fix issue #255 and #256 empty collection defaulting to flow-style0.15.76

an empty collection now defaults to best style. I.e. if elements are inserted
their style is not forced to flow-style

*When this change indeed resolves your problem, please **Close** this issue*.
*(You can do so using the WorkFlow pull-down (close to the top right of this page))*

8 files changed, 32 insertions, 21 deletions

diff --git a/CHANGES b/CHANGES
index 51b6be3..2c8e973 100644
--- a/CHANGES
+++ b/CHANGES
@@ -1,3 +1,8 @@
+[0, 15, 76]: 2018-11-01
+  - fix issue with empty mapping and sequence loaded as flow-style
+    (mapping reported by `Min RK <https://bitbucket.org/minrk/>`__, sequence
+    by `Maged Ahmed <https://bitbucket.org/maged2/>`__)
+
 [0, 15, 75]: 2018-10-27
   - fix issue with single '?' scalar (reported by `Terrance 
     <https://bitbucket.org/OllieTerrance/>`__)
diff --git a/README.rst b/README.rst
index a05eb5f..09960a4 100644
--- a/README.rst
+++ b/README.rst
@@ -4,8 +4,8 @@ ruamel.yaml
 
 ``ruamel.yaml`` is a YAML 1.2 loader/dumper package for Python.
 
-:version:       0.15.75
-:updated:       2018-10-27
+:version:       0.15.76
+:updated:       2018-11-01
 :documentation: http://yaml.readthedocs.io
 :repository:    https://bitbucket.org/ruamel/
 :pypi:          https://pypi.org/project/ruamel.yaml/
@@ -54,6 +54,11 @@ ChangeLog
 
 .. should insert NEXT: at the beginning of line for next key (with empty line)
 
+0.15.76 (2018-11-01):
+  - fix issue with empty mapping and sequence loaded as flow-style
+    (mapping reported by `Min RK <https://bitbucket.org/minrk/>`__, sequence
+    by `Maged Ahmed <https://bitbucket.org/maged2/>`__)
+
 0.15.75 (2018-10-27):
   - fix issue with single '?' scalar (reported by `Terrance 
     <https://bitbucket.org/OllieTerrance/>`__)
diff --git a/__init__.py b/__init__.py
index 764755b..9b28251 100644
--- a/__init__.py
+++ b/__init__.py
@@ -7,8 +7,8 @@ if False:  # MYPY
 
 _package_data = dict(
     full_package_name='ruamel.yaml',
-    version_info=(0, 15, 75),
-    __version__='0.15.75',
+    version_info=(0, 15, 76),
+    __version__='0.15.76',
     author='Anthon van der Neut',
     author_email='a.van.der.neut@ruamel.eu',
     description='ruamel.yaml is a YAML parser/emitter that supports roundtrip preservation of comments, seq/map flow style, and map key order',  # NOQA
diff --git a/_doc/_static/pypi.svg b/_doc/_static/pypi.svg
index 2aa33a3..ef9b8ff 100644
--- a/_doc/_static/pypi.svg
+++ b/_doc/_static/pypi.svg
@@ -1 +1 @@
-<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="86" height="20"><linearGradient id="b" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><clipPath id="a"><rect width="86" height="20" rx="3" fill="#fff"/></clipPath><g clip-path="url(#a)"><path fill="#555" d="M0 0h33v20H0z"/><path fill="#007ec6" d="M33 0h53v20H33z"/><path fill="url(#b)" d="M0 0h86v20H0z"/></g><g fill="#fff" text-anchor="middle" font-family="DejaVu Sans,Verdana,Geneva,sans-serif" font-size="110"> <text x="175" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="230">pypi</text><text x="175" y="140" transform="scale(.1)" textLength="230">pypi</text><text x="585" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="430">0.15.75</text><text x="585" y="140" transform="scale(.1)" textLength="430">0.15.75</text></g> </svg>
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="86" height="20"><linearGradient id="b" x2="0" y2="100%"><stop offset="0" stop-color="#bbb" stop-opacity=".1"/><stop offset="1" stop-opacity=".1"/></linearGradient><clipPath id="a"><rect width="86" height="20" rx="3" fill="#fff"/></clipPath><g clip-path="url(#a)"><path fill="#555" d="M0 0h33v20H0z"/><path fill="#007ec6" d="M33 0h53v20H33z"/><path fill="url(#b)" d="M0 0h86v20H0z"/></g><g fill="#fff" text-anchor="middle" font-family="DejaVu Sans,Verdana,Geneva,sans-serif" font-size="110"> <text x="175" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="230">pypi</text><text x="175" y="140" transform="scale(.1)" textLength="230">pypi</text><text x="585" y="150" fill="#010101" fill-opacity=".3" transform="scale(.1)" textLength="430">0.15.76</text><text x="585" y="140" transform="scale(.1)" textLength="430">0.15.76</text></g> </svg>
diff --git a/_test/data/duplicate-merge-key.former-loader-error.data b/_test/data/duplicate-merge-key.former-loader-error.data
deleted file mode 100644
index cebc3a1..0000000
--- a/_test/data/duplicate-merge-key.former-loader-error.data
+++ /dev/null
@@ -1,4 +0,0 @@
----
-<<: {x: 1, y: 2}
-foo: bar
-<<: {z: 3, t: 4}
diff --git a/anchor.py b/anchor.py
index e1b5432..4d77ad3 100644
--- a/anchor.py
+++ b/anchor.py
@@ -1,8 +1,8 @@
 
