Python in a Nutshell

The mmap Module

The mmap module supplies memory-mapped file objects. An mmap object behaves similarly to a plain (not Unicode) string, so you can often pass an mmap object where a plain string is expected. However, there are differences:

• An mmap object does not supply the methods of a string object

• An mmap object is mutable, while string objects are immutable

• An mmap object also corresponds to an open file and behaves polymorphically to a Python file object (as covered in Chapter 10)

An mmap object m can be indexed or sliced, yielding plain strings. Since m is mutable, you can also assign to an indexing or slicing of m. However, when you assign to a slice of m, the right-hand side of the assignment statement must be a string of exactly the same length as the slice you’re assigning to. Therefore, many of the useful tricks available with list slice assignment (covered in Chapter 4) do not apply to mmap slice assignment.

Module mmap supplies a factory function that is different on Unix-like systems and Windows.

Reference Section

Methods of mmap Objects

An mmap object m supplies the following methods.

Using mmap Objects for IPC

The way in which processes communicate using mmap is similar to IPC using files: one process can write data, and another process can later read the same data back. Since an mmap object rests on an underlying file, you can also have some processes doing I/O directly on the file, as covered in Chapter 10, while others use mmap to access the same file. You can choose ...