Question

我最近一直想知道是否有办法检测上下文管理器是否嵌套。

我创建了Timer和TimerGroup类：

class Timer:
    def __init__(self, name="Timer"):
        self.name = name
        self.start_time = clock()

    @staticmethod
    def seconds_to_str(t):
        return str(timedelta(seconds=t))

    def end(self):
        return clock() - self.start_time

    def print(self, t):
        print(("{0:<" + str(line_width - 18) + "} >> {1}").format(self.name, self.seconds_to_str(t)))

    def __enter__(self):
        return self

    def __exit__(self, exc_type, value, traceback):
        self.print(self.end())


class TimerGroup(Timer):
    def __enter__(self):
        print(('= ' + self.name + ' ').ljust(line_width, '='))
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        total_time = self.seconds_to_str(self.end())
        print(" Total: {0}".format(total_time).rjust(line_width, '='))
        print()

此代码以可读格式打印时间：

with TimerGroup("Collecting child documents for %s context" % context_name):
    with Timer("Collecting context features"):
        # some code...
    with Timer("Collecting child documents"):
        # some code...


= Collecting child documents for Global context ============
Collecting context features                >> 0:00:00.001063
Collecting child documents                 >> 0:00:10.611130
====================================== Total: 0:00:10.612292

然而，当我嵌套TimerGroups时，它搞砸了：

with TimerGroup("Choosing the best classifier for %s context" % context_name):
    with Timer("Splitting datasets"):
        # some code...
    for cname, cparams in classifiers.items():
        with TimerGroup("%s classifier" % cname):
            with Timer("Training"):
                # some code...
            with Timer("Calculating accuracy on testing set"):
                # some code


= Choosing the best classifier for Global context ==========
Splitting datasets                         >> 0:00:00.002054
= Naive Bayes classifier ===================================
Training                                   >> 0:00:34.184903
Calculating accuracy on testing set        >> 0:05:08.481904
====================================== Total: 0:05:42.666949

====================================== Total: 0:05:42.669078

我需要做的就是以某种方式缩进嵌套的Timers和TimerGroups。我应该将任何参数传递给他们的构造函数吗？或者我可以从班级内部检测出来吗？

Answer 1

没有特殊的工具来检测嵌套的上下文管理器，没有。你必须自己处理这个问题。您可以在自己的上下文管理器中执行此操作：

import threading


class TimerGroup(Timer):
    _active_group = threading.local()

    def __enter__(self):
        if getattr(TimerGroup._active_group, 'current', False):
            raise RuntimeError("Can't nest TimerGroup context managers")
        TimerGroup._active_group.current = self
        print(('= ' + self.name + ' ').ljust(line_width, '='))
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        TimerGroup._active_group.current = None
        total_time = self.seconds_to_str(self.end())
        print(" Total: {0}".format(total_time).rjust(line_width, '='))
        print()

然后，您可以在其他位置使用TimerGroup._active_group属性来抓取当前活动的群组。我使用thread-local object来确保可以在多个执行线程中使用它。

或者，您可以将堆栈计数器设置为嵌套__enter__调用中的递增和递减，或者堆栈列表并将self推送到该堆栈上，然后弹出它再次__exit__：

import threading


class TimerGroup(Timer):
    _active_group = threading.local()

    def __enter__(self):
        if not hasattr(TimerGroup._active_group, 'current'):
            TimerGroup._active_group.current = []
        stack = TimerGroup._active_group.current
        if stack:
            # nested context manager.
            # do something with stack[-1] or stack[0]
        TimerGroup._active_group.current.append(self)

        print(('= ' + self.name + ' ').ljust(line_width, '='))
        return self

    def __exit__(self, exc_type, exc_val, exc_tb):
        last = TimerGroup._active_group.current.pop()
        assert last == self, "Context managers being exited out of order"
        total_time = self.seconds_to_str(self.end())
        print(" Total: {0}".format(total_time).rjust(line_width, '='))
        print()

Answer 2

import this：

显式优于隐式

清洁设计明确允许指定一个组：

with TimerGroup('Doing big task') as big_task_tg:
    with Timer('Foo', big_task_tg):
      foo_result = foo()
    with Timer('Bar', big_task_tg):
      bar(baz(foo_result))

另一方面，您始终可以使用traceback.extract_stack并查找上游特定函数的调用。它对于日志记录和错误报告非常有用，并且可以适度地用于确保仅在特定上下文中调用特定函数。但它往往会产生非常难以跟踪的依赖关系。

我会避免将它用于分组定时器，尽管你可以试试。如果你非常需要自动分组，@ Martijn-Pieters的方法要优越得多。

Answer 3

如果你需要做的就是根据你执行的嵌套上下文管理器的数量来调整缩进级别，那么就有一个名为indent_level的类属性，并在每次进入和退出时调整它上下文经理。如下所示：

class Context:
    indent_level = 0

    def __init__(self, name):
        self.name = name

    def __enter__(self):
        print(' '*4*self.indent_level + 'Entering ' + self.name)
        self.adjust_indent_level(1)
        return self

    def __exit__(self, *a, **k):
        self.adjust_indent_level(-1)
        print(' '*4*self.indent_level + 'Exiting ' + self.name)

    @classmethod
    def adjust_indent_level(cls, val):
        cls.indent_level += val

并将其用作：

>>> with Context('Outer') as outer_context:
        with Context('Inner') as inner_context:
            print(' '*inner_context.indent_level*4 + 'In the inner context')


Entering Outer
    Entering Inner
        In the inner context
    Exiting Inner
Exiting Outer

检测上下文管理器嵌套

3 个答案: