Can only concatenate str not int to str питон как исправить

Это значит, что вы пытаетесь сложить строки с числами

Программист освоил JavaScript и начал изучать Python. Чтобы освоиться в языке, он переносит проекты с одного языка на другой — убирает точки с запятой, добавляет отступы и меняет команды одного языка на такие же команды из другого языка.

Один из фрагментов его кода после перевода в Python выглядит так:

# зарплата в месяц
month = 14200
# плата за ЖКХ
regular_cost = 5800

# функция, которая считает и возвращает долю ЖКХ в бюджете
def calculate(budget,base):
    message = 'На коммунальные платежи уходит ' + base + 'р. - это ' + base/budget*100 + ' процентов от ежемесячного бюджета'
    return message
    
# отправляем в функцию переменные и выводим результат на экран
print(calculate(month,regular_cost))

Но после запуска программист получает ошибку:

❌ TypeError: can only concatenate str (not “int”) to str

Странно, но в JavaScript всё работало, почему же здесь код сломался?

Что это значит: компилятор не смог соединить строки и числа, поэтому выдал ошибку.

Когда встречается: в языках со строгой типизацией, например в Python, когда у всех переменных в выражении должен быть один и тот же тип данных. А вот в JavaScript, который изучал программист до этого, типизация нестрогая, и компилятор сам мог привести все части выражения к одному типу.

Что делать с ошибкой TypeError: can only concatenate str (not “int”) to str

Раз это проблема строгой типизации, то для исправления этой ошибки нужно просто привести все части к одному типу. 

В нашем случае мы хотим получить на выходе строку, поэтому все слагаемые должны быть строкового типа. Но base и base/budget*100 — это числа, поэтому просто так их сложить со строками не получится. Чтобы выйти из ситуации, явно преобразуем их в строки командой str():

message = 'На коммунальные платежи уходит ' + str(base) + 'р. - это ' + str(base/budget*100) + ' процентов от ежемесячного бюджета'

Команда str() делает так, что всё внутри неё приводится к строке и она отдаёт дальше строку. В итоге операция склейки строк проходит как ожидаемо: строка к строке даёт строку. Нет повода для беспокойства. 

Your Python program crashed and displayed a traceback like this one:

Traceback (most recent call last):
  File "copyright.py", line 4, in <module>
    statement = "Copyright " + year
TypeError: can only concatenate str (not "int") to str

Or maybe you saw a variation on that message:

Traceback (most recent call last):
  File "names.py", line 3, in <module>
    print("Hello" + names)
TypeError: can only concatenate str (not "list") to str

Or you may have seen a different error message that mentions + and int and str:

Traceback (most recent call last):
  File "add.py", line 5, in <module>
    print(start + amount)
TypeError: unsupported operand type(s) for +: 'int' and 'str'

What do these mean?
And how can you fix your code?

What does that error message mean?

The last line is in a Python traceback message is the most important one.

TypeError: can only concatenate str (not "int") to str

That last line says the exception type (TypeError) and the error message (can only concatenate str (not "int") to str).

This error message says something similar:

TypeError: unsupported operand type(s) for +: 'int' and 'str'

Both of these error messages are trying to tell us that we’re trying to use the + operator (used for string concatenation in Python) between a string and a number.

You can use + between a string and a string (to concatenate them) or a number and a number (to add them).
But in Python, you cannot use + between a string and a number because Python doesn’t do automatic type coercion.
Type conversions are usually done manually in Python.

How do we fix this?

How we fix this will depend on our needs.

Look at the code the error occurred on (remember that tracebacks are read from the bottom-up in Python).

  File "copyright.py", line 4, in <module>
    statement = "Copyright " + year

Ask yourself “what am I trying to do here?”
You’re likely either trying to:

1. Build a bigger string (via concatenation)
2. Add two numbers together
3. Use + in another way (e.g. to concatenate lists)

Once you’ve figured out what you’re trying to do, then you’ll then need to figure out how to fix your code to accomplish your goal.

Converting numbers to strings to concatenate them

If the problematic code is meant to concatenate two strings, we’ll need to explicitly convert our non-string object to a string.
For numbers and many other objects, that’s as simple as using the built-in str function.

