Python 3 TypeError: must be str, not list

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If you’re getting a TypeError: must be str, not list error in Python 3, don’t worry! You’re not alone. This is a common error that many people encounter when they start using Python 3. In this blog post, we will discuss what causes this error and how to fix it. We will also provide some tips on how to avoid it in the future.

TypeError: must be str, not list is caused by trying to concatenate or compare a string with a list. This can happen when you’re using the + operator to concatenate strings, or when you’re using the == operator to compare two strings.

TypeError: must be str, not list

For example, take a look at the following code:

#python with wholeblogs

new_var = 'abc' + ['def']
print(new_var)
Traceback (most recent call last):

File "<stdin", line in, in module

TypeError: must be str, not list

As you can see, we get an error message telling us that we can’t concatenate a string and a list. The same thing if we try to compare two strings using the == operator:


#python with wholeblogs

if 'abc' == ['def']:
    print("Error")

Traceback (most recent call last):

File "<stdin", line in, in module

TypeError: must be str, not list

If you’re getting this error, it means that you’re probably trying to do something like this. So how do we fix it?

The easiest way to fix this error is to make sure that both sides of the equation are either strings or lists. For example, if we want to concatenate two strings, we can use the + operator:


#python with Wholeblogs

var_whole = 'abc' + 'def'
print(var_whole)
'abcdef'

Compare two lists

Or, if we want to compare two lists, we can use the == operator:

# python with wholeblogs

['abc'] == ['def']
print(['abc'])
False

If you’re not sure whether something is a string or a list, you can use the type() function to check:

#python with wholeblogs

type('abc')

str


type(['abc']) # Note that this is a list with one element!

list

Now that we know how to fix the TypeError: must be str, not list error, let’s talk about how to avoid it in the first place.

The best way to avoid this error is to be careful when using the + operator with strings. Remember that the + operator can be used to concatenate two strings, but it will also add two numbers together if both sides are numbers.

So, if you’re not careful, you might accidentally try to concatenate a string and a number:

#python with wholeblogs

var_wholeblogs  = 'abc' + 123
print(var_wholeblogs)

Traceback (most recent call last):

File “Conclusion

Hopefully, this blog post has helped you understand what causes the TypeError: must be str, not list error, and how to fix it. Remember to be careful when using the + operator with strings, and when using the == operator to compare strings. And if you’re ever not sure whether something is a string or a list, use the type() function to check. Read further: TypeError: Must be str, not tuple in Python?

Thanks for reading!

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