvwrapper(_lazy|).sh]}
+      /usr/share/virtualenvwrapper/virtualenvwrapper(_lazy|).sh(OnN)
+    )
+    source "${virtenv_sources[1]}"
+    unset virtenv_sources
   fi
 fi
 
@@ -109,7 +115,7 @@ fi
 if (( $#commands[(i)pip(|[23])] )); then
   cache_file="${0:h}/cache.zsh"
 
-  # Detect and use first one available among 'pip', 'pip2', 'pip3' variants
+  # Detect and use one available from among 'pip', 'pip2', 'pip3' variants
   pip_command="$commands[(i)pip(|[23])]"
 
   if [[ "$pip_command" -nt "$cache_file" || ! -s "$cache_file" ]]; then
@@ -119,8 +125,7 @@ if (( $#commands[(i)pip(|[23])] )); then
   fi
 
   source "$cache_file"
-  unset cache_file
-  unset pip_command
+  unset cache_file pip_command
 fi
 
 #
@@ -128,3 +133,5 @@ fi
 #
 
 alias py='python'
+alias py2='python2'
+alias py3='python3'

runcoms/zpreztorc

@@ -123,6 +123,9 @@ zstyle ':prezto:module:prompt' theme 'sorin'
 # Auto switch the Python virtualenv on directory change.
 # zstyle ':prezto:module:python:virtualenv' auto-switch 'yes'
 
+# Automatically initialize virtualenvwrapper if pre-requisites are met.
+# zstyle ':prezto:module:python:virtualenv' initialize 'yes'
+
 #
 # Screen
 #