通过django提高python 编码(一) – Python量化投资

通过django提高python 编码(一)

通过django来进阶python,看看django对常用问题的写法,提升自己。

本文方法来之django/core/cache/backends/filebased.py

  • 字典先取key1的value,如果key不存在,则取key2的value,如果都不在取默认值

    // params 字典
    timeout = params.get('timeout', params.get('TIMEOUT', 300))
    
  • 用户传递的函数对象或函数名都能执行

    def get_key_func(key_func):
        if key_func is not None:
            if callable(key_func):
                return key_func
            else:
                return import_string(key_func)
        return default_key_func
    
  • 验证可移植的memcached秘钥

    MEMCACHE_MAX_KEY_LENGTH = 250
    def validate_key(self, key):
        if len(key) > MEMCACHE_MAX_KEY_LENGTH:
            warnings.warn(
                'Cache key will cause errors if used with memcached: %r '
                '(longer than %s)' % (key, MEMCACHE_MAX_KEY_LENGTH), CacheKeyWarning
            )
        for char in key:
            if ord(char) < 33 or ord(char) == 127:
                warnings.warn(
                    'Cache key contains characters that will cause errors if '
                    'used with memcached: %r' % key, CacheKeyWarning
                )
                break
    
  • 创建目录

        def _createdir(self):
            if not os.path.exists(self._dir):
                try:
                    os.makedirs(self._dir, 0o700)
                except FileExistsError:
                    pass
    
  • 列出目录下的文件夹

    # 自己改了下
    import os
    import glob
    dir_ = os.path.abspath('./')
    cache_suffix = '.py'
    def _list_cache_files(_dir):
        """
        Get a list of paths to all the cache files. These are all the files
        in the root cache dir that end on the cache_suffix.
        """
        if not os.path.exists(_dir):
            return []
        filelist = [os.path.join(_dir, fname) for fname
                    in glob.glob1(_dir, '*%s' % cache_suffix)]
        return filelist
    print(_list_cache_files(dir_))
    
  • 删除文件

    def _delete(self, fname):
        if not fname.startswith(self._dir) or not os.path.exists(fname):
            return
        try:
            os.remove(fname)
        except FileNotFoundError:
            # The file may have been removed by another process.
            pass
    
  • 当缓存满的时候,移除一部分缓存算法

    _max_entries = 300
    _cull_frequency = 3
    def _cull():
        """
        清理缓存的触发点是超过做大缓存数目
        随机清除缓存三分之一
        _cull_frequency 为0 表示清空
        """
        filelist = _list_cache_files()
        num_entries = len(filelist)
        if num_entries < _max_entries:
            return  # return early if no culling is required
        if _cull_frequency == 0:
            return self.clear()  # Clear the cache when CULL_FREQUENCY = 0
        # Delete a random selection of entries
        filelist = random.sample(filelist,
                                 int(num_entries / self._cull_frequency))
        for fname in filelist:
            self._delete(fname)
    
  • 用文件作为缓存后端,设置值的逻辑

    
    def set(self, key, value, timeout=DEFAULT_TIMEOUT, version=None):
        """
            先检查缓存文件所在文件夹在不在
            生成缓存文件的目标名字
            确认是否清理缓存
            生成中间文件,内容写入中间文件,移动中间文件到目标文件
            如果失败移除中间文件
        """
        self._createdir()  # Cache dir can be deleted at any time.
        fname = self._key_to_file(key, version)
        self._cull()  # make some room if necessary
        fd, tmp_path = tempfile.mkstemp(dir=self._dir)
        renamed = False
        try:
            with open(fd, 'wb') as f:
                self._write_content(f, timeout, value)
            file_move_safe(tmp_path, fname, allow_overwrite=True)
            renamed = True
        finally:
            if not renamed:
                os.remove(tmp_path)
    

本文方法来之django/core/files/move.py

  • 移动或拷贝文件兼容win和unix

    def _samefile(src, dst):
        # Mac, Unix.使用自带的os.path.samefile
        if hasattr(os.path, 'samefile'):
            try:
                return os.path.samefile(src, dst)
            except OSError:
                return False
        # 其他系统,检查路径
        return (os.path.normcase(os.path.abspath(src)) ==
                os.path.normcase(os.path.abspath(dst)))
    def file_move_safe(old_file_name, new_file_name, chunk_size=1024 * 64, allow_overwrite=False):
        """
        以安全的方式移动文件,首先使用 os.rename移动,如果失败,手动拷贝。
        如果目标文件存在取决于 allow_overwrite 的值
        拷贝到时候加锁,这个锁也是跨平台
        """
        # 如果文件名一样,直接返回
        if _samefile(old_file_name, new_file_name):
            return
        try:
            # os.access 用于检测权限以及路径是否存在
            if not allow_overwrite and os.access(new_file_name, os.F_OK):
                raise IOError("Destination file %s exists and allow_overwrite is False" % new_file_name)
            # 以便宜的方式拷贝
            os.rename(old_file_name, new_file_name)
            return
        except OSError:
            # OSError发生有两种情况:
            #     移动到不同的文件系统上(ext3,ext4,tffs)
            #     移动打开文件
            pass
        # first open the old file, so that it won't go away
        with open(old_file_name, 'rb') as old_file:
            # 此处使用os打开文件标准做法(O_BINARY window必须):
            fd = os.open(new_file_name, (os.O_WRONLY | os.O_CREAT | getattr(os, 'O_BINARY', 0) |
                                        (os.O_EXCL if not allow_overwrite else 0)))
            try:
                # 上锁
                locks.lock(fd, locks.LOCK_EX)
                current_chunk = None
                while current_chunk != b'':
                    current_chunk = old_file.read(chunk_size)
                    os.write(fd, current_chunk)
            finally:
                # 释放锁,关闭文件句柄
                locks.unlock(fd)
                os.close(fd)
        try:
            # 复制权限
            copystat(old_file_name, new_file_name)
        except PermissionError as e:
            # 某些文件系统在拷贝文件元数据的时候会失败(CIFS),
            # 如果old_file_name, new_file_name 两个文件系统不同,忽略
            if e.errno != errno.EPERM:
                raise
        try:
            os.remove(old_file_name)
        except PermissionError as e:
            # 当删除文件的时候,某些操作系统(Cygwin and Windows))会失败,直接忽略就行。因为临时文件会自动删除
            if getattr(e, 'winerror', 0) != 32:
                raise
    
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