Text file programs and pickle

Creating and reading text files in programs:

• Opening an existing file in w mode erases its contents; opening it in a mode adds after the existing data. Both modes create the file if it does not exist.
• To read a file line by line, call readline() in a loop, and stop when it returns an empty string (the end of the file).
• In w+ mode, one file object can write, then seek(0) back to the start, and read what was written.

Storing Python objects: the pickle module. Text files can only store strings. To save objects such as lists or dictionaries exactly as they are, Python uses the pickle module.

• Pickling (also called serialization) turns a Python object into a stream of bytes, which can be stored in a binary file.
• Unpickling (deserialization) turns the bytes back into the original object.

Two methods do the work:

• pickle.dump(object, file_object) writes the object to a file opened in binary write (wb) or append (ab) mode.
• pickle.load(file_object) reads one object back from a file opened in binary read (rb) mode.

To read every object stored in a file, call pickle.load() in a loop. When there is nothing left, load() raises an EOFError, so the loop is placed in a try block and stops when that exception is caught.