### 3.14.5.2 Pickling and unpickling extension types

When the `Pickler` encounters an object of a type it knows nothing about -- such as an extension type -- it looks in two places for a hint of how to pickle it. One alternative is for the object to implement a `__reduce__()` method. If provided, at pickling time `__reduce__()` will be called with no arguments, and it must return either a string or a tuple.

If a string is returned, it names a global variable whose contents are pickled as normal. When a tuple is returned, it must be of length two or three, with the following semantics:

- A callable object, which in the unpickling environment must be either a class, a callable registered as a ``safe constructor'' (see below), or it must have an attribute `__safe_for_unpickling__` with a true value. Otherwise, an `UnpicklingError` will be raised in the unpickling environment. Note that as usual, the callable itself is pickled by name.

- A tuple of arguments for the callable object, or `None`.

- Optionally, the object's state, which will be passed to the object's `__setstate__()` method as described in section 3.14.5. If the object has no `__setstate__()` method, then, as above, the value must be a dictionary and it will be added to the object's `__dict__`.

Upon unpickling, the callable will be called (provided that it meets the above criteria), passing in the tuple of arguments; it should return the unpickled object. If the second item was `None`, then instead of calling the callable directly, its `__basicnew__()` method is called without arguments. It should also return the unpickled object.

An alternative to implementing a `__reduce__()` method on the object to be pickled, is to register the callable with the `copy_reg` module. This module provides a way for programs to register ``reduction functions'' and constructors for user-defined types. Reduction functions have the same semantics and interface as the `__reduce__()` method described above, except that they are called with a single argument, the object to be pickled.

The registered constructor is deemed a ``safe constructor'' for purposes of unpickling as described above.