惰性模块变量——可以做到吗? [英] Lazy module variables--can it be done?

问题描述

我正在尝试找到一种延迟加载模块级变量的方法.

I'm trying to find a way to lazily load a module-level variable.

具体来说，我编写了一个小型 Python 库来与 iTunes 对话，并且我想要一个 DOWNLOAD_FOLDER_PATH 模块变量.不幸的是，iTunes 不会告诉你它的下载文件夹在哪里，所以我写了一个函数来获取一些播客曲目的文件路径并爬回目录树，直到找到下载"目录.

Specifically, I've written a tiny Python library to talk to iTunes, and I want to have a DOWNLOAD_FOLDER_PATH module variable. Unfortunately, iTunes won't tell you where its download folder is, so I've written a function that grabs the filepath of a few podcast tracks and climbs back up the directory tree until it finds the "Downloads" directory.

这需要一两秒钟，所以我希望延迟评估它，而不是在模块导入时.

This takes a second or two, so I'd like to have it evaluated lazily, rather than at module import time.

有没有办法在第一次访问模块变量时懒惰地分配它，或者我必须依赖一个函数?

Is there any way to lazily assign a module variable when it's first accessed or will I have to rely on a function?

推荐答案

你不能用模块来做到这一点，但你可以把一个类伪装"成一个模块，例如，在 itun.py 中, 代码...:

You can't do it with modules, but you can disguise a class "as if" it was a module, e.g., in itun.py, code...:

import sys

class _Sneaky(object):
  def __init__(self):
    self.download = None

  @property
  def DOWNLOAD_PATH(self):
    if not self.download:
      self.download = heavyComputations()
    return self.download

  def __getattr__(self, name):
    return globals()[name]

# other parts of itun that you WANT to code in
# module-ish ways

sys.modules[__name__] = _Sneaky()

现在任何人都可以import itun...并获得你的itun._Sneaky() 实例.__getattr__ 允许您访问 itun.py 中的任何其他内容，这些内容可能比在 内部更方便您将其编码为顶级模块对象_偷偷摸摸的！_)

Now anybody can import itun... and get in fact your itun._Sneaky() instance. The __getattr__ is there to let you access anything else in itun.py that may be more convenient for you to code as a top-level module object, than inside _Sneaky!_)

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