标签: Python - 思考者

2019-05-28发表2025-08-20更新 ChenyangGao 编程思想9 分钟读完 (大约1370个字)

异常是一类消息

在面向对象的世界里，对象之间的交互表现为消息的传递。

异常也是一种消息，就我的理解，异常并非错误，它经常用来表明存在错误，但本质上它是一类特殊的消息，关联着一种跳出局部范围的机制。

It is my understanding that exceptions are not errors, and that they should only be used for exceptional conditions (e.g. I try to write a file into disk and there is no more space, or maybe I do not have permission), and not for flow control.

—— Is it a good practice to use try-except-else in Python?

而在Python的世界里，所有异常都必须是BaseException的子类。
所谓异常是消息，指的是一个异常对象可以携带信息，捕获异常（就得到了一个异常对象）或者传递一个异常对象，你就可以从这个异常对象中提取信息。这个消息有时会被认为是一个指令或者信号，即便仅仅只是捕获到了一个特定类型的异常对象，例如，某个无限循环中有一个捕获异常的语句，当捕获到某个类型的异常对象时，它就认为接收到了退出的信号，快速、优雅地退出。

2019-04-30发表2025-08-20更新 ChenyangGao 编程思想34 分钟读完 (大约5134个字)

CPython中函数的非本地变量

函数对象的特殊属性

NOTE 当前测试的CPython解释器版本为

1
2
3

>>> print(__import__('sys').version)
3.7.3 (default, Mar 27 2019, 16:54:48)
[Clang 4.0.1 (tags/RELEASE_401/final)]

以当前版本的解释器下执行下面的代码，可得到一个Markdown表格

def outer(freevar=None):
    def inner(a, b: int=1, *args, c: str='1', **kwds) -> tuple:
        'a test function'
        return freevar, a, b, args, c, kwds
    return inner

fn = outer()

print('attribute', 'type', 'setable', 'reason', sep=' | ')
print(*'-'*4, sep=' | ')
for attr in dir(fn):
    try:
        obj = getattr(fn, attr)
        setattr(fn, attr, obj)
        print('`%s`'%attr, type(obj), True, '', sep=' | ')
    except Exception as exc: 
        print('`%s`'%attr, type(obj), False, '`%s`'%exc, sep=' | ')

attribute	type	setable	reason
`__annotations__`	<class ‘dict’>	True
`__call__`	<class ‘method-wrapper’>	True
`__class__`	<class ‘type’>	False	`__class__ assignment only supported for heap types or ModuleType subclasses`
`__closure__`	<class ‘tuple’>	False	`readonly attribute`
`__code__`	<class ‘code’>	True
`__defaults__`	<class ‘tuple’>	True
`__delattr__`	<class ‘method-wrapper’>	True
`__dict__`	<class ‘dict’>	True
`__dir__`	<class ‘builtin_function_or_method’>	True
`__doc__`	<class ‘str’>	True
`__eq__`	<class ‘method-wrapper’>	True
`__format__`	<class ‘builtin_function_or_method’>	True
`__ge__`	<class ‘method-wrapper’>	True
`__get__`	<class ‘method-wrapper’>	True
`__getattribute__`	<class ‘method-wrapper’>	True
`__globals__`	<class ‘dict’>	False	`readonly attribute`
`__gt__`	<class ‘method-wrapper’>	True
`__hash__`	<class ‘method-wrapper’>	True
`__init__`	<class ‘method-wrapper’>	True
`__init_subclass__`	<class ‘builtin_function_or_method’>	True
`__kwdefaults__`	<class ‘dict’>	True
`__le__`	<class ‘method-wrapper’>	True
`__lt__`	<class ‘method-wrapper’>	True
`__module__`	<class ‘str’>	True
`__name__`	<class ‘str’>	True
`__ne__`	<class ‘method-wrapper’>	True
`__new__`	<class ‘builtin_function_or_method’>	True
`__qualname__`	<class ‘str’>	True
`__reduce__`	<class ‘builtin_function_or_method’>	True
`__reduce_ex__`	<class ‘builtin_function_or_method’>	True
`__repr__`	<class ‘method-wrapper’>	True
`__setattr__`	<class ‘method-wrapper’>	True
`__sizeof__`	<class ‘builtin_function_or_method’>	True
`__str__`	<class ‘method-wrapper’>	True
`__subclasshook__`	<class ‘builtin_function_or_method’>	True

异常是一类消息

函数对象的特殊属性

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