python错误:'str'对象没有属性'upper()'

问题描述

我在 Python 3 中发现了使用 .format( ) 方法进行字符串格式化的可能性，但我提出了一个我不理解的错误.

I'm discovering possibilities of string formatting with .format( ) method in Python 3 but i raised an error that i do not understand.

那么，为什么下面这行没问题[我认为0"可以完全像传递给 format() 的参数一样使用]:

So, why the following line is ok [wich let me think that "0" can be used exactly like the argument passed to format()]:

s = 'First letter of {0} is {0[0]}'.format("hello")  
#gives as expected: 'First letter of hello is h'

但不是这个[对{0}中的0应用方法或函数不起作用?]:

but not this one [applying a method or a function to 0 in {0} doesn't work?]:

s = '{0} becomes {0.upper()} with .upper() method'.format("hello")

引发以下错误:

AttributeError: 'str' object has no attribute 'upper()'

为什么出现的错误说我使用了 upper 作为属性而不是方法?还有其他方法吗:

Why the raised error says i've used upper as an attribute and not as a method? And is there another way to do it than:

s = '{} becomes {} with .upper() method'.format("hello","hello".upper())
#gives as expected: 'hello becomes HELLO with .upper() method'

谢谢！

推荐答案

字符串格式使用有限的类似 Python 的语法.它没有将它们视为实际的 Python 表达式.此语法不支持调用，仅支持订阅(按数字或不带引号的 (!) 名称索引)，并支持属性访问.

String formatting uses a limited Python-like syntax. It is not treating them as actual Python expressions. Calls are not supported in this syntax, only subscription (indexing by number or by unquoted (!) name), and attribute access is supported.

请参阅 格式字符串语法 文档，将字段命名部分限制为:

See the Format String Syntax documentation, which limits the field naming part to:

field_name        ::=  arg_name ("." attribute_name | "[" element_index "]")*

您看到的错误源于 attribute_name 值已设置为 'upper()'，因此标识符 包含括号.字符串对象只有一个名为 upper 的属性，而在实际的 Python 表达式中，() 部分是一个单独的 call 表达式，应用于属性查找:

The error you see stems from the attribute_name value having been set to 'upper()', so the identifier includes the parentheses. String objects only have an attribute named upper, and in actual Python expressions the () part is a separate call expression applied to the result of the attribute lookup:

>>> value = "hello"
>>> getattr(value, 'upper()')   # what the template engine tries to do
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'str' object has no attribute 'upper()'
>>> getattr(value, 'upper')    # what an actual Python expression does
<built-in method upper of str object at 0x10e08d298>
>>> getattr(value, 'upper')()  # you can call the object that is returned
'HELLO'

从 Python 3.6 开始，您可以使用新的 f-string 格式支持完整表达式的字符串文字，因为它们在编译 Python 代码时直接由解释器解析.使用这样的文字你可以这样做:

As of Python 3.6 you can use the new f-string formatted string literals that support full expressions, because those are parsed directly by the interpreter when compiling the Python code. Using such literals you can do:

value = 'hello'
s = f'{value} becomes {value.upper()} with .upper() method'

演示:

>>> f'{value} becomes {value.upper()} with .upper() method'
'hello becomes HELLO with .upper() method'

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