18.9. "mmap" — Memory-mapped file support
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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).

Note: If you want to create a memory-mapping for a writable,
  buffered file, you should "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: "ACCESS_READ", "ACCESS_WRITE", or "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 "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])

   **(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.

class mmap.mmap(fileno, length, flags=MAP_SHARED, prot=PROT_WRITE|PROT_READ, access=ACCESS_DEFAULT[, offset])

   **(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".

   *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 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 "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!")

   New 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()

   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.

      New 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]])

      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.

      **(Windows version)** A nonzero value returned indicates
      success; zero indicates failure.

      **(Unix version)** A zero value is returned to indicate success.
      An exception is raised when the call failed.

   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.

   resize(newsize)

      Resizes the map and the underlying file, if any. If the mmap was
      created with "ACCESS_READ" or "ACCESS_COPY", resizing the map
      will raise a "TypeError" exception.

   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).

   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(bytes)

      Write the bytes in *bytes* 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 raise a "TypeError"
      exception.

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

   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.
