Refactor Inspector._get_info to make it easier to subclass and modify behavior.

This factors out the logic to append an info field to the inspect_reply mimebundle and the logic for what information is in the mimebundle into separate functions that can easily be overridden by an inspector subclass. This allows a subclass to easily:

* format information into yet another mimetype besides text/plain and text/html
* modify or add to the default information

without having to copy the default implementation.

IPython/core/oinspect.py

@@ -552,9 +552,10 b' class Inspector(Colorable):'
 
 
     def format_mime(self, bundle):
-
+        """Format a mimebundle being created by _make_info_unformatted into a real mimebundle"""
+        # First, format the field names and values for the text/plain field
         text_plain = bundle['text/plain']
-
+        if isinstance(text_plain, (list, tuple)):
         text = ''
         heads, bodies = list(zip(*text_plain))
         _len = max(len(h) for h in heads)
@@ -565,43 +566,32 b' class Inspector(Colorable):'
             text += self.__head(head+':') + (_len - len(head))*' ' +delim + body +'\n'
 
         bundle['text/plain'] = text
-        return bundle
-
-    def _get_info(
-        self, obj, oname="", formatter=None, info=None, detail_level=0, omit_sections=()
-    ):
-        """Retrieve an info dict and format it.
-
-        Parameters
-        ----------
-        obj : any
-            Object to inspect and return info from
-        oname : str (default: ''):
-            Name of the variable pointing to `obj`.
-        formatter : callable
-        info
-            already computed information
-        detail_level : integer
-            Granularity of detail level, if set to 1, give more information.
-        omit_sections : container[str]
-            Titles or keys to omit from output (can be set, tuple, etc., anything supporting `in`)
-        """
-
-        info = self.info(obj, oname=oname, info=info, detail_level=detail_level)
 
-        _mime = {
-            'text/plain': [],
-            'text/html': '',
-        }
+        # Next format the text/html value by joining strings if it is a list of strings
+        if isinstance(bundle['text/html'], (list, tuple)):
+            bundle['text/html'] = '\n'.join(bundle['text/html'])
+        return bundle
 
-        def append_field(bundle, title:str, key:str, formatter=None):
+    def _append_info_field(self, bundle, title:str, key:str, info, omit_sections, formatter):
+        """Append an info value to the unformatted mimebundle being constructed by _make_info_unformatted"""
             if title in omit_sections or key in omit_sections:
                 return
             field = info[key]
             if field is not None:
                 formatted_field = self._mime_format(field, formatter)
                 bundle['text/plain'].append((title, formatted_field['text/plain']))
-                bundle['text/html'] += '<h1>' + title + '</h1>\n' + formatted_field['text/html'] + '\n'
+            bundle['text/html'] += '<h1>' + title + '</h1>\n' + formatted_field['text/html']
+
+    def _make_info_unformatted(self, info, formatter, detail_level, omit_sections):
+        """Assemble the mimebundle as unformatted lists of information"""
+        bundle = {
+            'text/plain': [],
+            'text/html': [],
+        }
+
+        # A convenience function to simplify calls below
+        def append_field(bundle, title:str, key:str, formatter=None):
+            self._append_info_field(bundle, title=title, key=key, info=info, omit_sections=omit_sections, formatter=formatter)
 
         def code_formatter(text):
             return {
@@ -610,56 +600,79 b' class Inspector(Colorable):'
             }
 
         if info['isalias']:
-            append_field(_mime, 'Repr', 'string_form')
+            append_field(bundle, 'Repr', 'string_form')
 
         elif info['ismagic']:
             if detail_level > 0:
-                append_field(_mime, 'Source', 'source', code_formatter)
+                append_field(bundle, 'Source', 'source', code_formatter)
             else:
-                append_field(_mime, 'Docstring', 'docstring', formatter)
-            append_field(_mime, 'File', 'file')
+                append_field(bundle, 'Docstring', 'docstring', formatter)
+            append_field(bundle, 'File', 'file')
 