 
-
 anchor_attrib = '_yaml_anchor'
 
+
 class Anchor(object):
     __slots__ = 'value', 'always_dump'
     attrib = anchor_attrib
@@ -11,4 +11,3 @@ class Anchor(object):
         # type: () -> None
         self.value = None
         self.always_dump = False
-
diff --git a/constructor.py b/constructor.py
index 05fbb34..bf484e1 100644
--- a/constructor.py
+++ b/constructor.py
@@ -846,7 +846,7 @@ class Constructor(SafeConstructor):
             lobject_name = [name]
         try:
             __import__(module_name)
-        except ImportError:
+        except ImportError as exc:
             raise ConstructorError(
                 'while constructing a Python object',
                 mark,
@@ -1289,7 +1289,7 @@ class RoundTripConstructor(SafeConstructor):
             return value
 
         # merge = []
-        merge_map_list = []
+        merge_map_list = []  # type: List[Any]
         index = 0
         while index < len(node.value):
             key_node, value_node = node.value[index]
@@ -1494,25 +1494,28 @@ class RoundTripConstructor(SafeConstructor):
         # type: (Any) -> Any
         data = CommentedSeq()
         data._yaml_set_line_col(node.start_mark.line, node.start_mark.column)
-        if node.flow_style is True:
-            data.fa.set_flow_style()
-        elif node.flow_style is False:
-            data.fa.set_block_style()
         if node.comment:
             data._yaml_add_comment(node.comment)
         yield data
         data.extend(self.construct_rt_sequence(node, data))
+        self.set_collection_style(data, node)
 
     def construct_yaml_map(self, node):
         # type: (Any) -> Any
         data = CommentedMap()
         data._yaml_set_line_col(node.start_mark.line, node.start_mark.column)
+        yield data
+        self.construct_mapping(node, data)
+        self.set_collection_style(data, node)
+
+    def set_collection_style(self, data, node):
+        # type: (Any, Any) -> None
+        if len(data) == 0:
+            return
         if node.flow_style is True:
             data.fa.set_flow_style()
         elif node.flow_style is False:
             data.fa.set_block_style()
-        yield data
-        self.construct_mapping(node, data)
 
     def construct_yaml_object(self, node, cls):
         # type: (Any, Any) -> Any
diff --git a/representer.py b/representer.py
index 28ec9a2..e0ac31d 100644
--- a/representer.py
+++ b/representer.py
@@ -895,7 +895,7 @@ class RoundTripRepresenter(SafeRepresenter):
                 best_style = False
             value.append(node_item)
         if flow_style is None:
-            if self.default_flow_style is not None:
+            if len(sequence) != 0 and self.default_flow_style is not None:
                 node.flow_style = self.default_flow_style
             else:
                 node.flow_style = best_style
@@ -961,11 +961,13 @@ class RoundTripRepresenter(SafeRepresenter):
         except AttributeError:
             item_comments = {}
         merge_list = [m[1] for m in getattr(mapping, merge_attrib, [])]
+        item_count = 0
         if bool(merge_list):
             items = mapping.non_merged_items()
         else:
             items = mapping.items()
         for item_key, item_value in items:
+            item_count += 1
             node_key = self.represent_key(item_key)
             node_value = self.represent_data(item_value)
             item_comment = item_comments.get(item_key)
@@ -984,7 +986,8 @@ class RoundTripRepresenter(SafeRepresenter):
                 best_style = False
             value.append((node_key, node_value))
         if flow_style is None:
-            if self.default_flow_style is not None:
+            if ((item_count != 0) or bool(merge_list)) \
+               and self.default_flow_style is not None:
                 node.flow_style = self.default_flow_style
             else:
                 node.flow_style = best_style