How to fix “TypeError: can only concatenate str (not “list”) to str” in Python

The Python error “TypeError: can only concatenate str (not “list”) to str” occurs if you concatenate a string with a list. 

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

 File "/dwd/sandbox/test.py", line 3, in 
    print('Resources: ' + books)
          ~~~~~~~~~~~~~~^~~~~~~
TypeError: can only concatenate str (not "list") to str

On Python 2.7, the error is slightly different, though: 

Traceback (most recent call last):
  File "test.py", line 3, in 
    print('Resources: ' + books)
TypeError: cannot concatenate 'str' and 'list' objects

But it occurs for the same reason.

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Why does it happen?

As you probably know, programming languages are either strongly typed or loosely typed.

Strongly-typed programming languages such as Python have a strict type system to help the programmer avoid data-type-related mistakes - meaning some operations aren't allowed on some data types. For instance, you can't divide 20 by '4' (because '4' is a string value) - depending on the operation, the error messages might vary.

Whenever you declare a variable like name = 'John', as a dynamically-typed language, Python determines the data type to be a string. 

Now, if you try to concatenate it with a list, you'll get the "TypeError: can only concatenate str (not "list") to str" error.

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How to fix it?

Fixing type errors such as this one is easy. If you need to use the + operator, check if the operands on either side have the same type. Remember: birds of a feather flock together 🦜 + 🦜

But if you want to insert the values into a string, there are multiple ways of doing it:

1. Convert the list into a string value with the str.join() method
2. Access list items individually
3. Use print() with multiple arguments - ideal for debugging
4. Use an f-string
5. Use printf-style formatting

Let's explore each method.

1. Convert the list into a string with the str.join() method: If you want to concatenate the items of a list to a string value, use str.join() like so:

books = ['Fluent Python', 'Head First Python']
output = 'Resources: ' + ', '.join(books)

print(output)
# output: Resources: Fluent Python, Head First Python

In the above code, we separate the list items by a comma. Please note we call the join method on the separator string (in this case: ', '). It seems weird, but that's how it works!

2. Access list items individually: Sometimes, you need to concatenate a single list item with a string value, but you use the whole list object by mistake.

In that case, you need to access the item by its index:

books = ['Fluent Python', 'Head First Python']
output = 'Top Pick: ' + books[1]

print(output)
# output: Top Pick: Head First Python

3. Use print() with multiple arguments - ideal for debugging: If you're concatenating a string with a list and readability isn't a concern, you can pass the string and the list 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.

books = ['Fluent Python', 'Head First Python']

print('Fetched these books:', books)
# output: Fetched these books: ['Fluent Python', 'Head First Python']

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

4. Use an f-string: Formatted string literals (a.k.a f-strings) are a robust way of formatting strings because they allow you to use Python expressions directly in string values (in a pair of curly brackets {}). 

You create an f-string by prefixing it with f or F and writing expressions inside curly braces:

books = ['Fluent Python', 'Head First Python']

print(f'Fetched these books: {books}')
# output: Fetched these books: ['Fluent Python', 'Head First Python']

Additionally, you can join the list items before using it in the string:

books = ['Fluent Python', 'Head First Python']
book_list = ', '.join(books)

print(f'Fetched these books: {book_list}')
# output: Fetched these books: Fluent Python, Head First Python

5. Use 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.

books = ['Fluent Python', 'Head First Python']

print('Fetched these books: %s' % books)
# output: Fetched these books: ['Fluent Python', 'Head First Python']

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

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

Thanks for reading!

Reza Lavarian Hey 👋 I'm a software engineer, an author, and an open-source contributor. I enjoy helping people (including myself) decode the complex side of technology. I share my findings on Twitter: @rezalavarian