18.9. "mmap" --- Suporte a arquivos com memória mapeada
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Objetos de arquivo mapeados na memória se comportam como "bytearray" e
como *objetos de arquivo*. Você pode usar objetos mmap na maioria dos
lugares onde "bytearray" é esperado; por exemplo, você pode usar o
módulo "re" para pesquisar um arquivo mapeado na memória. Você também
pode alterar um único byte executando "obj[index] = 97" ou alterar uma
subsequência atribuindo a uma fatia: "obj[i1:i2] = b'...'". Você
também pode ler e gravar dados começando na posição atual do arquivo e
"seek()" através do arquivo para diferentes posições.

Um arquivo mapeado na memória é criado pelo construtor "mmap", que é
diferente no Unix e no Windows. Nos dois casos, você deve fornecer um
descritor de arquivo para um arquivo aberto para atualização. Se você
deseja mapear um objeto de arquivo Python existente, use o método
"fileno()" para obter o valor correto para o parâmetro *fileno*. Caso
contrário, você pode abrir o arquivo usando a função "os.open()", que
retorna um descritor de arquivo diretamente (o arquivo ainda precisa
ser fechado quando terminar).

Nota:

  Se você deseja criar um mapeamento de memória para um arquivo
  gravável e armazenado em buffer, deve usar "flush()" no arquivo
  primeiro. Isso é necessário para garantir que as modificações locais
  nos buffers estejam realmente disponíveis para o mapeamento.

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.

Para mapear a memória anônima, -1 deve ser passado como o fileno junto
com o comprimento.

class mmap.mmap(fileno, length, tagname=None, access=ACCESS_DEFAULT[, offset])

   **(Versão Windows)** Mapeia *length* bytes do arquivo especificado
   pelo identificador de arquivo *fileno* e cria um objeto mmap. Se
   *length* for maior que o tamanho atual do arquivo, o arquivo será
   estendido para conter *length* bytes. Se *length* for "0", o
   tamanho máximo do mapa será o tamanho atual do arquivo, exceto que,
   se o arquivo estiver vazio, o Windows gerará uma exceção (você não
   poderá criar um mapeamento vazio no Windows).

   *tagname*, se especificado e não "None", é uma string que fornece
   um nome de tag para o mapeamento. O Windows permite que você tenha
   muitos mapeamentos diferentes no mesmo arquivo. Se você especificar
   o nome de uma marca existente, essa marca será aberta; caso
   contrário, uma nova marca com esse nome será criada. Se este
   parâmetro for omitido ou >>``<<None'', o mapeamento será criado sem
   um nome. Evitar o uso do parâmetro tag ajudará a manter seu código
   portátil entre o Unix e o 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
   "ALLOCATIONGRANULARITY" which is equal to "PAGESIZE" on Unix
   systems.

   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!")

   Novo na versão 3.2: Suporte a gerenciador de contexto.

   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.

      Novo na versão 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.

      Alterado na versão 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.  *offset* must be a multiple
      of the "PAGESIZE" or "ALLOCATIONGRANULARITY".

      **(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.

      Alterado na versão 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.

      Alterado na versão 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 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.

      Alterado na versão 3.5: Writable *bytes-like object* is now
      accepted.

      Alterado na versão 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.
