Add decorator to parse function type hints

docs/guide.md

@@ -692,6 +692,56 @@ flag (as in `--obj=True`), or by making sure there's another flag after any
 boolean flag argument.
 
 
+#### Type hints
+
+Fire can be configured to use type hints information by decorating functions with `UseTypeHints()` decorator.
+Only `int`, `float` and `str` type hints are respected by default, everything else is ignored (parsed as usual).
+Quite common usecase is to instruct fire not to convert strings to integer/floats by supplying `str` 
+type annotation.
+
+See minimal example below:
+
+```python
+import fire
+
+from fire.decorators import UseTypeHints
+
+
+@UseTypeHints()  # () are mandatory here
+def main(a: str, b: float):
+  print(type(a), type(b))
+
+
+if __name__ == "__main__":
+  fire.Fire(main)
+```
+
+When invoked with `python command.py 1 2` this code will print `str float`.
+
+You can set custom parsers for type hints via decorator argument, following example shows how to parse string to `pathlib.Path` object:
+
+```python
+import fire
+
+from pathlib import Path
+from fire.decorators import UseTypeHints
+
+
+@UseTypeHints({Path: Path})
+def main(a: Path, b: str):
+  print(a)
+
+
+if __name__ == "__main__":
+  fire.Fire(main)
+```
+
+This code will convert argument `a` to `pathlib.Path`.
+
+To override default behavior for `int`, `str`, and `float` type hints you need to add them into dictionary supplied to 
+`UseTypeHints` decorator. 
+
+
 ### Using Fire Flags
 
 Fire CLIs all come with a number of flags. These flags should be separated from

fire/decorators.py

@@ -29,6 +29,46 @@
 ACCEPTS_POSITIONAL_ARGS = 'ACCEPTS_POSITIONAL_ARGS'
 
 
+def UseTypeHints(type_hints_mapping=None):
+  """Instruct fire to use type hints information when parsing args for this
+  function.
+
+  Args:
+    type_hints_mapping: mapping of type hints into parsing functions, by
+      default floats, ints and strings are treated, and all other type
+      hints are ignored (parsed as usual)
+  Returns:
+    The decorated function, which now has metadata telling Fire how to perform
+    according to type hints.
+
+  Examples:
+    @UseTypeHints()
+    def main(a, b:int, c:float=2.0)
+      assert isinstance(b, int)
+      assert isinstance(c, float)
+
+    @UseTypeHints({list: lambda s: s.split(";")})
+    def main(a, c: list):
+      assert isinstance(c, list)
+  """
+  mapping = {float: float, int: int, str: str}
+  if type_hints_mapping is not None:
+    mapping.update(type_hints_mapping)
+  type_hints_mapping = mapping
+
+  def _Decorator(fn):
+    signature = inspect.signature(fn)
+    named = {}
+    for name, param in signature.parameters.items():
+      has_type_hint = param.annotation is not param.empty
+      if has_type_hint and param.annotation in type_hints_mapping:
+        named[name] = type_hints_mapping[param.annotation]
+    decorator = SetParseFns(**named)
+    decorated_func = decorator(fn)
+    return decorated_func
+  return _Decorator
+
+
 def SetParseFn(fn, *arguments):
   """Sets the fn for Fire to use to parse args when calling the decorated fn.
 

fire/decorators_test.py

@@ -17,6 +17,8 @@
 from __future__ import absolute_import
 from __future__ import division
 from __future__ import print_function
+import sys
+import unittest
 
 from fire import core
 from fire import decorators
@@ -169,6 +171,44 @@ def testSetParseFn(self):
                   command=['example7', '1', '--arg2=2', '3', '4', '--kwarg=5']),
         ('1', '2', ('3', '4'), {'kwarg': '5'}))
 
+  @unittest.skipIf(sys.version_info < (3, 5),
+                   'Type hints were introduced in python 3.5')
+  def testDefaultTypeHints(self):
+    # need to hide type hints syntax behind exec
+    # otherwise old python parser will fail
+    #pylint: disable=exec-used
+    exec("""@decorators.UseTypeHints()
+def exampleWithSimpleTypeHints(a: int, b: str, c, d : float = None):
+  return a, b, c, d""")
+
+
+    self.assertEqual(
+        core.Fire(locals()['exampleWithSimpleTypeHints'],
+                  command=['1', '2', '3', '--d=4']),
+        (1, '2', 3, 4)
+    )
+
+  @unittest.skipIf(sys.version_info < (3, 5),
+                   'Type hints were introduced in python 3.5')
+  def testCustomTypeHints(self):
+    # need to hide type hints syntax behind exec
+    # otherwise old python parser will fail
+    #pylint: disable=exec-used
+    exec("""from pathlib import Path
+
+
+@decorators.UseTypeHints({
+      list: lambda arg: list(map(int, arg.split(";"))),
+      Path: Path})
+def exampleWithComplexHints(a: Path, b, c: list, d : list = None):
+  return a, b, c, d""")
+
+    self.assertEqual(
+        core.Fire(locals()['exampleWithComplexHints'],
+                  command=['1', '2', '3', '--d=4;5;6']),
+        (locals()['Path']('1'), 2, [3], [4, 5, 6])
+    )
+
 
 if __name__ == '__main__':
   testutils.main()