Django文件存储 默认存储系统解析

Django默认使用的文件存储系统'django.core.files.storage.FileSystemStorage'是一个本地存储系统，由settings中的DEFAULT_FILE_STORAGE值确定。

class FileSystemStorage(location=None, base_url=None, file_permissions_mode=None, directory_permissions_mode=None)

FileSystemStorage类继承自Storage类，location是存储文件的绝对路径，默认值是settings中的MEDIA_ROOT值，base_url默认值是settings中的MEDIA_URL值。

当定义location参数时，可以无视MEDIA_ROOT值来存储文件：

from django.db import models
from django.core.files.storage import FileSystemStorage
fs = FileSystemStorage(location='/media/photos')
class Car(models.Model):
  ...
  photo = models.ImageField(storage=fs)

这样文件会存储在/media/photos文件夹。

可以直接使用Django的文件存储系统来存储文件：

>>> from django.core.files.storage import default_storage
>>> from django.core.files.base import ContentFile

>>> path = default_storage.save('/path/to/file', ContentFile('new content'))
>>> path
'/path/to/file'

>>> default_storage.size(path)
11
>>> default_storage.open(path).read()
'new content'

>>> default_storage.delete(path)
>>> default_storage.exists(path)
False

可以从FileSystemStorage类的_save方法看下上传文件是怎么存储的：

def _save(self, name, content):
  full_path = self.path(name)

  # Create any intermediate directories that do not exist.
  # Note that there is a race between os.path.exists and os.makedirs:
  # if os.makedirs fails with EEXIST, the directory was created
  # concurrently, and we can continue normally. Refs #16082.
  directory = os.path.dirname(full_path)
  if not os.path.exists(directory):
    try:
      if self.directory_permissions_mode is not None:
        # os.makedirs applies the global umask, so we reset it,
        # for consistency with file_permissions_mode behavior.
        old_umask = os.umask(0)
        try:
          os.makedirs(directory, self.directory_permissions_mode)
        finally:
          os.umask(old_umask)
      else:
        os.makedirs(directory)
    except OSError as e:
      if e.errno != errno.EEXIST:
        raise
  if not os.path.isdir(directory):
    raise IOError("%s exists and is not a directory." % directory)

  # There's a potential race condition between get_available_name and
  # saving the file; it's possible that two threads might return the
  # same name, at which point all sorts of fun happens. So we need to
  # try to create the file, but if it already exists we have to go back
  # to get_available_name() and try again.

  while True:
    try:
      # This file has a file path that we can move.
      if hasattr(content, 'temporary_file_path'):
        file_move_safe(content.temporary_file_path(), full_path)

      # This is a normal uploadedfile that we can stream.
      else:
        # This fun binary flag incantation makes os.open throw an
        # OSError if the file already exists before we open it.
        flags = (os.O_WRONLY | os.O_CREAT | os.O_EXCL |
             getattr(os, 'O_BINARY', 0))
        # The current umask value is masked out by os.open!
        fd = os.open(full_path, flags, 0o666)
        _file = None
        try:
          locks.lock(fd, locks.LOCK_EX)
          for chunk in content.chunks():
            if _file is None:
              mode = 'wb' if isinstance(chunk, bytes) else 'wt'
              _file = os.fdopen(fd, mode)
            _file.write(chunk)
        finally:
          locks.unlock(fd)
          if _file is not None:
            _file.close()
          else:
            os.close(fd)
    except OSError as e:
      if e.errno == errno.EEXIST:
        # Ooops, the file exists. We need a new file name.
        name = self.get_available_name(name)
        full_path = self.path(name)
      else:
        raise
    else:
      # OK, the file save worked. Break out of the loop.
      break

  if self.file_permissions_mode is not None:
    os.chmod(full_path, self.file_permissions_mode)

  # Store filenames with forward slashes, even on Windows.
  return force_text(name.replace('\\', '/'))

方法中可以看出，先判断文件存储的目录是否存在，如果不存在，使用os.mkdirs()依次创建目录。

根据directory_permissions_mode参数来确定创建的目录的权限，应该为(0777 &~umask)。

然后使用os.open()创建文件，flags参数为(os.O_WRONLY | os.O_CREAT | os.O_EXCL | getattr(os, 'O_BINARY', 0))，

这样当文件已存在时，则报EEXIST异常，使用get_available_name()方法重新确定文件的名字。

mode为0o666，权限为(0666 &~umask)。

content为FILE对象，如一切正常，使用FILE.chunks()依次将内容写入文件。

最后，根据file_permissions_mode参数，修改创建文件的权限。