fix(decorators): avoid mutating original function metadata on functools.wraps

When decorators like SetParseFn and SetParseFns are applied to functions
wrapped with functools.wraps, copying existing parse_fns and metadata
dictionaries prevents mutating the underlying function's metadata in place.

Fixes #728.

Signed-off-by: Aryan Sachin <aryansachinixd@gmail.com>

Author: Aryan9891

fire/decorators.py

@@ -37,7 +37,12 @@ def SetParseFn(fn, *arguments):
     The decorated function, which now has metadata telling Fire how to perform.
   """
   def _Decorator(func):
-    parse_fns = GetParseFns(func)
+    existing = GetParseFns(func)
+    parse_fns = {
+        'default': existing.get('default'),
+        'positional': list(existing.get('positional', [])),
+        'named': dict(existing.get('named', {})),
+    }
     if not arguments:
       parse_fns['default'] = fn
     else:
@@ -66,8 +71,12 @@ def SetParseFns(*positional, **named):
     The decorated function, which now has metadata telling Fire how to perform.
   """
   def _Decorator(fn):
-    parse_fns = GetParseFns(fn)
-    parse_fns['positional'] = positional
+    existing = GetParseFns(fn)
+    parse_fns = {
+        'default': existing.get('default'),
+        'positional': positional if positional else tuple(existing.get('positional', ())),
+        'named': dict(existing.get('named', {})),
+    }
     parse_fns['named'].update(named)
     _SetMetadata(fn, FIRE_PARSE_FNS, parse_fns)
     return fn
@@ -76,7 +85,7 @@ def _Decorator(fn):
 
 
 def _SetMetadata(fn, attribute, value):
-  metadata = GetMetadata(fn)
+  metadata = dict(GetMetadata(fn))
   metadata[attribute] = value
   setattr(fn, FIRE_METADATA, metadata)
 

fire/decorators_test.py

@@ -14,6 +14,7 @@
 
 """Tests for the decorators module."""
 
+import functools
 from fire import core
 from fire import decorators
 from fire import testutils
@@ -165,6 +166,32 @@ def testSetParseFn(self):
                   command=['example7', '1', '--arg2=2', '3', '4', '--kwarg=5']),
         ('1', '2', ('3', '4'), {'kwarg': '5'}))
 
+  def testSetParseFnsFunctoolsWraps(self):
+    @decorators.SetParseFns(value=int)
+    def original(value):
+      return type(value).__name__
+
+    @decorators.SetParseFns(value=str)
+    @functools.wraps(original)
+    def wrapped(value):
+      return original(value)
+
+    self.assertEqual(core.Fire(original, command=['01']), 'int')
+    self.assertEqual(core.Fire(wrapped, command=['01']), 'str')
+
+  def testSetParseFnFunctoolsWraps(self):
+    @decorators.SetParseFn(int, 'value')
+    def original(value):
+      return type(value).__name__
+
+    @decorators.SetParseFn(str, 'value')
+    @functools.wraps(original)
+    def wrapped(value):
+      return original(value)
+
+    self.assertEqual(core.Fire(original, command=['01']), 'int')
+    self.assertEqual(core.Fire(wrapped, command=['01']), 'str')
+
 
 if __name__ == '__main__':
   testutils.main()