django “如何”系列5：如何编写自定义存储系统

如果你需要提供一个自定义的文件存储-一个常见的例子便是在远程系统上存储文件-你可以通过定义一个自己的存储类来做这件事情，你将通过一下步骤：

• 你自定义的存储系统一定是django.core.files.storage.Storage的子类

from django.core.files.storage import Storage

class MyStorage(Storage):
    ...

• django必须可以实例化你的存储系统（不使用任何参数），这意味着任何的设置都必须从django.conf.settings中拿

from django.conf import settings
from django.core.files.storage import Storage

class MyStorage(Storage):
    def __init__(self, option=None):
        if not option:
            option = settings.CUSTOM_STORAGE_OPTIONS

• 你的存储类必须实现_open()和_save()方法，以及其他的和你存储类相关的方法，下面会讲到
• 另外，如果你的类提供本地文件存储，你必须覆盖path()方法，如果不，可以忽略这个方法

这是基类Storage的源码，我会在源码的注释中讲解一些内容一些要注意的内容

class Storage(object):
    """    存储基类，提供一些默认的行为供其他的存储系统继承或者覆盖（如果需要的话）    """
    # 下面的方法代表了私有方法的一个公共接口，除非绝对的需要，这些方法不应该被子类覆盖
    def open(self, name, mode='rb'):
        """        从存储中检索特定的文件        """
        return self._open(name, mode)

    def save(self, name, content):
        """        用给定的文件名保存给定的新内容，内容应该是一个合适的可以从头开始读取的File对象        """
        # Get the proper name for the file, as it will actually be saved.
        if name is None:
            name = content.name

        name = self.get_available_name(name)
        name = self._save(name, content)

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

    # 这些方法是一部分的公共API（已经实现好的，当然，你可以覆盖）

    def get_valid_name(self, name):
        """        Returns a filename, based on the provided filename, that's suitable for
        use in the target storage system.        """
        return get_valid_filename(name)

    def get_available_name(self, name):
        """        Returns a filename that's free on the target storage system, and
        available for new content to be written to.        """
        dir_name, file_name = os.path.split(name)
        file_root, file_ext = os.path.splitext(file_name)
        # If the filename already exists, add an underscore and a number (before
        # the file extension, if one exists) to the filename until the generated
        # filename doesn't exist.
        count = itertools.count(1)
        while self.exists(name):
            # file_ext includes the dot.
            name = os.path.join(dir_name, "%s_%s%s" % (file_root, count.next(), file_ext))

        return name

    def path(self, name):
        """
        Returns a local filesystem path where the file can be retrieved using
        Python's built-in open() function. Storage systems that can't be
        accessed using open() should *not* implement this method.
        """
        raise NotImplementedError("This backend doesn't support absolute paths.")

    # 下面的这些方法是没有提供默认实现的公共API，子类一定要实现这些方法

    def delete(self, name):
        """        Deletes the specified file from the storage system.        """
        raise NotImplementedError()

    def exists(self, name):
        """        Returns True if a file referened by the given name already exists in the
        storage system, or False if the name is available for a new file.        """
        raise NotImplementedError()

    def listdir(self, path):
        """        Lists the contents of the specified path, returning a 2-tuple of lists;
        the first item being directories, the second item being files.        """
        raise NotImplementedError()

    def size(self, name):
        """        Returns the total size, in bytes, of the file specified by name.        """
        raise NotImplementedError()

    def url(self, name):
        """        Returns an absolute URL where the file's contents can be accessed
        directly by a Web browser.        """
        raise NotImplementedError()

    def accessed_time(self, name):
        """        Returns the last accessed time (as datetime object) of the file
        specified by name.        """
        raise NotImplementedError()

    def created_time(self, name):
        """        Returns the creation time (as datetime object) of the file
        specified by name.        """
        raise NotImplementedError()

    def modified_time(self, name):
        """        Returns the last modified time (as datetime object) of the file
        specified by name.        """
        raise NotImplementedError()

看完这个源码，相信你已经知道该如何如写一个自己的存储系统类了（那些方法一定要有的，那些是可以直接用的，那些是一定要写的），下面我们看一下django自带的一个实现吧

class FileSystemStorage(Storage):
    """    Standard filesystem storage    """
    def __init__(self, location=None, base_url=None):
        if location is None:
            location = settings.MEDIA_ROOT
        self.base_location = location
        self.location = abspathu(self.base_location)
        if base_url is None:
            base_url = settings.MEDIA_URL
        self.base_url = base_url

    def _open(self, name, mode='rb'):
        return File(open(self.path(name), mode))

    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:
                os.makedirs(directory)
            except OSError, 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)
                    content.close()

                # 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.
                    fd = os.open(full_path, os.O_WRONLY | os.O_CREAT | os.O_EXCL | getattr(os, 'O_BINARY', 0))
                    try:
                        locks.lock(fd, locks.LOCK_EX)
                        for chunk in content.chunks():
                            os.write(fd, chunk)
                    finally:
                        locks.unlock(fd)
                        os.close(fd)
            except OSError, 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 settings.FILE_UPLOAD_PERMISSIONS is not None:
            os.chmod(full_path, settings.FILE_UPLOAD_PERMISSIONS)

        return name

    def delete(self, name):
        name = self.path(name)
        # If the file exists, delete it from the filesystem.
        # Note that there is a race between os.path.exists and os.remove:
        # if os.remove fails with ENOENT, the file was removed
        # concurrently, and we can continue normally.
        if os.path.exists(name):
            try:
                os.remove(name)
            except OSError, e:
                if e.errno != errno.ENOENT:
                    raise

    def exists(self, name):
        return os.path.exists(self.path(name))

    def listdir(self, path):
        path = self.path(path)
        directories, files = [], []
        for entry in os.listdir(path):
            if os.path.isdir(os.path.join(path, entry)):
                directories.append(entry)
            else:
                files.append(entry)
        return directories, files

    def path(self, name):
        try:
            path = safe_join(self.location, name)
        except ValueError:
            raise SuspiciousOperation("Attempted access to '%s' denied." % name)
        return os.path.normpath(path)

    def size(self, name):
        return os.path.getsize(self.path(name))

    def url(self, name):
        if self.base_url is None:
            raise ValueError("This file is not accessible via a URL.")
        return urlparse.urljoin(self.base_url, filepath_to_uri(name))

    def accessed_time(self, name):
        return datetime.fromtimestamp(os.path.getatime(self.path(name)))

    def created_time(self, name):
        return datetime.fromtimestamp(os.path.getctime(self.path(name)))

    def modified_time(self, name):
        return datetime.fromtimestamp(os.path.getmtime(self.path(name)))