Issue #27137: align Python & C implementations of functools.partial

The pure Python fallback implementation of functools.partial
now matches the behaviour of its accelerated C counterpart for
subclassing, pickling and text representation purposes.

Patch by Emanuel Barry and Serhiy Storchaka.

Lib/functools.py

@@ -21,6 +21,7 @@ from abc import get_cache_token
 from collections import namedtuple
 from types import MappingProxyType
 from weakref import WeakKeyDictionary
+from reprlib import recursive_repr
 try:
     from _thread import RLock
 except ImportError:
@@ -237,26 +238,83 @@ except ImportError:
 ################################################################################
 
 # Purely functional, no descriptor behaviour
-def partial(func, *args, **keywords):
+class partial:
     """New function with partial application of the given arguments
     and keywords.
     """
-    if hasattr(func, 'func'):
-        args = func.args + args
-        tmpkw = func.keywords.copy()
-        tmpkw.update(keywords)
-        keywords = tmpkw
-        del tmpkw
-        func = func.func
 
-    def newfunc(*fargs, **fkeywords):
-        newkeywords = keywords.copy()
-        newkeywords.update(fkeywords)
-        return func(*(args + fargs), **newkeywords)
-    newfunc.func = func
-    newfunc.args = args
-    newfunc.keywords = keywords
-    return newfunc
+    __slots__ = "func", "args", "keywords", "__dict__", "__weakref__"
+
+    def __new__(*args, **keywords):
+        if not args:
+            raise TypeError("descriptor '__new__' of partial needs an argument")
+        if len(args) < 2:
+            raise TypeError("type 'partial' takes at least one argument")
+        cls, func, *args = args
+        if not callable(func):
+            raise TypeError("the first argument must be callable")
+        args = tuple(args)
+
+        if hasattr(func, "func"):
+            args = func.args + args
+            tmpkw = func.keywords.copy()
+            tmpkw.update(keywords)
+            keywords = tmpkw
+            del tmpkw
+            func = func.func
+
+        self = super(partial, cls).__new__(cls)
+
+        self.func = func
+        self.args = args
+        self.keywords = keywords
+        return self
+
+    def __call__(*args, **keywords):
+        if not args:
+            raise TypeError("descriptor '__call__' of partial needs an argument")
+        self, *args = args
+        newkeywords = self.keywords.copy()
+        newkeywords.update(keywords)
+        return self.func(*self.args, *args, **newkeywords)
+
+    @recursive_repr()
+    def __repr__(self):
+        qualname = type(self).__qualname__
+        args = [repr(self.func)]
+        args.extend(repr(x) for x in self.args)
+        args.extend(f"{k}={v!r}" for (k, v) in self.keywords.items())
+        if type(self).__module__ == "functools":
+            return f"functools.{qualname}({', '.join(args)})"
+        return f"{qualname}({', '.join(args)})"
+
+    def __reduce__(self):
+        return type(self), (self.func,), (self.func, self.args,
+               self.keywords or None, self.__dict__ or None)
+
+    def __setstate__(self, state):
+        if not isinstance(state, tuple):
+            raise TypeError("argument to __setstate__ must be a tuple")
+        if len(state) != 4:
+            raise TypeError(f"expected 4 items in state, got {len(state)}")
+        func, args, kwds, namespace = state
+        if (not callable(func) or not isinstance(args, tuple) or
+           (kwds is not None and not isinstance(kwds, dict)) or
+           (namespace is not None and not isinstance(namespace, dict))):
+            raise TypeError("invalid partial state")
+
+        args = tuple(args) # just in case it's a subclass
+        if kwds is None:
+            kwds = {}
+        elif type(kwds) is not dict: # XXX does it need to be *exactly* dict?
+            kwds = dict(kwds)
+        if namespace is None:
+            namespace = {}
+
+        self.__dict__ = namespace
+        self.func = func
+        self.args = args
+        self.keywords = kwds
 
 try:
     from _functools import partial