Added 'when' as a shortcut around only_if.

examples/playbooks/conditionals_part2.yml

 ---
-# this is a demo of conditional executions using 'only_if', which can skip
-# certain tasks on machines/platforms/etc where they do not apply.
+# this is a demo of conditional executions using 'when' statements, which can skip
+# certain tasks on machines/platforms/etc where they do not apply.  
 
 - hosts: all
   user: root
 
   vars:
      favcolor: "red"
+     dog: "fido"
+     cat: "whiskers"
      ssn: 8675309
 
-# Below we're going to define some expressions.
+# These are the types of when statemnets available
+#     when_set: $variable_name
+#     when_unset: $variable_name
+#     when_str: $x == "test"
+#     when_int: $y > 2
+#     when_float: $z => 2.3
 #
-# Not only can we assign variables for reuse, but we can also assign conditional
-# expressions.  By keeping these in 'vars', the task section remains
-# extraordinarily clean, and not littered with programming language
-# constructs -- so it's easily skimmed by humans.
-#
-# Remember to quote any variables if they are not numbers!
-#
-# Interesting fact: aside from the $variables, these expressions are actually
-# tiny bits of Python.  They are evaluated in the context of each host, so different
-# steps can be skipped on different hosts!  They should evaluate to either True
-# or False
-
-     is_favcolor_blue: "'$favcolor' == 'blue'"
-     is_centos: "'$facter_operatingsystem' == 'CentOS'"
-
-# NOTE:
-#
-# facter and ohai variables can be used in only_if statements too
-# ex:  "'$facter_operatingsystem' == 'CentOS'", which bubble up automatically
-# from the managed machines
-#
-# this example won't do that though, as you might not have facter or ohai,
-# but you get the idea...
+# when using 'when', take care to make sure any variables given are surrounded by spaces
+# as an example, $z>3 will not do what you want, use "$z > 3"
 
   tasks:
 
-     - name: "do this if my favcolor is blue"
+     - name: "do this if my favcolor is blue, and my dog is named fido"
        action: shell /bin/false
-       only_if: '$is_favcolor_blue'
+       when_string: $favcolor == 'blue' and $dog == 'fido'
+
+     - name: "do this if my favcolor is not blue, and my dog is named fido"
+       action: shell /bin/true
+       when_string: $favcolor != 'blue' and $dog == 'fido'
+
+     - name: "do this if my SSN is over 9000"
+       action: shell /bin/true
+       when_integer: $ssn > 9000
 
-     - name: "do this if my favcolor is not blue"
+     - name: "do this if I have one of these SSNs"
+       action: shell /bin/true
+       when_integer:  $ssn in [ 8675309, 8675310, 8675311 ]
+     
+     - name: "do this if a variable named hippo is NOT defined"
        action: shell /bin/true
-       only_if: 'not ($is_favcolor_blue)'
+       when_unset: $hippo
+
+     - name: "do this if a variable named hippo is defined"
+       action: shell /bin/true
+       when_set: $hippo
+
 

examples/playbooks/conditionals_part3.yml

+---
+# this is a demo of conditional executions using 'only_if', which can skip
+# certain tasks on machines/platforms/etc where they do not apply.  This is
+# the more 'raw' version of the 'when' statement, most users will be able to
+# use 'when' directly.  'only_if' is an older feature, and useful for when
+# you need more advanced expression control.
+
+- hosts: all
+  user: root
+
+  vars:
+     favcolor: "red"
+     ssn: 8675309
+
+# Below we're going to define some expressions.
+#
+# Not only can we assign variables for reuse, but we can also assign conditional
+# expressions.  By keeping these in 'vars', the task section remains
+# extraordinarily clean, and not littered with programming language
+# constructs -- so it's easily skimmed by humans.
+#
+# Remember to quote any variables if they are not numbers!
+#
+# Interesting fact: aside from the $variables, these expressions are actually
+# tiny bits of Python.  They are evaluated in the context of each host, so different
+# steps can be skipped on different hosts!  They should evaluate to either True
+# or False
+
+     is_favcolor_blue: "'$favcolor' == 'blue'"
+     is_centos: "'$facter_operatingsystem' == 'CentOS'"
+
+# NOTE:
+#
+# setup module values, facter and ohai variables can be used in only_if statements too
+# ex:  "'$facter_operatingsystem' == 'CentOS'", which bubble up automatically
+# from the managed machines.  This example doesn't do that though.
+
+  tasks:
+
+     - name: "do this if my favcolor is blue"
+       action: shell /bin/false
+       only_if: '$is_favcolor_blue'
+
+     - name: "do this if my favcolor is not blue"
+       action: shell /bin/true
+       only_if: 'not ($is_favcolor_blue)'
+

