7.7 mmap -- Memory-mapped file support
Keywords: lib

# 7.7 `mmap` -- Memory-mapped file support

Memory-mapped file objects behave like both strings and like file objects. Unlike normal string objects, however, these are mutable. You can use mmap objects in most places where strings are expected; for example, you can use the `re` module to search through a memory-mapped file. Since they're mutable, you can change a single character by doing `obj[index] = 'a'`, or change a substring by assigning to a slice: `obj[i1:i2] = '...'`. 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()` function, 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 function, access may be specified as an optional keyword parameter. access accepts one of three values: `ACCESS_READ`, `ACCESS_WRITE`, or `ACCESS_COPY` to specify readonly, 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 `ACCESS_READ` memory map raises a `TypeError` exception. Assignment to an `ACCESS_WRITE` memory map affects both memory and the underlying file. Assigment to an `ACCESS_COPY` memory map affects memory but does not update the underlying file.

`mmap`(fileno, length[, tagname[, access]])
  (Windows version) Maps length bytes from the file specified by the file handle 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.

  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.

`mmap`(fileno, length[, flags[, prot[, access]]])
  (Unix version) Maps length bytes from the file specified by the file descriptor fileno, and returns a mmap object.

  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`.

  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.

Memory-mapped file objects support the following methods:

`close`()
  Close the file. Subsequent calls to other methods of the object will result in an exception being raised.

`find`(string[, start])
  Returns the lowest index in the object where the substring string is found. Returns `-1` on failure. start is the index at which the search begins, and defaults to zero.

`flush`([offset, size])
  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.

`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 throw a `TypeError` exception.

`read`(num)
  Return a string containing up to num bytes starting from the current file position; the file position is updated to point after the bytes that were returned.

`read_byte`()
  Returns a string of length 1 containing the character at the current file position, and advances the file position by 1.

`readline`()
  Returns a single line, starting at the current file position and up to the next newline.

`resize`(newsize)
  If the mmap was created with `ACCESS_READ` or `ACCESS_COPY`, resizing the map will throw a `TypeError` exception.

`seek`(pos[, whence])
  Set the file's current position. whence argument is optional and defaults to `0` (absolute file positioning); other values are `1` (seek relative to the current position) and `2` (seek relative to the file's end).

`size`()
  Return the length of the file, which can be larger than the size of the memory-mapped area.

`tell`()
  Returns the current position of the file pointer.

`write`(string)
  Write the bytes in string into memory at the current position of the file pointer; 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 throw a `TypeError` exception.

`write_byte`(byte)
  Write the single-character string 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 throw a `TypeError` exception.