1 files changed, 66 insertions, 0 deletions
diff --git a/factory/declarations.py b/factory/declarations.py
index f0dbfe5..895f2ac 100644
--- a/factory/declarations.py
+++ b/factory/declarations.py
@@ -57,6 +57,23 @@ class OrderedDeclaration(object):
         raise NotImplementedError('This is an abstract method')
 
 
+class LazyFunction(OrderedDeclaration):
+    """Simplest OrderedDeclaration computed by calling the given function.
+
+    Attributes:
+        function (function): a function without arguments and
+            returning the computed value.
+    """
+
+    def __init__(self, function, *args, **kwargs):
+        super(LazyFunction, self).__init__(*args, **kwargs)
+        self.function = function
+
+    def evaluate(self, sequence, obj, create, extra=None, containers=()):
+        logger.debug("LazyFunction: Evaluating %r on %r", self.function, obj)
+        return self.function()
+
+
 class LazyAttribute(OrderedDeclaration):
     """Specific OrderedDeclaration computed using a lambda.
 
@@ -423,6 +440,55 @@ class List(SubFactory):
             **params)
 
 
+# Parameters
+# ==========
+
+
+class ComplexParameter(object):
+    """A complex parameter, to be used in a Factory.Params section.
+
+    Must implement:
+    - A "compute" function, performing the actual declaration override
+    - Optionally, a get_revdeps() function (to compute other parameters it may alter)
+    """
+
+    def compute(self, field_name, declarations):
+        """Compute the overrides for this parameter.
+
+        Args:
+        - field_name (str): the field this parameter is installed at
+        - declarations (dict): the global factory declarations
+
+        Returns:
+            dict: the declarations to override
+        """
+        raise NotImplementedError()
+
+    def get_revdeps(self, parameters):
+        """Retrieve the list of other parameters modified by this one."""
+        return []
+
+
+class Trait(ComplexParameter):
+    """The simplest complex parameter, it enables a bunch of new declarations based on a boolean flag."""
+    def __init__(self, **overrides):
+        self.overrides = overrides
+
+    def compute(self, field_name, declarations):
+        if declarations.get(field_name):
+            return self.overrides
+        else:
+            return {}
+
+    def get_revdeps(self, parameters):
+        """This might alter fields it's injecting."""
+        return [param for param in parameters if param in self.overrides]
+
+
+# Post-generation
+# ===============
+
+
 class ExtractionContext(object):
     """Private class holding all required context from extraction to postgen."""
     def __init__(self, value=None, did_extract=False, extra=None, for_field=''):