mmap — Memory-mapped file support¶

Memory-mapped file objects behave like both bytearray and like file objects. You can use mmap objects in most places where bytearray are expected; for example, you can use the 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 seek() through the file to different positions.

A memory-mapped file is created by the 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 fileno() method to obtain the correct value for the fileno parameter. Otherwise, you can open the file using the os.open() function, which returns a file descriptor directly (the file still needs to be closed when done).

For both the Unix and Windows versions of the constructor, access may be specified as an optional keyword parameter. access accepts one of four values: ACCESS_READ, ACCESS_WRITE, or ACCESS_COPY to specify read-only, write-through or copy-on-write memory respectively, or ACCESS_DEFAULT to defer to prot. 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 ACCESS_READ memory map raises a TypeError exception. Assignment to an ACCESS_WRITE memory map affects both memory and the underlying file. Assignment to an 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.

class mmap.mmap(fileno, length, tagname=None, access=ACCESS_DEFAULT, offset=0, *, trackfd=True)¶

(Windows version) Maps length bytes from the file specified by the file descriptor 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 tagname 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.

If trackfd is False, the file handle corresponding to fileno will not be duplicated, and the resulting mmap object will not be associated with the map’s underlying file. This means that the size() and resize() methods will fail. This mode is useful to limit the number of open file handles. The original file can be renamed (but not deleted) after closing fileno.

Changed in version 3.15: The trackfd parameter was added.

Raises an auditing event mmap.__new__ with arguments fileno, length, access, offset.

class mmap.mmap(fileno, length, flags=MAP_SHARED, prot=PROT_WRITE | PROT_READ, access=ACCESS_DEFAULT, offset=0, *, trackfd=True)

(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 mmap is called.

flags specifies the nature of the mapping. 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 MAP_SHARED creates a mapping that’s shared with all other processes mapping the same areas of the file. The default value is MAP_SHARED. Some systems have additional possible flags with the full list specified in MAP_* constants.

prot, if specified, gives the desired memory protection; the two most useful values are PROT_READ and PROT_WRITE, to specify that the pages may be read or written. prot defaults to 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 ALLOCATIONGRANULARITY which is equal to PAGESIZE on Unix systems.

If trackfd is False, the file descriptor specified by fileno will not be duplicated, and the resulting mmap object will not be associated with the map’s underlying file. This means that the size() and resize() methods will fail. This mode is useful to limit the number of open file descriptors.

To ensure validity of the created memory mapping the file specified by the descriptor fileno is internally automatically synchronized with the physical backing store on macOS.

Changed in version 3.13: The trackfd parameter was added.

This example shows a simple way of using 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()

mmap can also be used as a context manager in a with statement:

import mmap

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

Added in version 3.2: Context manager support.

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()

Raises an auditing event mmap.__new__ with arguments fileno, length, access, offset.

Memory-mapped file objects support the following methods:

close()¶

Closes the mmap. Subsequent calls to other methods of the object will result in a ValueError exception being raised. This will not close the open file.

closed¶

True if the file is closed.

Added in version 3.2.

find(sub[, start[, end]])¶

Returns the lowest index in the object where the subsequence sub is found, such that sub is contained in the range [start, end]. Optional arguments start and end are interpreted as in slice notation. Returns -1 on failure.

Changed in version 3.5: Writable bytes-like object is now accepted.

flush([offset, [size, ]]*, flags=MS_SYNC)¶

Flushes changes made to the in-memory copy of a file back to disk. Without use of this call there is no guarantee that changes are written back before the object is destroyed. If offset and size are specified, only changes to the given range of bytes will be flushed to disk; otherwise, the whole extent of the mapping is flushed. offset must be a multiple of the PAGESIZE or ALLOCATIONGRANULARITY.

The flags parameter specifies the synchronization behavior. flags must be one of the MS_* constants available on the system.

On Windows, the flags parameter is ignored.

None is returned to indicate success. An exception is raised when the call failed.

