Omit Header with None value from request

Fixes #167.

CHANGELOG.rst

@@ -6,6 +6,13 @@ All notable changes to this project will be documented in this file.
 The format is based on `Keep a Changelog`_, and this project adheres to the
 `Semantic Versioning`_ scheme.
 
+# Unreleased_
+=============
+Changed
+-------
+- Omit ``Header`` argument from request when its value is ``None``.
+  (`#167`_, `#169`_)
+
 0.9.0_ - 2019-06-05
 ===================
 Added
@@ -29,7 +36,7 @@ Fixed
 
 Changed
 -------
-- Renamed ``uplink.retry.stop.DISABLE`` to ``uplink.retry.stop.NEVER``
+- Rename ``uplink.retry.stop.DISABLE`` to ``uplink.retry.stop.NEVER``
 
 0.8.0_ - 2019-02-16
 ===================
@@ -294,6 +301,7 @@ Added
 .. _`Semantic Versioning`: https://packaging.python.org/tutorials/distributing-packages/#semantic-versioning-preferred
 
 .. Releases
+.. _Unreleased: https://github.com/prkumar/uplink/compare/v0.9.0...HEAD
 .. _0.9.0: https://github.com/prkumar/uplink/compare/v0.8.0...v0.9.0
 .. _0.8.0: https://github.com/prkumar/uplink/compare/v0.7.0...v0.8.0
 .. _0.7.0: https://github.com/prkumar/uplink/compare/v0.6.1...v0.7.0

tests/unit/test_arguments.py

@@ -327,6 +327,10 @@ def test_modify_request(self, request_builder):
         arguments.Header("hello").modify_request(request_builder, "world")
         assert request_builder.info["headers"] == {"hello": "world"}
 
+    def test_skip_none(self, request_builder):
+        arguments.Header("hello").modify_request(request_builder, None)
+        assert request_builder.info["headers"] == {}
+
 
 class TestHeaderMap(ArgumentTestCase, FuncDecoratorTestCase):
     type_cls = arguments.HeaderMap

uplink/arguments.py

@@ -225,6 +225,23 @@ def converter_key(self):  # pragma: no cover
         raise NotImplementedError
 
 
+class EncodeNoneMixin(object):
+    #: Identifies how a `None` value should be encoded in the request.
+    _encode_none = None  # type: str
+
+    def _modify_request(self, request_builder, value):  # pragma: no cover
+        raise NotImplementedError
+
+    def modify_request(self, request_builder, value):
+        if value is None:
+            if self._encode_none is None:
+                # ignore value if it is None and shouldn't be encoded
+                return
+            else:
+                value = self._encode_none
+        super(EncodeNoneMixin, self).modify_request(request_builder, value)
+
+
 class FuncDecoratorMixin(object):
     @classmethod
     def _is_static_call(cls, *args_, **kwargs):
@@ -304,7 +321,7 @@ def _modify_request(self, request_builder, value):
         request_builder.url.set_variable({self.name: value})
 
 
-class Query(FuncDecoratorMixin, NamedArgument):
+class Query(FuncDecoratorMixin, EncodeNoneMixin, NamedArgument):
     """
     Set a dynamic query parameter.
 
@@ -392,14 +409,6 @@ def _modify_request(self, request_builder, value):
             request_builder.info, {self.name: value}, self._encoded
         )
 
-    def modify_request(self, request_builder, value):
-        if value is None:
-            # ignore value if it is None and shouldn't be encoded
-            if self._encode_none is not None:
-                self._modify_request(request_builder, self._encode_none)
-        else:
-            super(Query, self).modify_request(request_builder, value)
-
 
 class QueryMap(FuncDecoratorMixin, TypedArgument):
     """
@@ -438,20 +447,30 @@ def _modify_request(self, request_builder, value):
         Query.update_params(request_builder.info, value, self._encoded)
 
 
-class Header(FuncDecoratorMixin, NamedArgument):
+class Header(FuncDecoratorMixin, EncodeNoneMixin, NamedArgument):
     """
     Pass a header as a method argument at runtime.
 
-    While :py:class:`uplink.headers` attaches static headers
-    that define all requests sent from a consumer method, this
-    class turns a method argument into a dynamic header value.
+    While :py:class:`uplink.headers` attaches request headers values
+    that are static across all requests sent from the decorated
+    consumer method, this annotation turns a method argument into a
+    dynamic request header.
 
     Example:
         .. code-block:: python
 
             @get("/user")
-            def (self, session_id: Header("Authorization")):
-                \"""Get the authenticated user\"""
+            def me(self, session_id: Header("Authorization")):
+                \"""Get the authenticated user.\"""
+
+        To define an optional header, use the default value of `None`:
+
+            @get("/repositories")
+            def fetch_repos(self, auth: Header("Authorization") = None):
+                \"""List all public repositories.\"""
+
+        When the argument is not used, the header will not be added
+        to the request.
     """
 
     @property