default: new 'init' option

The purpose is to replace colletive.recipe.shelloutput

/reviewed-on !11

README.rst

@@ -4,12 +4,13 @@
 
 .. contents::
 
-Examples
---------
+Default
+-------
 
-Recipe to build the software.
+The default recipe can be used to execute ad-hoc Python code at both
+init & install phases.
 
-Example buildout::
+Example buildout that builds software::
 
   [buildout]
   parts =
@@ -83,6 +84,19 @@ For example::
     print('%(foo)s')
     # will print “%(foo)s”
 
+Using the init option::
+
+  [section-one]
+  recipe = slapos.recipe.build
+  init =
+    import os
+    options['foo'] = os.uname()[4]
+
+  [section-two]
+  bar = ${section-one:foo}
+
+It is also possible to use both `init` & `script` in the same section.
+
 Pure download
 ~~~~~~~~~~~~~
 

slapos/recipe/build/__init__.py

@@ -268,18 +268,28 @@ class Script(EnvironMixin):
     except KeyError:
       self.options['location'] = os.path.join(
         buildout['buildout']['parts-directory'], self.name)
-    self.script = self.options['script']
+    format = options.get('format', 'yes') in TRUE_LIST
+    init = options.get('init')
+    self.script = options.get('script')
     if self.script:
-      if self.options.get('format', 'yes') in TRUE_LIST:
+      if format:
         self.script = self.script % self.options
+    elif not init:
+      raise zc.buildout.UserError('either init or script is required')
     if self.options.get('keep-on-error', '').strip().lower() in TRUE_LIST:
       self.logger.debug('Keeping directories in case of errors')
       self.keep_on_error = True
     else:
       self.keep_on_error = False
     EnvironMixin.__init__(self, False)
+    if init:
+      if format:
+        init = init % options
+      exec(init, {'self': self, 'options': options, 'env': self.environ})
 
   def install(self):
+    if not self.script:
+      return ""
     location = self.options['location']
     if os.path.lexists(location):
       self.logger.warning('Removing already existing path %r', location)