We could replace this:

statement = "Copyright " + year

With this:

statement = "Copyright " + str(year)

Though in many cases it may be more readable to use string interpolation (via f-strings):

statement = f"Copyright {year}"

This works because Python automatically calls the str function on all objects within the replacement fields in an f-string.

Converting other objects to strings

Using the str function (or an f-string) will work for converting a number to a string, but it won’t always produce nice output for other objects.

For example when trying to concatenate a string and a list:

>>> names = ["Judith", "Andrea", "Verena"]
>>> user_message = "Users include: " + names
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "list") to str

You might think to use str to convert the list to a string:

>>> user_message = "Users include: " + str(names)

This does work, but the resulting string might not look quite the way you’d expect:

>>> print(user_message)
Users include: ['Judith', 'Andrea', 'Verena']

When converting a list to a string you’ll likely want to join together the list items using the string join method.

>>> user_message = "Users include: " + ", ".join(names)
>>> print(user_message)
Users include: Judith, Andrea, Verena

Other types of objects may require a different approach: the string conversion technique you use will depend on the object you’re working with and how you’d like it to look within your string.

Converting strings to numbers to add them

What if our code isn’t supposed to concatenate strings?
What if it’s meant to add numbers together?

If we’re seeing one of these two error messages while trying to add numbers together:

• TypeError: can only concatenate (not "int") to str
• TypeError: unsupported operand type(s) for +: 'int' and 'str'

That means one of our “numbers” is currently a string!

This often happens when accepting a command-line argument that should represent a number:

$ python saving.py 100
Traceback (most recent call last):
  File "saving.py", line 7, in <module>
    total += sys.argv[1]
TypeError: unsupported operand type(s) for +=: 'int' and 'str'

Command-line arguments are often an issue because all command-line arguments come into Python as strings.
So the sys.argv list in that saving.py program might look like this:

>>> sys.argv
['saving.py', '100']

Note: If command-line arguments are the issue, you may want to look into using argparse to parse your command-line arguments into the correct types.

This numbers-represented-as-strings issue can also occur when prompting the user with the built-in input function.

>>> total = 0
>>> user_input = input("Also add: ")
Also add: 100
>>> total += user_input
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +=: 'int' and 'str'

The input function stores everything the user entered into a string:

Regardless of how it happened, if you want to convert your string which represents a number to an actual number, you can use either the built-in float function or the built-in int function:

>>> user_input
'100'
>>> deposit = float(user_input)
>>> deposit
100.0
>>> deposit = int(user_input)
>>> deposit
100

The float function will accept numbers with a decimal point.
The int function will only accept integers (numbers without a decimal point).

Remember: type conversions usually need to be explicit

That can only concatenate str (not "int") to str error message shows up frequently in Python because Python does not have type coercion.

Whenever you have a string and a non-string and you’d like to either sum them with + or concatenate them with +, you’ll need to explicitly convert your object to a different type.

All data that comes from outside of your Python process starts as binary data (bytes) and Python typically converts that data to strings.
Whether you’re reading command-line arguments, reading from a file, or prompting a user for input, you’ll likely end up with strings.

If your data represents something deeper than a string, you’ll need to convert those strings to numbers (or whatever type you need):

>>> c = a + int(b)
>>> a, b, c
(3, '4', 7)

Likewise, if you have non-strings (whether numbers, lists, or pretty much any other object) and you’d like to concatenate those objects with strings, you’ll need to convert those non-strings to strings:

>>> year = 3000
>>> message = "Welcome to the year " + str(year) + "!"
>>> print(message)
Welcome to the year 3000!

Though remember that f-strings might be a better option than concatenation:

>>> year = 3000
>>> message = f"Welcome to the year {year}!"
>>> print(message)
Welcome to the year 3000!

And remember that friendly string conversions sometimes require a bit more than a str call (e.g. converting lists to strings).

✋ Update: This post was originally published on my blog decodingweb.dev, where you can read the latest version for a 💯 user experience. ~reza

TypeError: can only concatenate str (not “int”) to str” occurs if you try to concatenate a string with an integer (int object).

Here’s what the error looks like on Python 3.

Traceback (most recent call last):
File "/dwd/sandbox/test.py", line 6, in 
output = 'User: ' + user['name'] + ', Score: ' + user['score']
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~
TypeError: can only concatenate str (not "int") to str

Python 2.7 displays a slightly different error:

Traceback (most recent call last):
 File "test.py", line 6, in 
  output = 'User: ' + user['name'] + ', Score: ' + user['score']
TypeError: cannot concatenate 'str' and 'int' objects

But it occurs for the same reason: concatenating a string with an integer.

