Fix Mermaid printer rendering double underscores as bold formatting

doc/whatsnew/fragments/10402.bugfix

@@ -0,0 +1,3 @@
+Fix double underscores rendering as bold in pyreverse Mermaid output.
+
+Closes #10402

pylint/pyreverse/mermaidjs_printer.py

@@ -32,6 +32,14 @@ def _open_graph(self) -> None:
         self.emit("classDiagram")
         self._inc_indent()
 
+    def _escape_mermaid_text(self, text: str) -> str:
+        """Escape text to prevent Mermaid from interpreting it as Markdown
+        formatting.
+        """
+        # Escape characters that conflict with Markdown formatting
+        text = text.replace("__", r"\_\_")  # Double underscore → escaped
+        return text
+
     def emit_node(
         self,
         name: str,
@@ -48,14 +56,24 @@ def emit_node(
         nodetype = self.NODES[type_]
         body = []
         if properties.attrs:
-            body.extend(properties.attrs)
+            # Escape attribute names to prevent Markdown formatting issues
+            escaped_attrs = [
+                self._escape_mermaid_text(attr) for attr in properties.attrs
+            ]
+            body.extend(escaped_attrs)
         if properties.methods:
             for func in properties.methods:
                 args = self._get_method_arguments(func)
-                line = f"{func.name}({', '.join(args)})"
+                # Escape method name and arguments
+                escaped_method_name = self._escape_mermaid_text(func.name)
+                escaped_args = [self._escape_mermaid_text(arg) for arg in args]
+                line = f"{escaped_method_name}({', '.join(escaped_args)})"
                 line += "*" if func.is_abstract() else ""
                 if func.returns:
-                    line += f" {get_annotation_label(func.returns)}"
+                    # Escape return type annotation
+                    return_type = get_annotation_label(func.returns)
+                    escaped_return_type = self._escape_mermaid_text(return_type)
+                    line += f" {escaped_return_type}"
                 body.append(line)
         name = name.split(".")[-1]
         self.emit(f"{nodetype} {name} {{")
@@ -80,7 +98,7 @@ def emit_edge(
         to_node = to_node.split(".")[-1]
         edge = f"{from_node} {self.ARROWS[type_]} {to_node}"
         if label:
-            edge += f" : {label}"
+            edge += f" : {self._escape_mermaid_text(label)}"
         self.emit(edge)
 
     def _close_graph(self) -> None:

tests/pyreverse/functional/class_diagrams/aggregation_filtering/all.mmd

@@ -1,25 +1,25 @@
 classDiagram
   class P {
     name : str
-    __init__(name: str)
+    \_\_init\_\_(name: str)
   }
   class PrivateAttr {
-    __x
-    __init__()
+    \_\_x
+    \_\_init\_\_()
   }
   class ProtectedAttr {
     _x
-    __init__()
+    \_\_init\_\_()
   }
   class PublicAttr {
     x
-    __init__()
+    \_\_init\_\_()
   }
   class SpecialAttr {
-    __x__
-    __init__()
+    \_\_x\_\_
+    \_\_init\_\_()
   }
-  P --* PrivateAttr : __x
+  P --* PrivateAttr : \_\_x
   P --* ProtectedAttr : _x
   P --* PublicAttr : x
-  P --* SpecialAttr : __x__
+  P --* SpecialAttr : \_\_x\_\_

tests/pyreverse/functional/class_diagrams/aggregation_filtering/other.mmd

@@ -1,21 +1,21 @@
 classDiagram
   class P {
     name : str
-    __init__(name: str)
+    \_\_init\_\_(name: str)
   }
   class PrivateAttr {
-    __init__()
+    \_\_init\_\_()
   }
   class ProtectedAttr {
-    __init__()
+    \_\_init\_\_()
   }
   class PublicAttr {
     x
-    __init__()
+    \_\_init\_\_()
   }
   class SpecialAttr {
-    __x__
-    __init__()
+    \_\_x\_\_
+    \_\_init\_\_()
   }
   P --* PublicAttr : x
-  P --* SpecialAttr : __x__
+  P --* SpecialAttr : \_\_x\_\_

tests/pyreverse/functional/class_diagrams/aggregation_filtering/special.mmd

@@ -3,7 +3,7 @@ classDiagram
     name : str
   }
   class PrivateAttr {
-    __x
+    \_\_x
   }
   class ProtectedAttr {
     _x
@@ -13,6 +13,6 @@ classDiagram
   }
   class SpecialAttr {
   }
-  P --* PrivateAttr : __x
+  P --* PrivateAttr : \_\_x
   P --* ProtectedAttr : _x
   P --* PublicAttr : x