lib/ansible/playbook/task.py

@@ -22,7 +22,7 @@ from ansible import utils
 class Task(object):
 
     __slots__ = [
-        'name', 'action', 'only_if', 'async_seconds', 'async_poll_interval',
+        'name', 'action', 'only_if', 'when', 'async_seconds', 'async_poll_interval',
         'notify', 'module_name', 'module_args', 'module_vars',
         'play', 'notified_by', 'tags', 'register',
         'delegate_to', 'first_available_file', 'ignore_errors',
@@ -58,6 +58,11 @@ class Task(object):
                 else:
                     raise errors.AnsibleError("cannot find lookup plugin named %s for usage in with_%s" % (plugin_name, plugin_name))
 
+            elif x.startswith("when_"):
+                when_name = x.replace("when_","")
+                ds['when'] = "%s %s" % (when_name, ds[x])
+                ds.pop(x)
+
             elif not x in Task.VALID_KEYS:
                 raise errors.AnsibleError("%s is not a legal parameter in an Ansible task or handler" % x)
 
@@ -109,6 +114,8 @@ class Task(object):
 
         # load various attributes
         self.only_if = ds.get('only_if', 'True')
+        self.when    = ds.get('when', None)
+
         self.async_seconds = int(ds.get('async', 0))  # not async by default
         self.async_poll_interval = int(ds.get('poll', 10))  # default poll = 10 seconds
         self.notify = ds.get('notify', [])
@@ -169,3 +176,58 @@ class Task(object):
                 self.tags.extend(apply_tags)
         self.tags.extend(import_tags)
 
+        if self.when is not None:
+            if self.only_if != 'True':
+                raise errors.AnsibleError('when obsoletes only_if, only use one or the other')
+            self.only_if = self.compile_when_to_only_if(self.when)
+
+    def compile_when_to_only_if(self, expression):
+        ''' 
+        when is a shorthand for writing only_if conditionals.  It requires less quoting
+        magic.  only_if is retained for backwards compatibility.
+        '''
+
+        # when: set $variable
+        # when: unset $variable
+        # when: int $x >= $z and $y < 3
+        # when: int $x in $alist
+        # when: float $x > 2 and $y <= $z
+        # when: str $x != $y
+
+        if type(expression) not in [ str, unicode ]:
+             raise errors.AnsibleError("invalid usage of when_ operator: %s" % expression)
+        tokens = expression.split()
+        if len(tokens) < 2:
+             raise errors.AnsibleError("invalid usage of when_ operator: %s" % expression)
+
+        # when_set / when_unset
+        if tokens[0] in [ 'set', 'unset' ]:
+             if len(tokens) != 2:
+                 raise errors.AnsibleError("usage: when: <set|unset> <$variableName>")
+             return "is_%s('%s')" % (tokens[0], tokens[1])
+
+        # when_integer / when_float / when_string
+        elif tokens[0] in [ 'integer', 'float', 'string' ]:
+             cast = None
+             if tokens[0] == 'integer':
+                 cast = 'int'
+             elif tokens[0] == 'string':
+                 cast = 'str'
+             elif tokens[0] == 'float':
+                 cast = 'float'
+             tcopy = tokens[1:]
+             for (i,t) in enumerate(tokens[1:]):
+                 if t.find("$") != -1:
+                     # final variable substitution will happen in Runner code
+                     tcopy[i] = "%s('%s')" % (cast, t)
+                 else:
+                     tcopy[i] = t
+             return " ".join(tcopy)
+ 
+        else:
+             raise errors.AnsibleError("invalid usage of when_ operator: %s" % expression)
+ 
+
+
+
+