18.9. `mmap` — Suporte a arquivos com memória mapeada¶

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.

18.9. `mmap` — Suporte a arquivos com memória mapeada¶

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18.9. mmap — Suporte a arquivos com memória mapeada¶

18.9. `mmap` — Suporte a arquivos com memória mapeada¶