Changed in version 3.8: Previously, a nonzero value was returned on success; zero was returned on error under Windows. A zero value was returned on success; an exception was raised on error under Unix.

Changed in version 3.15: Allow specifying offset without size. Previously, both offset and size parameters were required together. Now offset can be specified alone, and the flush operation will extend from offset to the end of the mmap.

Changed in version 3.15: Added flags parameter to control synchronization behavior.

madvise(option[, start[, length]])¶

Send advice option to the kernel about the memory region beginning at start and extending length bytes. option must be one of the MADV_* constants available on the system. If start and length are omitted, the entire mapping is spanned. On some systems (including Linux), start must be a multiple of the PAGESIZE.

Availability: Systems with the madvise() system call.

Added in version 3.8.

move(dest, src, count)¶

Copy the count bytes starting at offset src to the destination index dest. If the mmap was created with ACCESS_READ, then calls to move will raise a TypeError exception.

read([n])¶

Return a bytes containing up to n bytes starting from the current file position. If the argument is omitted, None or negative, return all bytes from the current file position to the end of the mapping. The file position is updated to point after the bytes that were returned.

Changed in version 3.3: Argument can be omitted or None.

read_byte()¶

Returns a byte at the current file position as an integer, and advances the file position by 1.

readline()¶

Returns a single line, starting at the current file position and up to the next newline. The file position is updated to point after the bytes that were returned.

resize(newsize)¶

Resizes the map and the underlying file, if any.

Resizing a map created with access of ACCESS_READ or ACCESS_COPY, will raise a TypeError exception. Resizing a map created with trackfd set to False, will raise a ValueError exception.

On Windows: Resizing the map will raise an OSError if there are other maps against the same named file. Resizing an anonymous map (ie against the pagefile) will silently create a new map with the original data copied over up to the length of the new size.

Availability: Windows and systems with the mremap() system call.

Changed in version 3.11: Correctly fails if attempting to resize when another map is held Allows resize against an anonymous map on Windows

rfind(sub[, start[, end]])¶

Returns the highest index in the object where the subsequence sub is found, such that sub is contained in the range [start, end]. Optional arguments start and end are interpreted as in slice notation. Returns -1 on failure.

Changed in version 3.5: Writable bytes-like object is now accepted.

seek(pos[, whence])¶

Set the file’s current position. whence argument is optional and defaults to os.SEEK_SET or 0 (absolute file positioning); other values are os.SEEK_CUR or 1 (seek relative to the current position) and os.SEEK_END or 2 (seek relative to the file’s end).

Changed in version 3.13: Return the new absolute position instead of None.

seekable()¶

Return whether the file supports seeking, and the return value is always True.

Added in version 3.13.

set_name(name, /)¶

Annotate the memory mapping with the given name for easier identification in /proc/<pid>/maps if the kernel supports the feature and -X dev is passed to Python or if Python is built in debug mode. The length of name must not exceed 67 bytes including the '\0' terminator.

Availability: Linux >= 5.17 (kernel built with CONFIG_ANON_VMA_NAME option)

Added in version 3.15.

size()¶

Return the length of the file, which can be larger than the size of the memory-mapped area. For an anonymous mapping, return its size.

Changed in version 3.15: Anonymous mappings are now supported on Unix.

tell()¶

Returns the current position of the file pointer.

write(bytes)¶

Write the bytes in bytes into memory at the current position of the file pointer and return the number of bytes written (never less than len(bytes), since if the write fails, a ValueError will be raised). The file position is updated to point after the bytes that were written. If the mmap was created with ACCESS_READ, then writing to it will raise a TypeError exception.

Changed in version 3.5: Writable bytes-like object is now accepted.

Changed in version 3.6: The number of bytes written is now returned.

write_byte(byte)¶

Write the integer byte into memory at the current position of the file pointer; the file position is advanced by 1. If the mmap was created with ACCESS_READ, then writing to it will raise a TypeError exception.