如何连接 str 和 int 对象？

如果我尝试做以下事情:

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

我在 Python3.x 中得到以下错误:

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "int") to str

... 在 Python2.x 中也有类似的错误:

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

我怎样才能解决这个问题呢？

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最佳答案

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

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

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

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'.

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

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

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.'

... 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).'

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).

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.

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

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

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