Add custom formatter for Fire result (#345)

Fixes #344 (see issue for more details)

This lets you define a function that will take the result from the Fire component and allows the user to alter it before fire looks at it to render it.

fire/core.py

@@ -78,7 +78,7 @@ def main(argv):
   import asyncio  # pylint: disable=import-error,g-import-not-at-top  # pytype: disable=import-error
 
 
-def Fire(component=None, command=None, name=None):
+def Fire(component=None, command=None, name=None, serialize=None):
   """This function, Fire, is the main entrypoint for Python Fire.
 
   Executes a command either from the `command` argument or from sys.argv by
@@ -164,7 +164,7 @@ def Fire(component=None, command=None, name=None):
     raise FireExit(0, component_trace)
 
   # The command succeeded normally; print the result.
-  _PrintResult(component_trace, verbose=component_trace.verbose)
+  _PrintResult(component_trace, verbose=component_trace.verbose, serialize=serialize)
   result = component_trace.GetResult()
   return result
 
@@ -241,12 +241,19 @@ def _IsHelpShortcut(component_trace, remaining_args):
   return show_help
 
 
-def _PrintResult(component_trace, verbose=False):
+def _PrintResult(component_trace, verbose=False, serialize=None):
   """Prints the result of the Fire call to stdout in a human readable way."""
   # TODO(dbieber): Design human readable deserializable serialization method
   # and move serialization to its own module.
   result = component_trace.GetResult()
 
+  # Allow users to modify the return value of the component and provide 
+  # custom formatting.
+  if serialize:
+    if not callable(serialize):
+      raise FireError("serialize argument {} must be empty or callable.".format(serialize))
+    result = serialize(result)
+
   if value_types.HasCustomStr(result):
     # If the object has a custom __str__ method, rather than one inherited from
     # object, then we use that to serialize the object.

fire/core_test.py

@@ -194,6 +194,30 @@ def testClassMethod(self):
         7,
     )
 
+  def testCustomSerialize(self):
+    def serialize(x):
+      if isinstance(x, list):
+        return ', '.join(str(xi) for xi in x)
+      if isinstance(x, dict):
+        return ', '.join('{}={!r}'.format(k, v) for k, v in x.items())
+      if x == 'special':
+        return ['SURPRISE!!', "I'm a list!"]
+      return x
+
+    ident = lambda x: x
+
+    with self.assertOutputMatches(stdout='a, b', stderr=None):
+      result = core.Fire(ident, command=['[a,b]'], serialize=serialize)
+    with self.assertOutputMatches(stdout='a=5, b=6', stderr=None):
+      result = core.Fire(ident, command=['{a:5,b:6}'], serialize=serialize)
+    with self.assertOutputMatches(stdout='asdf', stderr=None):
+      result = core.Fire(ident, command=['asdf'], serialize=serialize)
+    with self.assertOutputMatches(stdout="SURPRISE!!\nI'm a list!\n", stderr=None):
+      result = core.Fire(ident, command=['special'], serialize=serialize)
+    with self.assertRaises(core.FireError):
+      core.Fire(ident, command=['asdf'], serialize=55)
+
+
   @testutils.skipIf(six.PY2, 'lru_cache is Python 3 only.')
   def testLruCacheDecoratorBoundArg(self):
     self.assertEqual(