Python TypeError:传递给 object.__format__ 的非空格式字符串 [英] Python TypeError: non-empty format string passed to object.__format__

问题描述

我最近遇到了这个 TypeError 异常，我发现它很难调试.我最终将其简化为这个小测试用例:

<预><代码>>>>"{:20}".format(b"hi")回溯(最近一次调用最后一次):文件<stdin>"，第 1 行，在 <module> 中类型错误:传递给 object.__format__ 的非空格式字符串

无论如何，这对我来说非常不明显.我的代码的解决方法是将字节字符串解码为 un​​icode:

<代码> >>>"{:20}".format(b"hi".decode("ascii"))'你好                  '

这个异常是什么意思?有什么方法可以说得更清楚吗?

解决方案

bytes 对象没有自己的 __format__ 方法，所以默认来自 使用对象:

<预><代码>>>>bytes.__format__ 是 object.__format__真的>>>'{:20}'.format(object())回溯(最近一次调用最后一次):文件<stdin>"，第 1 行，在 <module> 中类型错误:传递给 object.__format__ 的非空格式字符串

这只是意味着除了直接的、未格式化的未对齐格式之外，您不能使用任何其他格式.显式转换为字符串对象(就像您通过将 bytes 解码为 str 所做的那样)以获取 格式规范支持.

您可以使用 !s 字符串转换使转换显式:

<预><代码>>>>'{!s:20s}'.format(b"嗨")你好">>>'{!s:20s}'.format(object())'<0x1100b9080处的对象>'

object.__format__ 明确拒绝格式化字符串以避免隐式字符串转换，特别是因为格式化指令是特定于类型的.

I hit this TypeError exception recently, which I found very difficult to debug. I eventually reduced it to this small test case:

>>> "{:20}".format(b"hi")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: non-empty format string passed to object.__format__

This is very non-obvious, to me anyway. The workaround for my code was to decode the byte string into unicode:

 >>> "{:20}".format(b"hi".decode("ascii"))
 'hi                  '

What is the meaning of this exception? Is there a way it can be made more clear?

解决方案

bytes objects do not have a __format__ method of their own, so the default from object is used:

>>> bytes.__format__ is object.__format__
True
>>> '{:20}'.format(object())
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: non-empty format string passed to object.__format__

It just means that you cannot use anything other than straight up, unformatted unaligned formatting on these. Explicitly convert to a string object (as you did by decoding bytes to str) to get format spec support.

You can make the conversion explicit by using the !s string conversion:

>>> '{!s:20s}'.format(b"Hi")
"b'Hi'               "
>>> '{!s:20s}'.format(object())
'<object object at 0x1100b9080>'

object.__format__ explicitly rejects format strings to avoid implicit string conversions, specifically because formatting instructions are type specific.

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