Merge pull request #29 from harlowja/optional-reqs

Allow for optional task requirements.

taskflow/decorators.py

@@ -68,6 +68,10 @@ def task(*args, **kwargs):
         requires_what = kwargs.pop('requires', [])
         f = requires(*requires_what, **kwargs)(f)
 
+        # Attach any optional requirements this function needs for running.
+        optional_what = kwargs.pop('optional', [])
+        f = optional(*optional_what, **kwargs)(f)
+
         # Attach any items this function provides as output
         provides_what = kwargs.pop('provides', [])
         f = provides(*provides_what, **kwargs)(f)
@@ -111,6 +115,33 @@ def versionize(major, minor=None):
     return decorator
 
 
+def optional(*args, **kwargs):
+    """Attaches a set of items that the decorated function would like as input
+    to the functions underlying dictionary."""
+
+    def decorator(f):
+        if not hasattr(f, 'optional'):
+            f.optional = set()
+
+        f.optional.update([a for a in args if _take_arg(a)])
+
+        @wraps(f)
+        def wrapper(*args, **kwargs):
+            return f(*args, **kwargs)
+
+        return wrapper
+
+    # This is needed to handle when the decorator has args or the decorator
+    # doesn't have args, python is rather weird here...
+    if kwargs or not args:
+        return decorator
+    else:
+        if isinstance(args[0], collections.Callable):
+            return decorator(args[0])
+        else:
+            return decorator
+
+
 def requires(*args, **kwargs):
     """Attaches a set of items that the decorated function requires as input
     to the functions underlying dictionary."""

taskflow/patterns/graph_flow.py

@@ -50,17 +50,31 @@ class Flow(ordered_flow.Flow):
         self._connected = False
 
     def _fetch_task_inputs(self, task):
-        inputs = collections.defaultdict(list)
 
-        for n in getattr(task, 'requires', []):
+        def extract_inputs(place_where, would_like, is_optional=False):
+            for n in would_like:
                 for (them, there_result) in self.results:
-                if (not self._graph.has_edge(them, task) or
-                    not n in getattr(them, 'provides', [])):
+                    if not n in set(getattr(them, 'provides', [])):
+                        continue
+                    if (not is_optional and
+                        not self._graph.has_edge(them, task)):
                         continue
                     if there_result and n in there_result:
-                    inputs[n].append(there_result[n])
-                else:
-                    inputs[n].append(None)
+                        place_where[n].append(there_result[n])
+                        if is_optional:
+                            # Take the first task that provides this optional
+                            # item.
+                            break
+                    elif not is_optional:
+                        place_where[n].append(None)
+
+        required_inputs = set(getattr(task, 'requires', []))
+        optional_inputs = set(getattr(task, 'optional', []))
+        optional_inputs = optional_inputs - required_inputs
+
+        task_inputs = collections.defaultdict(list)
+        extract_inputs(task_inputs, required_inputs)
+        extract_inputs(task_inputs, optional_inputs, is_optional=True)
 
         def collapse_functor(k_v):
             (k, v) = k_v
@@ -68,7 +82,7 @@ class Flow(ordered_flow.Flow):
                 v = v[0]
             return (k, v)
 
-        return dict(map(collapse_functor, inputs.iteritems()))
+        return dict(map(collapse_functor, task_inputs.iteritems()))
 
     def order(self):
         self.connect()

taskflow/patterns/linear_flow.py

@@ -30,16 +30,19 @@ class Flow(ordered_flow.Flow):
         self._tasks = []
 
     def _fetch_task_inputs(self, task):
+        would_like = set(getattr(task, 'requires', []))
+        would_like.update(getattr(task, 'optional', []))
+
         inputs = {}
-        for r in getattr(task, 'requires', []):
+        for n in would_like:
             # Find the last task that provided this.
             for (last_task, last_results) in reversed(self.results):
-                if r not in getattr(last_task, 'provides', []):
+                if n not in getattr(last_task, 'provides', []):
                     continue
-                if last_results and r in last_results:
-                    inputs[r] = last_results[r]
+                if last_results and n in last_results:
+                    inputs[n] = last_results[n]
                 else:
-                    inputs[r] = None
+                    inputs[n] = None
                 # Some task said they had it, get the next requirement.
                 break
         return inputs

taskflow/task.py

@@ -32,6 +32,10 @@ class Task(object):
         # An *immutable* input 'resource' name set this task depends
         # on existing before this task can be applied.
         self.requires = set()
+        # An *immutable* input 'resource' name set this task would like to
+        # depends on existing before this task can be applied (but does not
+        # strongly depend on existing).
+        self.optional = set()
         # An *immutable* output 'resource' name set this task
         # produces that other tasks may depend on this task providing.
         self.provides = set()