The following types can be pickled:
- integers, long integers, floating point numbers, complex numbers
- normal and Unicode strings
- tuples, lists, and dictionaries containing only picklable objects
- functions defined at the top level of a module
- built-in functions defined at the top level of a module
- classes that are defined at the top level of a module
- instances of such classes whose __dict__ or __setstate__()
is picklable (see section 3.14.5
Attempts to pickle unpicklable objects will raise the PicklingError
exception; when this happens, an unspecified number of bytes may have already been written to
the underlying file.
Note that functions (built-in and user-defined) are pickled by ``fully qualified'' name
reference, not by value. This means that only the function name is pickled, along with the
name of module the function is defined in. Neither the function's code, nor any of its
function attributes are pickled. Thus the defining module must be importable in the unpickling
environment, and the module must contain the named object, otherwise an exception will be
Similarly, classes are pickled by named reference, so the same restrictions in the
unpickling environment apply. Note that none of the class's code or data is pickled, so in the
following example the class attribute
attr is not restored in the unpickling
attr = 'a class attr'
picklestring = pickle.dumps(Foo)
These restrictions are why picklable functions and classes must be defined in the top level
of a module.
Similarly, when class instances are pickled, their class's code and data are not pickled
along with them. Only the instance data are pickled. This is done on purpose, so you can fix
bugs in a class or add methods to the class and still load objects that were created with an
earlier version of the class. If you plan to have long-lived objects that will see many
versions of a class, it may be worthwhile to put a version number in the objects so that
suitable conversions can be made by the class's __setstate__() method.
- ... raised3.5
- The exception raised will likely be an ImportError or an AttributeError but it could be something else.