Django默认使用的文件存储系统

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。