如何连接str和int对象？ [英] How can I concatenate str and int objects?

问题描述

如果我尝试执行以下操作：

If I try to do the following:

things = 5
print("You have " + things + " things.")

我在Python 3.x中遇到以下错误：

I get the following error in Python 3.x:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: must be str, not int

...以及Python中的类似错误2.x：

... and a similar error in Python 2.x:

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'int' objects

我该如何解决

推荐答案

这里的问题是 + 运算符具有（至少）在Python中有两种不同的含义：对于数字类型，它的意思是将数字加在一起：

The problem here is that the + operator has (at least) two different meanings in Python: for numeric types, it means "add the numbers together":

>>> 1 + 2
3
>>> 3.4 + 5.6
9.0

...对于序列类型，其含义是连接序列：

... and for sequence types, it means "concatenate the sequences":

>>> [1, 2, 3] + [4, 5, 6]
[1, 2, 3, 4, 5, 6]
>>> 'abc' + 'def'
'abcdef'

通常，Python不会 t隐式地将对象从一种类型转换为另一种 ¹ ，以使操作有意义，因为这会造成混淆：例如，您可能会认为'3'+ 5 应该表示 '35'，但是其他人可能会认为它应该表示 8 或甚至'8'。

As a rule, Python doesn't implicitly convert objects from one type to another¹ in order to make operations "make sense", because that would be confusing: for instance, you might think that '3' + 5 should mean '35', but someone else might think it should mean 8 or even '8'.

类似地，Python不允许您连接两种不同类型的序列：

Similarly, Python won't let you concatenate two different types of sequence:

>>> [7, 8, 9] + 'ghi'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate list (not "str") to list

因此，无论您要连接还是添加，都需要显式进行转换。

Because of this, you need to do the conversion explicitly, whether what you want is concatenation or addition:

>>> 'Total: ' + str(123)
'Total: 123'
>>> int('456') + 789
1245

但是，有一个更好的方法。根据所使用的Python版本，可以使用三种不同的字符串格式 ² ，它们不仅可以避免多次 + 操作：

However, there is a better way. Depending on which version of Python you use, there are three different kinds of string formatting available², which not only allow you to avoid multiple + operations:

>>> things = 5

>>> 'You have %d things.' % things  # % interpolation
'You have 5 things.'

>>> 'You have {} things.'.format(things)  # str.format()
'You have 5 things.'

>>> f'You have {things} things.'  # f-string (since Python 3.6)
'You have 5 things.'

...但还可以控制值的显示方式：

... but also allow you to control how values are displayed:

>>> value = 5
>>> sq_root = value ** 0.5
>>> sq_root
2.23606797749979

>>> 'The square root of %d is %.2f (roughly).' % (value, sq_root)
'The square root of 5 is 2.24 (roughly).'

>>> 'The square root of {v} is {sr:.2f} (roughly).'.format(v=value, sr=sq_root)
'The square root of 5 is 2.24 (roughly).'

>>> f'The square root of {value} is {sq_root:.2f} (roughly).'
'The square root of 5 is 2.24 (roughly).'

是否使用％内插， str.format（） 或 f字符串由您决定：％内插法最长（并且对于C语言背景的人来说是很熟悉的）， str.format（）通常更强大，而f字符串仍然更强大（但仅在Python 3.6和更高版本中可用）。

Whether you use % interpolation, str.format(), or f-strings is up to you: % interpolation has been around the longest (and is familiar to people with a background in C), str.format() is often more powerful, and f-strings are more powerful still (but available only in Python 3.6 and later).

另一种选择是利用事实如果您给 print 多个位置参数，它将使用 sep 关键字参数将字符串表示形式连接在一起（默认到' ’）：

Another alternative is to use the fact that if you give print multiple positional arguments, it will join their string representations together using the sep keyword argument (which defaults to ' '):

>>> things = 5
>>> print('you have', things, 'things.')
you have 5 things.
>>> print('you have', things, 'things.', sep=' ... ')
you have ... 5 ... things.

...但这通常不如使用Python的内置字符串格式化功能灵活。

... but that's usually not as flexible as using Python's built-in string formatting abilities.

¹ 尽管数字类型是一个例外，但大多数人会同意'要做的事情：

¹ Although it makes an exception for numeric types, where most people would agree on the 'right' thing to do:

>>> 1 + 2.3
3.3
>>> 4.5 + (5.6+7j)
(10.1+7j)

² 实际上是四个...，但是模板字符串很少使用并且有点尴尬。

² Actually four ... but template strings are rarely used and somewhat awkward.

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