How to fix “TypeError: can only concatenate str (not “int”) to str”

If you need to use the + operator, check if the operands on either side are compatible. Remember: birds of a feather flock together 🦜 + 🦜

Python – as a strongly-typed programming language – doesn’t allow some operations on specific data types. For instance, it disallows adding 4 with '4' because one is an integer (4) and the other is a string containing a numeric character ('4').

This TypeError might happen in two scenarios:

1. Scenario #1: when concatenating a string with a numeric value
2. Scenario #2: when adding two or more numbers

Let’s explore the solutions.

⚠️ Scenario #1: when concatenating a string with a numeric value

Python syntax disallows the use of a string and a number with the addition (+) operator:

# ⛔ Raises TypeError: can only concatenate str (not “int”) to str
user = {
    'name': 'John',
    'score': 75
}

output = 'User: ' + user['name'] + ', Score: ' + user['score']
print(output)

In the above example, user['score'] contains a numeric value (75) and can’t be directly concatenated to the string.

Python provides various methods for inserting integer values into strings:

1. The str() function
2. Formatted string literals (a.k.a f-strings)
3. Printf-style formatting
4. Using print() with multiple arguments (ideal for debugging)

The str() function: This is the easiest (and probably the laziest) method of concatenating strings with integers. All you need to do is to convert the numbers into string values with the str() function:

user = {
    'name': 'John',
    'score': 75
}

print('User: ' + user['name'] +', Score: ' + str(user['score']))
# output: User: John, Score: 75

Although it’s easy to use, it might make your code harder to read if you have multiple numbers to convert.

Formatted string literals (a.k.a f-strings): f-strings provide a robust way of inserting integer values into string literals. You create an f-string by prefixing it with f or F and writing expressions inside curly braces ({}):

user = {
    'name': 'John',
    'score': 75
}

print(f'User: {user["name"]}, Score: {user["score"]}')
# output: User: John, Score: 75

Please note that f-string was added to Python from version 3.6. For older versions, check out the str.format() function.

Printf-style formatting: In the old string formatting (a.k.a printf-style string formatting), we use the % (modulo) operator to generate dynamic strings (string % values).

The string operand is a string literal containing one or more placeholders identified with %, while the values operand can be a single value or a tuple of values.

user = {
    'name': 'John',
    'score': 75
}

print('User: %s, Score: %s' % (user['name'], user['score']))
# output: User: John, Score: 75

When using the old-style formatting, check if your format string is valid. Otherwise, you’ll get another TypeError: not all arguments converted during string formatting.

Using print() with multiple arguments (ideal for debugging): If you only want to debug your code, you can pass the strings and the integers as separate arguments to the print() function.

All the positional arguments passed to the print() function are automatically converted to strings – like how str() works.

user = {
    'name': 'John',
    'score': 75
}

print('User:', user)
# output: User: {'name': 'John', 'score': 75}

As you can see, the print() function outputs the arguments separated by a space. You can change the separator via the sep keyword argument.

⚠️ Scenario #2: when adding two or more numbers

This error doesn’t only happen when you intend to concatenate strings. It can also occur when you try to add two or more numbers, but it turned out one of them was a string. For instance, you get a value from a third-party API or even the input() function:

devices = input('How many iphones do you want to order:  ')
unit_price = 900

# total cost with shipping (25)
total_cost = devices * unit_price + 25

The above code is a super-simplified program that takes iPhone pre-orders. It prompts the user to input the number of devices they need and outputs the total cost – including shipping.

But if you run the code, it raises TypeError: can only concatenate str (not “int”) to str.

The reason is the input() reads an input line and converts it to a string. And once we try to add the input value (which is now a string) to 25, we get a TypeError.

The reason is Python’s interpreter assumes we’re trying to concatenate strings.

To fix the issue, you need to ensure all operands are numbers. And for those that aren’t, you can cast them to an integer with the int() function:

devices = input('Number of iphones: ')
unit_price = 900

# total cost with shipping (25)
total_cost = int(devices) * unit_price + 25

It should solve the problem.

Alright, I think that does it! I hope this quick guide helped you fix your problem.

Thanks for reading!

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