转换为字符串会引发错误吗？ [英] Can a conversion to string raise an error?

问题描述

我想知道是否转换为字符串，即 str（sth）是否会引发异常，例如 float（sth）是吗？
我想问一下是否需要将我的代码包装在：

I was wondering if converting to string i.e. str(sth) can raise an exception like for example float(sth) does? I am asking this to know if it is necessary to wrap my code in:

try:
    x = str(value)
except ValueError:
    x = None

这在Python 2和3中也有所不同，因为 str 类与它们不同？

Also does this differ in Python 2 and 3, since the str class is different in them??

推荐答案

如果遇到自定义类，该类显式在 __ str __ （如果未定义 __ str __ ，则为 __ repr __ ）。或者，例如，一个类从 __ str __ 返回 bytes 个对象：

If you encounter a custom class that explicitly raises an exception in __str__ (or __repr__ if __str__ is not defined). Or, for example a class that returns a bytes object from __str__:

class Foo:
    def __str__(self):
        return b''

str(Foo()) # TypeError: __str__ returned non-string (type bytes)

但就我个人而言，我没有 看到了这一点，我很确定没有人知道。这样做很愚蠢。同样，在 __ str __ 的实现中发生愚蠢的错误或极端情况可能会导致另一个 Exception 。有趣的是，您知道如何将Python推入这些极端情况（请参阅 @ user2357112

But personally, I have never seen this and I'm pretty sure no one has; it would be daft to do it. Likewise, a silly mistake in the implementation of __str__ or edge cases might create another Exception. It is interesting to see how you could push Python to these edge cases (look at @user2357112 answer here).

在这种情况下，没有内置函数通常会引发异常，因为它是在中为所有它们定义的Py2 和 Py3 。

Other than that case, no built-ins generally raise an exception in this case since it is defined for all of them in Py2 and Py3.

对于用户定义的类 str 将使用 object .__ str __ 默认情况下，如果未在Python 3中定义，则在Python 2中，如果类是新样式类（继承自 object ），请使用它。

For user defined classes str will use object.__str__ by default if not defined in Python 3 and, in Python 2, use it if a class is a new style class (inherits from object).

如果一个类是我相信的旧类，则是 classobj .__ str __ 用于类和 instance .__ str __ （实例）。

If a class is an old style class I believe it is classobj.__str__ that is used for classes and instance.__str__ for instances.

总的来说，我不会理解这一点，特殊情况还不够特殊。

In general, I would not catch this, special cases aren't special enough for this.

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