In 3.6 typing types are pickled by names:

>>> import pickle, pickletools, typing
>>> pickletools.optimize(pickle.dumps(typing.List))
b'\x80\x03ctyping\nList\n.'
>>> pickletools.dis(pickletools.optimize(pickle.dumps(typing.List)))
    0: \x80 PROTO      3
    2: c    GLOBAL     'typing List'
   15: .    STOP
highest protocol among opcodes = 2

The side effect of this is that they are considered atomic by the copy module.

In 3.7 the pickle data contains all private attributes.

>>> pickletools.optimize(pickle.dumps(typing.List))
b'\x80\x03ctyping\n_GenericAlias\n)\x81}(X\x05\x00\x00\x00_inst\x89X\x08\x00\x00\x00_special\x88X\x05\x00\x00\x00_nameX\x04\x00\x00\x00ListX\n\x00\x00\x00__origin__cbuiltins\nlist\nX\x08\x00\x00\x00__args__ctyping\nTypeVar\n)\x81q\x00}(X\x04\x00\x00\x00nameX\x01\x00\x00\x00TX\x05\x00\x00\x00boundNX\x0b\x00\x00\x00constraints)X\x02\x00\x00\x00co\x89X\x06\x00\x00\x00contra\x89ub\x85X\x0e\x00\x00\x00__parameters__h\x00\x85X\t\x00\x00\x00__slots__Nub.'
>>> pickletools.dis(pickletools.optimize(pickle.dumps(typing.List)))
    0: \x80 PROTO      3
    2: c    GLOBAL     'typing _GenericAlias'
   24: )    EMPTY_TUPLE
   25: \x81 NEWOBJ
   26: }    EMPTY_DICT
   27: (    MARK
   28: X        BINUNICODE '_inst'
   38: \x89     NEWFALSE
   39: X        BINUNICODE '_special'
   52: \x88     NEWTRUE
   53: X        BINUNICODE '_name'
   63: X        BINUNICODE 'List'
   72: X        BINUNICODE '__origin__'
   87: c        GLOBAL     'builtins list'
  102: X        BINUNICODE '__args__'
  115: c        GLOBAL     'typing TypeVar'
  131: )        EMPTY_TUPLE
  132: \x81     NEWOBJ
  133: q        BINPUT     0
  135: }        EMPTY_DICT
  136: (        MARK
  137: X            BINUNICODE 'name'
  146: X            BINUNICODE 'T'
  152: X            BINUNICODE 'bound'
  162: N            NONE
  163: X            BINUNICODE 'constraints'
  179: )            EMPTY_TUPLE
  180: X            BINUNICODE 'co'
  187: \x89         NEWFALSE
  188: X            BINUNICODE 'contra'
  199: \x89         NEWFALSE
  200: u            SETITEMS   (MARK at 136)
  201: b        BUILD
  202: \x85     TUPLE1
  203: X        BINUNICODE '__parameters__'
  222: h        BINGET     0
  224: \x85     TUPLE1
  225: X        BINUNICODE '__slots__'
  239: N        NONE
  240: u        SETITEMS   (MARK at 27)
  241: b    BUILD
  242: .    STOP
highest protocol among opcodes = 2

Unpickling it creates a new object. And I'm not sure all invariants are satisfied.

In additional to lesses efficiency and lost of preserving identity, such pickle can be incompatible with old Python versions and future Python versions if the internal representation of typing types will be changed.

I think it would be nice it would be pickled by name so the pickles are
compatible between Python versions.

What would we do for List[int]?

How are regular ABCs pickled?

Here is the situation for 3.6 and before:

Generic classes are all actual class objects, so they are pickled as immutable. However this creates a problem, parameterized generics, such as List[int] cannot be pickled in 3.6 and before, see python/typing#511 (and this is something very hard to fix).

Here is the situation for 3.7:

Almost no generics are actual class objects, so they are pickled as usual. This also fixes the pickling problems in 3.6. However, there is one problematic thing, type variables, they should be pickled as immutable (i.e. by name reference), but I didn't have time to fix this, this is tracked in python/typing#512

What is interesting this issue adds here is an idea that we can treat special typing aliases that are conceptually "unique" also as immutable. For example, typing.List will be pickled as "typing.List", while typing.List[int] will be pickled as _GenericAlias(<builtins.list>, args=(<builtins.int>,), ...)

Conveniently, all the special typing aliases are already marked with _special=True, so the potential solution would be like this:

class _GenericAlias:
    ...
    def __reduce__(self):
        if self._special:
            return 'typing.' + self._name
        return super().__reduce__()

I think it would be better to pickle typing.List[int] as operator.getitem(typing.List, int).

    def __reduce__(self):
        if self._special:
            return self._name # __module__ = 'typing'
        index = self._args
        if len(index) == 1:
            index, = index
        return operator.getitem, (self._unparametrized, index)

And there may be a special case for Union. I tried to implement this, but it seems to me that parametrized type doesn't have a reference to unparametrized type, and I don't know this code enough for writing idiomatic code.

I'm honestly not too concerned about what happens with List[int] (though
doing a sensible thing here is not wrong :-), but I feel strongly that a
pickle containing a reference to typing.List should be compatible between
Python 3.6 and 3.7.

So we need a decision on this about what, if anything, to do for 3.7. The 3.7.0 ABI freeze is in 3.7.0b3; it would be better to get it resolved for 3.7.0b2.

I am sick now, so can't work on this. There is a small chance I will be able to work on this issue this week. Is it possible to fix this in 3.7b3?

Is it possible to fix this in 3.7b3?

Yes. Get well first!

Thank you, Ned!

New changeset 8349403 by Ivan Levkivskyi in branch 'master':
bpo-32873: Treat type variables and special typing forms as immutable by copy and pickle (GH-6216)
8349403

New changeset d0e04c8 by Miss Islington (bot) in branch '3.7':
bpo-32873: Treat type variables and special typing forms as immutable by copy and pickle (GH-6216)
d0e04c8

I believe I hit a bug with this fix (just pulled the code a few min ago):

In [10]: pickle.loads(pickle.dumps(typing.List))
Out[10]: typing.List

In [11]: pickle.loads(pickle.dumps(typing.FrozenSet))
\---------------------------------------------------------------------------

    PicklingError                             Traceback (most recent call last)
    <ipython-input-11-be060c6090e3> in <module>()
    ----> 1 pickle.loads(pickle.dumps(typing.FrozenSet))

PicklingError: Can't pickle typing.Frozenset: attribute lookup Frozenset on typing failed

The cause is in _GenericAlias.__init__

            name = orig_name[0].title() + orig_name[1:]

Maybe just pass the name explicitly?

For context I originally hit this trying to explicitly getattr(typing, alias_name) not by pickling but I'm pleased to see that's at least apparently intended to be valid use (I need to get the underlying special's parameter variance which is lost when you give it args).

Apparently there is another type with a similar problem -- DefaultDict.

Will fix this now.

New changeset 2a363d2 by Ivan Levkivskyi in branch 'master':
bpo-32873: Remove a name hack for generic aliases in typing module (GH-6376)
2a363d2

New changeset 04eac02 by Miss Islington (bot) in branch '3.7':
bpo-32873: Remove a name hack for generic aliases in typing module (GH-6376)
04eac02