Fix #3281: Unexpected result when using build args with default values

compose/utils.py

@@ -95,4 +95,4 @@ def microseconds_from_time_nano(time_nano):
 
 
 def build_string_dict(source_dict):
-    return dict((k, str(v)) for k, v in source_dict.items())
+    return dict((k, str(v if v is not None else '')) for k, v in source_dict.items())

tests/unit/config/config_test.py

@@ -715,7 +715,35 @@ def test_build_args_allow_empty_properties(self):
         ).services[0]
         assert 'args' in service['build']
         assert 'foo' in service['build']['args']
-        assert service['build']['args']['foo'] == 'None'
+        assert service['build']['args']['foo'] == ''
+
+    # If build argument is None then it will be converted to the empty
+    # string. Make sure that int zero kept as it is, i.e. not converted to
+    # the empty string
+    def test_build_args_check_zero_preserved(self):
+        service = config.load(
+            build_config_details(
+                {
+                    'version': '2',
+                    'services': {
+                        'web': {
+                            'build': {
+                                'context': '.',
+                                'dockerfile': 'Dockerfile-alt',
+                                'args': {
+                                    'foo': 0
+                                }
+                            }
+                        }
+                    }
+                },
+                'tests/fixtures/extends',
+                'filename.yml'
+            )
+        ).services[0]
+        assert 'args' in service['build']
+        assert 'foo' in service['build']['args']
+        assert service['build']['args']['foo'] == '0'
 
     def test_load_with_multiple_files_mismatched_networks_format(self):
         base_file = config.ConfigFile(