         elif info['isclass'] or is_simple_callable(obj):
             # Functions, methods, classes
-            append_field(_mime, 'Signature', 'definition', code_formatter)
-            append_field(_mime, 'Init signature', 'init_definition', code_formatter)
-            append_field(_mime, 'Docstring', 'docstring', formatter)
+            append_field(bundle, 'Signature', 'definition', code_formatter)
+            append_field(bundle, 'Init signature', 'init_definition', code_formatter)
+            append_field(bundle, 'Docstring', 'docstring', formatter)
             if detail_level > 0 and info['source']:
-                append_field(_mime, 'Source', 'source', code_formatter)
+                append_field(bundle, 'Source', 'source', code_formatter)
             else:
-                append_field(_mime, 'Init docstring', 'init_docstring', formatter)
+                append_field(bundle, 'Init docstring', 'init_docstring', formatter)
 
-            append_field(_mime, 'File', 'file')
-            append_field(_mime, 'Type', 'type_name')
-            append_field(_mime, 'Subclasses', 'subclasses')
+            append_field(bundle, 'File', 'file')
+            append_field(bundle, 'Type', 'type_name')
+            append_field(bundle, 'Subclasses', 'subclasses')
 
         else:
             # General Python objects
-            append_field(_mime, 'Signature', 'definition', code_formatter)
-            append_field(_mime, 'Call signature', 'call_def', code_formatter)
-            append_field(_mime, 'Type', 'type_name')
-            append_field(_mime, 'String form', 'string_form')
+            append_field(bundle, 'Signature', 'definition', code_formatter)
+            append_field(bundle, 'Call signature', 'call_def', code_formatter)
+            append_field(bundle, 'Type', 'type_name')
+            append_field(bundle, 'String form', 'string_form')
 
             # Namespace
             if info['namespace'] != 'Interactive':
-                append_field(_mime, 'Namespace', 'namespace')
+                append_field(bundle, 'Namespace', 'namespace')
 
-            append_field(_mime, 'Length', 'length')
-            append_field(_mime, 'File', 'file')
+            append_field(bundle, 'Length', 'length')
+            append_field(bundle, 'File', 'file')
 
             # Source or docstring, depending on detail level and whether
             # source found.
             if detail_level > 0 and info['source']:
-                append_field(_mime, 'Source', 'source', code_formatter)
+                append_field(bundle, 'Source', 'source', code_formatter)
             else:
-                append_field(_mime, 'Docstring', 'docstring', formatter)
+                append_field(bundle, 'Docstring', 'docstring', formatter)
+
+            append_field(bundle, 'Class docstring', 'class_docstring', formatter)
+            append_field(bundle, 'Init docstring', 'init_docstring', formatter)
+            append_field(bundle, 'Call docstring', 'call_docstring', formatter)
+        return bundle
 
-            append_field(_mime, 'Class docstring', 'class_docstring', formatter)
-            append_field(_mime, 'Init docstring', 'init_docstring', formatter)
-            append_field(_mime, 'Call docstring', 'call_docstring', formatter)
 
+    def _get_info(
+        self, obj, oname="", formatter=None, info=None, detail_level=0, omit_sections=()
+    ):
+        """Retrieve an info dict and format it.
+
+        Parameters
+        ----------
+        obj : any
+            Object to inspect and return info from
+        oname : str (default: ''):
+            Name of the variable pointing to `obj`.
+        formatter : callable
+        info
+            already computed information
+        detail_level : integer
+            Granularity of detail level, if set to 1, give more information.
+        omit_sections : container[str]
+            Titles or keys to omit from output (can be set, tuple, etc., anything supporting `in`)
+        """
 
-        return self.format_mime(_mime)
+        info = self.info(obj, oname=oname, info=info, detail_level=detail_level)
+        bundle = self._make_info_unformatted(info, formatter, detail_level=detail_level, omit_sections=omit_sections)
+        return self.format_mime(bundle)
 
     def pinfo(
         self,