Python - 检测我的对象何时写入标准输出? [英] Python - detect when my object is written to stdout?

问题描述

我有一个相当不寻常的要求，我想...我会在解释什么之后再解释原因.

I have a rather unusual request, I think... I'll explain the why after I explain the what.

什么

我想检测我的对象何时写入标准输出，以便我可以在那个时候执行副作用.例如，当我输入:

I want to detect whenever my object is written to stdout, so that I can perform side effects at that time. So, for example, when I type:

sys.stdout.write(instance_of_my_class)

它应该会产生副作用.我已经让我的类成为 str 的子类并覆盖了 __call__、__unicode__、__str__、__repr__、index、decode、encode、format、__format__、__getattribute__、__getitem__ 和 __len__ 以便他们每个人都打印一个声明，表明他们已被调用，但似乎 sys.stdout.write 不调用任何这些来打印对象.

it should perform side effects. I've made my class be a subclass of str and overrode __call__, __unicode__, __str__, __repr__, index, decode, encode, format, __format__, __getattribute__,__getitem__, and __len__ so that each of them prints a statement indicating that they've been called, but it seems that sys.stdout.write calls none of those in order to print an object.

请注意，我专门讨论的是 sys.stdout.write 而不是例如 print - 我发现 print对给定的任何内容调用 __str__.

Note that I'm specifically talking about sys.stdout.write and not, for example, print - I have found that print calls __str__ on whatever it is given.

为什么

此问题从 Windows 中的彩色 Python 提示? 的答案继续关闭.

This question continues from where the answer to Colored Python Prompt in Windows? left off.

我发现python每次需要显示交互提示时，都会调用sys.ps1和sys.ps2上的__str__，然后它保存结果以显示在命令行上.这意味着 sys.ps2.__str__ 中的任何副作用都是在 sys.ps1.__str__ 中的副作用之后引起的，但我想让这些等到显示的时候sys.ps2.

I have found that each time python needs to display an interactive prompt, it calls __str__ on sys.ps1 and sys.ps2, and then it saves the results to be displayed on the command line. This means any side effects in sys.ps2.__str__ are caused right after the ones in sys.ps1.__str__, but I want to have those wait until it's time to display sys.ps2.

因此，我没有返回 sys.ps2.__str__ 中的 str，而是返回了 str 的子类，我'我希望在调用 sys.stdout.write 时能够以某种方式捕获它.

So rather than return a str in sys.ps2.__str__, I've been returning my subclass of str, which I'm hoping will somehow be capable of catching when sys.stdout.write is called on it.

推荐答案

为什么不monkeypatch stdout.write?

Why not monkeypatch stdout.write?

stdoutRegistry = set()

class A(object):
    def __init__(self):
        self.stdoutRegistry.add(self)

    def stdoutNotify(self):
        pass

original_stdoutWrite = sys.stdout.write
def stdoutWrite(*a, **kw):
    if a in stdoutRegistry:
        a.stdoutNotify()
    original_stdoutWrite(*a, **kw)
sys.stdout.write = stdoutWrite

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