Allow multiple style references

README.rst

@@ -287,9 +287,9 @@ highest priority among all links in the group.
 ~~~~~~~~~~
 
 You can add style definitions to the ``styles`` section to create
-named styles that can be referred to by the terminals and links.  Each
-style definition consists of attribute key-value pairs.  For example,
-the following two terminals are drawn in the same color:
+named styles that can be referred to by the terminals, links and
+groups.  Each style definition consists of attribute key-value pairs.
+For example, the following two terminals are drawn in the same color:
 
 .. code:: yaml
 
@@ -306,13 +306,19 @@ the following two terminals are drawn in the same color:
        stroke_width: 3.0
        fill: "#FFDDDD"
 
-Attributes you define in the element itself override the attributes it
-inherits from the linked named style.
+You add style references to an element using the ``style`` attribute.
+The value of this attribute can be either a single style name or a
+list of style names.  When in a list, later styles override the ones
+coming before them.  Attributes you define in the element itself
+override the attributes it inherits from the referenced named styles.
 
 There are two special style names, ``default_terminal`` and
 ``default_link``, which you can use to set default values for all the
 terminals and links in the diagram respectively.
 
+Styles themselves *cannot* reference other styles, i.e. the program
+ignores the ``style`` attribute in style definitions.
+
 ``groups``
 ~~~~~~~~~~
 
@@ -335,10 +341,10 @@ attributes.  In the example that follows, all links are drawn in blue:
        end: d
        group: water
 
-A group definition may contain a reference to a named ``style`` if
-needed.  Note that creating an entry in the ``groups`` section is not
-necessary for the grouping of the links; a common ``group`` name in
-each link definition is sufficient.
+A group definition may contain references to named styles.  Note that
+creating an entry in the ``groups`` section is not necessary for the
+grouping of the links; a common ``group`` name in each link definition
+is sufficient.
 
