Compute sensible default values for unspecified image attributes

The goal is to enable the user to avoid specifying more attributes
than strictly needed.

examples/ex08.yml

@@ -9,8 +9,6 @@ connectors:
     show_pincount: false
     image:
       src: resources/stereo-phone-plug-TRS.png
-      scale: true
-      height: 100
       caption: Tip, Ring, and Sleeve
 
 cables:
@@ -23,6 +21,7 @@ cables:
     shield: SN
     image:
       src: resources/cable-WH+BN+GN+shield.png
+      height: 75
       caption: Cross-section
 
 connections:

src/wireviz/DataClasses.py

@@ -11,15 +11,30 @@ from wireviz import wv_colors
 class Image:
     # Attributes of the image object <img>:
     src: str
-    scale: Optional[str] = "false"  # false | true | width | height | both
+    scale: Optional[str] = None  # false | true | width | height | both
     # Attributes of the image cell <td> containing the image:
     width: Optional[int] = None
     height: Optional[int] = None
-    fixedsize: Optional[bool] = False
+    fixedsize: Optional[bool] = None
     # Contents of the cell <td> just below the image cell:
     caption: Optional[str] = None
     # See also HTML doc at https://graphviz.org/doc/info/shapes.html#html
 
+    def __post_init__(self):
+
+        if self.scale is None:
+            self.scale = "false" if self.width is None     and self.height is None \
+                else     "both"  if self.width is not None and self.height is not None \
+                else     "true" # When only one dimension is specified.
+
+        if self.fixedsize is None:
+            self.fixedsize = self.width is not None or self.height is not None
+
+        if self.width is None and self.height is not None:
+            self.width = self.height # Assuming 1:1 aspect ratio for now.
+        elif self.height is None and self.width is not None:
+            self.height = self.width # Assuming 1:1 aspect ratio for now.
+
 
 @dataclass
 class Connector:

src/wireviz/wv_helper.py

@@ -75,8 +75,8 @@ def html_caption(image):
 
 def html_size_attr(image):
     # Return Graphviz HTML attributes to specify minimum or fixed size of a TABLE or TD object
-    return ((f' width="{image.width}"'   if image.width else '')
+    return ((f' width="{image.width}"'   if image.width is not None else '')
-        +   (f' height="{image.height}"' if image.height else '')
+        +   (f' height="{image.height}"' if image.height is not None else '')
         +   ( ' fixedsize="true"'        if image.fixedsize else '')) if image else ''