Python 垃圾收集有时在 Jupyter Notebook 中不起作用

Question

我经常用完一些 Jupyter Notebooks 的 RAM，而且我似乎无法释放不再需要的内存。这是一个例子:

import gc
thing = Thing()
result = thing.do_something(...)
thing = None
gc.collect()

如您所料，thing 使用大量内存来做某事，然后我就不再需要它了。我应该能够释放它使用的内存。即使它没有写入我可以从我的笔记本访问的任何变量，垃圾收集器也没有正确释放空间。我发现的唯一解决方法是将 result 写入 pickle，重新启动内核，从 pickle 加载 result，然后继续。这在运行长笔记本时确实很不方便。如何正确释放内存？

Answer 1

这里有很多问题。第一个是 IPython(当你看到像 Out[67] 这样的东西时，Jupyter 在幕后使用的东西会保留对对象的额外引用。事实上，你可以使用该语法来调用对象并用它做一些事情. eg. str(Out[67]). 第二个问题是 Jupyter 似乎保留了它自己的输出变量引用，所以只有 IPython 的完全重置才能工作。但这并没有太大的不同只是重新启动笔记本电脑。

虽然有一个解决方案!我编写了一个您可以运行的函数，它将清除所有变量，除了您明确要求保留的变量。

def my_reset(*varnames):
    """
    varnames are what you want to keep
    """
    globals_ = globals()
    to_save = {v: globals_[v] for v in varnames}
    to_save['my_reset'] = my_reset  # lets keep this function by default
    del globals_
    get_ipython().magic("reset")
    globals().update(to_save)

你会像这样使用它:

x = 1
y = 2
my_reset('x')
assert 'y' not in globals()
assert x == 1

下面我写了一个笔记本，它向您展示了幕后发生的一些事情，以及您如何通过使用 weakref 模块来查看何时真正删除了某些内容。您可以尝试运行它，看看它是否有助于您了解正在发生的事情。

In [1]: class MyObject:
            pass

In [2]: obj = MyObject()

In [3]: # now lets try deleting the object
        # First, create a weak reference to obj, so we can know when it is truly deleted.
        from weakref import ref
        from sys import getrefcount
        r = ref(obj)
        print("the weak reference looks like", r)
        print("it has a reference count of", getrefcount(r()))
        # this prints a ref count of 2 (1 for obj and 1 because getrefcount
        # had a reference to obj)
        del obj
        # since obj was the only strong reference to the object, it should have been 
        # garbage collected now.
        print("the weak reference looks like", r)

the weak reference looks like <weakref at 0x7f29a809d638; to 'MyObject' at 0x7f29a810cf60>
it has a reference count of 2
the weak reference looks like <weakref at 0x7f29a809d638; dead>

In [4]: # lets try again, but this time we won't print obj, will just do "obj"
        obj = MyObject()

In [5]: print(getrefcount(obj))
        obj

2
Out[5]: <__main__.MyObject at 0x7f29a80a0c18>

In [6]: # note the "Out[5]". This is a second reference to our object
        # and will keep it alive if we delete obj
        r = ref(obj)
        del obj
        print("the weak reference looks like", r)
        print("with a reference count of:", getrefcount(r()))

the weak reference looks like <weakref at 0x7f29a809db88; to 'MyObject' at 0x7f29a80a0c18>
with a reference count of: 7

In [7]: # So what happened? It's that Out[5] that is keeping the object alive.
        # if we clear our Out variables it should go away...
        # As it turns out Juypter keeps a number of its own variables lying around, 
        # so we have to reset pretty everything.

In [8]: def my_reset(*varnames):
            """
            varnames are what you want to keep
            """
            globals_ = globals()
            to_save = {v: globals_[v] for v in varnames}
            to_save['my_reset'] = my_reset  # lets keep this function by default
            del globals_
            get_ipython().magic("reset")
            globals().update(to_save)

        my_reset('r') # clear everything except our weak reference to the object
        # you would use this to keep "thing" around.

Once deleted, variables cannot be recovered. Proceed (y/[n])? y

In [9]: print("the weak reference looks like", r)

the weak reference looks like <weakref at 0x7f29a809db88; dead>