 Attributes
 ----------
@@ -611,4 +617,10 @@ Release history
 0.2.1 (2020-12-15)
 ~~~~~~~~~~~~~~~~~~
 
-* Enforce the UTF-8 character encoding for the definition file.
+* Enforced the UTF-8 character encoding for the definition file.
+
+0.2.2 (2020-12-16)
+~~~~~~~~~~~~~~~~~~
+
+* Enabled multiple style references in definition files.
+* Made debug switch compatible with Python 3.8.

examples/powerplant.yaml

@@ -19,25 +19,34 @@ styles:
     buffer_fill: lightyellow
     stroke_width: 3
 
-  shaft:
-    arrow_forward: false
-    stroke: dimgrey
-    stroke_width: 8
-    stroke_dasharray: 8 8
+  cold_gas:
+    stroke: turquoise
+
+  combustion:
+    label: "Combustion\nChamber"
 
   compressor:
     label: Compressor
 
-  cold_gas:
-    stroke: turquoise
-
   electrical_generator:
     label: "Electrical\nGenerator"
 
+  gas_turbine:
+    label: Gas Turbine
+
   hot_gas:
     stroke: orangered
     fill: gold
 
+  low_priority:
+    drawing_priority: -1
+
+  shaft:
+    arrow_forward: false
+    stroke: dimgrey
+    stroke_width: 8
+    stroke_dasharray: 8 8
+
   steam:
     stroke: grey
 
@@ -47,7 +56,7 @@ styles:
 groups:
 
   gas_to_steam:
-    drawing_priority: -1
+    style: low_priority
 
 terminals:
 
@@ -67,12 +76,10 @@ terminals:
     style: compressor
 
   cmb1:
-    label: "Combustion\nChamber"
-    style: hot_gas
+    style: [combustion, hot_gas]
 
   cmb2:
-    label: "Combustion\nChamber"
-    style: hot_gas
+    style: [combustion, hot_gas]
 
   cool:
     label: "Cooling\nTower"
@@ -90,12 +97,10 @@ terminals:
     style: electrical_generator
 
   gtb1:
-    label: Gas Turbine
-    style: hot_gas
+    style: [gas_turbine, hot_gas]
 
   gtb2:
-    label: Gas Turbine
-    style: hot_gas
+    style: [gas_turbine, hot_gas]
 
   sgn:
     label: "Steam\nGenerator"
@@ -140,7 +145,7 @@ links:
 
   - start: fuel
     end: [cmb1, cmb2]
-    drawing_priority: -1
+    style: low_priority
     end_bias: vertical
     stroke: olive
 

orthogram/__init__.py

@@ -1,4 +1,4 @@
-__version__ = '0.2.1'
+__version__ = '0.2.2'
 
 from .build import Builder
 from .diagram import Diagram

orthogram/build.py

@@ -125,9 +125,8 @@ def add_group(self, name: str, group_def: _Definition) -> None:
 
         """
         attrs: AttributeDict = {}
-        # Merge attributes inherited from style reference.
-        style_name = group_def.get('style')
-        style_attrs = self._get_style(style_name, True)
+        # Merge attributes inherited from style references.
+        style_attrs = self._collect_style_attributes(group_def)
         self._merge_attributes(attrs, style_attrs)
         # Merge attributes defined here.
         own_attrs = self._collect_attributes(group_def)
@@ -175,9 +174,8 @@ def add_terminal(
         self._merge_attributes(attrs, def_attrs)
         # The terminal definition may be None: an empty definition.
         if terminal_def:
-            # Merge attributes inherited from style reference.
-            style_name = terminal_def.get('style')
-            style_attrs = self._get_style(style_name, True)
+            # Merge attributes inherited from style references.
+            style_attrs = self._collect_style_attributes(terminal_def)
             self._merge_attributes(attrs, style_attrs)
             # Merge attributes defined here.
             own_attrs = self._collect_attributes(terminal_def)
@@ -242,16 +240,25 @@ def add_link(self, link_def: _Definition) -> None:
         if group and group in self._group_styles:
             group_attrs = self._group_styles[group]
             self._merge_attributes(attrs, group_attrs)
-        # Merge attributes inherited from style reference.
-        style_name = link_def.get('style')
-        style_attrs = self._get_style(style_name, True)
+        # Merge attributes inherited from style references.
+        style_attrs = self._collect_style_attributes(link_def)
         self._merge_attributes(attrs, style_attrs)
         # Merge attributes defined here.
         own_attrs = self._collect_attributes(link_def)
         self._merge_attributes(attrs, own_attrs)
         # Create the object(s).
         self._diagram.add_links(start, end, **attrs)
 
+    def _collect_style_attributes(self, any_def: _Definition) -> AttributeMap:
+        """Collect the attributes of the named styles in the definition."""
+        style_value = any_def.get('style')
+        style_names = self._str_or_list(style_value)
+        attrs: AttributeDict = {}
+        for style_name in style_names:
+            style_attrs = self._get_style(style_name, True)
+            self._merge_attributes(attrs, style_attrs)
+        return attrs
+
     def _collect_attributes(self, any_def: _Definition) -> AttributeMap:
         """Collect the attributes from a definition."""
         attrs: AttributeDict = {}
@@ -418,7 +425,9 @@ def _get_style(
     @staticmethod
     def _str_or_list(text: Any) -> List[str]:
         """Take a string or list of strings and return a list of strings."""
-        result = []
+        result: List[str] = []
+        if not text:
+            return result
         if isinstance(text, list):
             result.extend(text)
         else:

pyproject.toml

@@ -1,6 +1,6 @@
 [tool.poetry]
 name = "orthogram"
-version = "0.2.1"
+version = "0.2.2"
 description = "Draw diagrams of graphs."
 license = "GPL-3.0-or-later"
 authors = ["Georgios Athanasiou <yorgath@gmail.com>"]