gh-90117: handle dict and mapping views in pprint (#30135)

* Teach pprint about dict views with PrettyPrinter._pprint_dict_view and ._pprint_dict_items_view.
* Use _private names for _dict_*_view attributes of PrettyPrinter.
* Use explicit 'items' keyword when calling _pprint_dict_view from _pprint_dict_items_view.
* 📜🤖 Added by blurb_it.
* Improve tests
* Add tests for collections.abc.[Keys|Items|Mapping|Values]View support in pprint.
* Add support for collections.abc.[Keys|Items|Mapping|Values]View in pprint.
* Split _pprint_dict_view into _pprint_abc_view, so pretty-printing normal dict views and ABC views is handled in two simple methods.
* Simplify redundant code.
* Add collections.abc views to some existing pprint tests.
* Test that views from collection.UserDict are correctly formatted by pprint.
* Handle recursive dict and ABC views.
* Test that subclasses of ABC views work in pprint.
* Test dict views coming from collections.Counter.
* Test ABC views coming from collections.ChainMap.
* Test odict views coming from collections.OrderedDict.
* Rename _pprint_abc_view to _pprint_mapping_abc_view.
* Add pprint test for mapping ABC views where ._mapping has a custom __repr__ and fix ChainMap test.
* When a mapping ABC view has a ._mapping that defines a custom __repr__, dispatch pretty-printing it by that __repr__.
* Add tests for ABC mapping views subclasses that don't replace __repr__, also handling those that delete ._mapping on instances.
* Simplify the pretty printing of ABC mapping views.
* Add a test for depth handling when pretty printing dict views.
* Fix checking whether the view type is a subclass of an items view, add a test.
* Move construction of the views __repr__ set out of _safe_repr.

---------

Co-authored-by: blurb-it[bot] <43283697+blurb-it[bot]@users.noreply.github.com>
Co-authored-by: Éric <merwok@netwok.org>
Co-authored-by: Oleg Iarygin <oleg@arhadthedev.net>
Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>
Co-authored-by: Gregory P. Smith <greg@krypto.org>

Lib/pprint.py

@@ -248,6 +248,49 @@ class PrettyPrinter:
 
     _dispatch[_collections.OrderedDict.__repr__] = _pprint_ordered_dict
 
+    def _pprint_dict_view(self, object, stream, indent, allowance, context, level):
+        """Pretty print dict views (keys, values, items)."""
+        if isinstance(object, self._dict_items_view):
+            key = _safe_tuple
+        else:
+            key = _safe_key
+        write = stream.write
+        write(object.__class__.__name__ + '([')
+        if self._indent_per_level > 1:
+            write((self._indent_per_level - 1) * ' ')
+        length = len(object)
+        if length:
+            if self._sort_dicts:
+                entries = sorted(object, key=key)
+            else:
+                entries = object
+            self._format_items(entries, stream, indent, allowance + 1,
+                               context, level)
+        write('])')
+
+    def _pprint_mapping_abc_view(self, object, stream, indent, allowance, context, level):
+        """Pretty print mapping views from collections.abc."""
+        write = stream.write
+        write(object.__class__.__name__ + '(')
+        # Dispatch formatting to the view's _mapping
+        self._format(object._mapping, stream, indent, allowance, context, level)
+        write(')')
+
+    _dict_keys_view = type({}.keys())
+    _dispatch[_dict_keys_view.__repr__] = _pprint_dict_view
+
+    _dict_values_view = type({}.values())
+    _dispatch[_dict_values_view.__repr__] = _pprint_dict_view
+
+    _dict_items_view = type({}.items())
+    _dispatch[_dict_items_view.__repr__] = _pprint_dict_view
+
+    _dispatch[_collections.abc.MappingView.__repr__] = _pprint_mapping_abc_view
+
+    _view_reprs = {cls.__repr__ for cls in
+                   (_dict_keys_view, _dict_values_view, _dict_items_view,
+                    _collections.abc.MappingView)}
+
     def _pprint_list(self, object, stream, indent, allowance, context, level):
         stream.write('[')
         self._format_items(object, stream, indent, allowance + 1,
@@ -610,6 +653,42 @@ class PrettyPrinter:
             del context[objid]
             return "{%s}" % ", ".join(components), readable, recursive
 
+        if issubclass(typ, _collections.abc.MappingView) and r in self._view_reprs:
+            objid = id(object)
+            if maxlevels and level >= maxlevels:
+                return "{...}", False, objid in context
+            if objid in context:
+                return _recursion(object), False, True
+            key = _safe_key
+            if issubclass(typ, (self._dict_items_view, _collections.abc.ItemsView)):
+                key = _safe_tuple
+            if hasattr(object, "_mapping"):
+                # Dispatch formatting to the view's _mapping
+                mapping_repr, readable, recursive = self.format(
+                    object._mapping, context, maxlevels, level)
+                return (typ.__name__ + '(%s)' % mapping_repr), readable, recursive
+            elif hasattr(typ, "_mapping"):
+                #  We have a view that somehow has lost its type's _mapping, raise
+                #  an error by calling repr() instead of failing cryptically later
+                return repr(object), True, False
+            if self._sort_dicts:
+                object = sorted(object, key=key)
+            context[objid] = 1
+            readable = True
+            recursive = False
+            components = []
+            append = components.append
+            level += 1
+            for val in object:
+                vrepr, vreadable, vrecur = self.format(
+                    val, context, maxlevels, level)
+                append(vrepr)
+                readable = readable and vreadable
+                if vrecur:
+                    recursive = True
+            del context[objid]
+            return typ.__name__ + '([%s])' % ", ".join(components), readable, recursive
+
         if (issubclass(typ, list) and r is list.__repr__) or \
            (issubclass(typ, tuple) and r is tuple.__repr__):
             if issubclass(typ, list):