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python-clinic / Doc / library / mmap.rst

:mod:`mmap` --- Memory-mapped file support

Memory-mapped file objects behave like both :class:`bytearray` and like :term:`file objects <file object>`. You can use mmap objects in most places where :class:`bytearray` are expected; for example, you can use the :mod:`re` module to search through a memory-mapped file. You can also change a single byte by doing obj[index] = 97, or change a subsequence by assigning to a slice: obj[i1:i2] = b'...'. You can also read and write data starting at the current file position, and :meth:`seek` through the file to different positions.

A memory-mapped file is created by the :class:`mmap` constructor, which is different on Unix and on Windows. In either case you must provide a file descriptor for a file opened for update. If you wish to map an existing Python file object, use its :meth:`fileno` method to obtain the correct value for the fileno parameter. Otherwise, you can open the file using the :func:`os.open` function, which returns a file descriptor directly (the file still needs to be closed when done).

Note

If you want to create a memory-mapping for a writable, buffered file, you should :func:`~io.IOBase.flush` the file first. This is necessary to ensure that local modifications to the buffers are actually available to the mapping.

For both the Unix and Windows versions of the constructor, access may be specified as an optional keyword parameter. access accepts one of three values: :const:`ACCESS_READ`, :const:`ACCESS_WRITE`, or :const:`ACCESS_COPY` to specify read-only, write-through or copy-on-write memory respectively. access can be used on both Unix and Windows. If access is not specified, Windows mmap returns a write-through mapping. The initial memory values for all three access types are taken from the specified file. Assignment to an :const:`ACCESS_READ` memory map raises a :exc:`TypeError` exception. Assignment to an :const:`ACCESS_WRITE` memory map affects both memory and the underlying file. Assignment to an :const:`ACCESS_COPY` memory map affects memory but does not update the underlying file.

To map anonymous memory, -1 should be passed as the fileno along with the length.

(Windows version) Maps length bytes from the file specified by the file handle fileno, and creates a mmap object. If length is larger than the current size of the file, the file is extended to contain length bytes. If length is 0, the maximum length of the map is the current size of the file, except that if the file is empty Windows raises an exception (you cannot create an empty mapping on Windows).

tagname, if specified and not None, is a string giving a tag name for the mapping. Windows allows you to have many different mappings against the same file. If you specify the name of an existing tag, that tag is opened, otherwise a new tag of this name is created. If this parameter is omitted or None, the mapping is created without a name. Avoiding the use of the tag parameter will assist in keeping your code portable between Unix and Windows.

offset may be specified as a non-negative integer offset. mmap references will be relative to the offset from the beginning of the file. offset defaults to 0. offset must be a multiple of the ALLOCATIONGRANULARITY.

(Unix version) Maps length bytes from the file specified by the file descriptor fileno, and returns a mmap object. If length is 0, the maximum length of the map will be the current size of the file when :class:`mmap` is called.

flags specifies the nature of the mapping. :const:`MAP_PRIVATE` creates a private copy-on-write mapping, so changes to the contents of the mmap object will be private to this process, and :const:`MAP_SHARED` creates a mapping that's shared with all other processes mapping the same areas of the file. The default value is :const:`MAP_SHARED`.

prot, if specified, gives the desired memory protection; the two most useful values are :const:`PROT_READ` and :const:`PROT_WRITE`, to specify that the pages may be read or written. prot defaults to :const:`PROT_READ \| PROT_WRITE`.

access may be specified in lieu of flags and prot as an optional keyword parameter. It is an error to specify both flags, prot and access. See the description of access above for information on how to use this parameter.

offset may be specified as a non-negative integer offset. mmap references will be relative to the offset from the beginning of the file. offset defaults to 0. offset must be a multiple of the PAGESIZE or ALLOCATIONGRANULARITY.

To ensure validity of the created memory mapping the file specified by the descriptor fileno is internally automatically synchronized with physical backing store on Mac OS X and OpenVMS.

This example shows a simple way of using :class:`mmap`:

import mmap

# write a simple example file
with open("hello.txt", "wb") as f:
    f.write(b"Hello Python!\n")

with open("hello.txt", "r+b") as f:
    # memory-map the file, size 0 means whole file
    mm = mmap.mmap(f.fileno(), 0)
    # read content via standard file methods
    print(mm.readline())  # prints b"Hello Python!\n"
    # read content via slice notation
    print(mm[:5])  # prints b"Hello"
    # update content using slice notation;
    # note that new content must have same size
    mm[6:] = b" world!\n"
    # ... and read again using standard file methods
    mm.seek(0)
    print(mm.readline())  # prints b"Hello  world!\n"
    # close the map
    mm.close()

:class:`mmap` can also be used as a context manager in a :keyword:`with` statement.:

import mmap

with mmap.mmap(-1, 13) as mm:
    mm.write("Hello world!")

The next example demonstrates how to create an anonymous map and exchange data between the parent and child processes:

import mmap
import os

mm = mmap.mmap(-1, 13)
mm.write(b"Hello world!")

pid = os.fork()

if pid == 0: # In a child process
    mm.seek(0)
    print(mm.readline())

    mm.close()

Memory-mapped file objects support the following methods: