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7.2.2 The `pickle` Module

Strings can easily be written to and read from a file. Numbers take a bit more effort, since the `read()` method only returns strings, which will have to be passed to a function like `string.atoi()`, which takes a string like `'123'` and returns its numeric value 123. However, when you want to save more complex data types like lists, dictionaries, or class instances, things get a lot more complicated.

Rather than have users be constantly writing and debugging code to save complicated data types, Python provides a standard module called `pickle`. This is an amazing module that can take almost any Python object (even some forms of Python code!), and convert it to a string representation; this process is called pickling. Reconstructing the object from the string representation is called unpickling. Between pickling and unpickling, the string representing the object may have been stored in a file or data, or sent over a network connection to some distant machine.

If you have an object `x`, and a file object `f` that's been opened for writing, the simplest way to pickle the object takes only one line of code:

      pickle.dump(x, f)

To unpickle the object again, if `f` is a file object which has been opened for reading:

      x = pickle.load(f)

(There are other variants of this, used when pickling many objects or when you don't want to write the pickled data to a file; consult the complete documentation for `pickle` in the Library Reference.)

`pickle` is the standard way to make Python objects which can be stored and reused by other programs or by a future invocation of the same program; the technical term for this is a persistent object. Because `pickle` is so widely used, many authors who write Python extensions take care to ensure that new data types such as matrices can be properly pickled and unpickled.