bpo-44649: Fix dataclasses(slots=True) with a field with a default, but init=False (GH-29692)

Special handling is needed, because for non-slots dataclasses the instance attributes are not set: reading from a field just references the class's attribute of the same name, which contains the default value. But this doesn't work for classes using __slots__: they don't read the class's attribute. So in that case (and that case only), initialize the instance attribute. Handle this for both normal defaults, and for fields using default_factory.

Lib/dataclasses.py

@@ -447,7 +447,7 @@ def _field_assign(frozen, name, value, self_name):
     return f'{self_name}.{name}={value}'
 
 
-def _field_init(f, frozen, globals, self_name):
+def _field_init(f, frozen, globals, self_name, slots):
     # Return the text of the line in the body of __init__ that will
     # initialize this field.
 
@@ -487,9 +487,15 @@ def _field_init(f, frozen, globals, self_name):
                 globals[default_name] = f.default
                 value = f.name
         else:
-            # This field does not need initialization.  Signify that
-            # to the caller by returning None.
-            return None
+            # If the class has slots, then initialize this field.
+            if slots and f.default is not MISSING:
+                globals[default_name] = f.default
+                value = default_name
+            else:
+                # This field does not need initialization: reading from it will
+                # just use the class attribute that contains the default.
+                # Signify that to the caller by returning None.
+                return None
 
     # Only test this now, so that we can create variables for the
     # default.  However, return None to signify that we're not going
@@ -521,7 +527,7 @@ def _init_param(f):
 
 
 def _init_fn(fields, std_fields, kw_only_fields, frozen, has_post_init,
-             self_name, globals):
+             self_name, globals, slots):
     # fields contains both real fields and InitVar pseudo-fields.
 
     # Make sure we don't have fields without defaults following fields
@@ -548,7 +554,7 @@ def _init_fn(fields, std_fields, kw_only_fields, frozen, has_post_init,
 
     body_lines = []
     for f in fields:
-        line = _field_init(f, frozen, locals, self_name)
+        line = _field_init(f, frozen, locals, self_name, slots)
         # line is None means that this field doesn't require
         # initialization (it's a pseudo-field).  Just skip it.
         if line:
@@ -1027,6 +1033,7 @@ def _process_class(cls, init, repr, eq, order, unsafe_hash, frozen,
                                     '__dataclass_self__' if 'self' in fields
                                             else 'self',
                                     globals,
+                                    slots,
                           ))
 
     # Get the fields as a list, and include only real fields.  This is