子类化异常有什么帮助？

Question

我没有直接调用异常，而是看到它被子类化，其中没有任何内容或语句pass。它如何帮助 Python 在内部以这种方式子类化基类？它会改变命名空间或签名吗？如何？

class ACustomException(Exception):
    pass

class BCustomException(Exception):
    pass

Answer 1

提高Exception就像告诉医生“出了什么问题”，然后拒绝回答任何问题。比较：

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try:\n    with open("foo.json", "rt") as r:\n        new_count = json.load(r)["count"] + 1\nexcept Exception:\n    # Is the file missing?\n    # Is the file there, but not readable?\n    # Is the file readable, but does not contain valid JSON?\n    # Is the file format okay, but the data\'s not a dict with `count`?\n    # Is the entry `count` there, but is not a number?\n    print("Something\'s wrong")\n    # I don\'t care. You figure it out.\n

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和

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try:\n    with open("data.json", "rt") as r:\n        new_count = json.load(r)["count"] + 1\nexcept FileNotFoundError:\n    print("File is missing.")\nexcept PermissionError:\n    print("File not readable.")\nexcept json.decoder.JSONDecoderError:\n    print("File is not valid JSON.")\nexcept KeyError:\n    print("Cannot find count.")\nexcept TypeError:\n    print("Count is not a number.")\n

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如果您正在创建一个库，则可以在适当的情况下使用预定义的异常类 \xe2\x80\x94，但有时您需要传达 Python 创建者从未考虑过的错误，或者需要比现有异常进行更精细的区分。这是您创建自定义异常的时候。

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例如，Django 将定义异常来传达代码尝试在数据库中django.contrib.auth.models.User.DoesNotExist查找，但找不到与给定条件匹配的情况。能够发现疾病就像是一名医生，接诊的病人不仅会告诉你疼痛情况，还会带来 X 光片和打印的家族史。UserUserdjango.contrib.auth.models.User.DoesNotExist

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