TypeError: 'str' object does not support item assignment

TypeError: 'str' object does not support item assignment #

The Python "TypeError: 'str' object does not support item assignment" occurs when we try to modify a character in a string. Strings are immutable in Python, so we have to convert the string to a list, replace the list item and join the list elements into a string.

Here is an example of how the error occurs.

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my_str = 'abc'

# ⛔️ TypeError: 'str' object does not support item assignment
my_str[0] = 'z'

We tried to change a specific character of a string which caused the error.

One way to replace a character at a specific index in a string is to:

1. Convert the string to a list.
2. Update the list item at the specific index.
3. Join the list items into a string.

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my_str = 'abc'

my_list = list(my_str)
print(my_list)  # 👉️ ['a', 'b', 'c']

my_list[0] = 'z'

new_str = ''.join(my_list)
print(new_str)  # 👉️ 'zbc'

We passed the string to the list() class to get a list where each character of the string is a list item.

The last step is to join the list items into a string with an empty string separator.

The str.join method takes an iterable as an argument and returns a string which is the concatenation of the strings in the iterable.

An alternative approach is to use string slicing.

Here is an example that replaces an underscore at a specific index with a space.

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my_str = 'hello_world'

idx = my_str.index('_')
print(idx)  # 👉️ 5

my_new_str = my_str[0:idx] + ' ' + my_str[idx+1:]
print(my_new_str)  # 👉️ "hello world"

The first piece of the string we need is up to, but not including the character we want to replace.

So, my_str[0:idx] is a slice of the string that starts at index 0 and goes up to, but not including idx.

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my_str = 'hello_world'

print(my_str[0:5]) # 👉️ "hello"

The next step is to use the addition + operator to add the replacement string (in our case - a space).

The last step is to concatenate the rest of the string. Notice that we start the slice at idx + 1 because we want to omit the character we are replacing.

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my_str = 'hello_world'

print(my_str[5 + 1:])  # 👉️ "world"

We don't specify an end index after the colon. This means - go til the end of the string.

We simply construct a new string excluding the character at the specific index and providing a replacement string.

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my_str = 'hello_world'

idx = my_str.index('_')
print(idx)  # 👉️ 5

my_new_str = my_str[0:idx] + ' ' + my_str[idx+1:]
print(my_new_str)  # "hello world"

Another alternative is to use the replace() method.

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my_str = 'apple, banana, apple'

my_new_str = my_str.replace('apple', 'melon')

print(my_new_str)  # 👉️ "melon, banana, melon"

The str.replace method returns a copy of the string with all occurrences of a substring replaced by the provided replacement.

The method takes the following parameters:

The difference with the previous two approaches is that the replace() method takes the substring we want to replace as a parameter (not an index).

The method also replaces all occurrences of the substring.