Summary
This chapter explains that strings are immutable and cannot be modified in place, and shows how to produce new string instances using methods like replace, concatenation, and other built-in operations.

General domain of usage
text processing

You may recall something about mutable and immutable data types.  **Immutable cannot be changed** .

Watch this video to further explore why strings are immutable in Python and how methods like replace() allow you to work with strings effectively. You'll see practical demonstrations of string immutability and learn strategies for modifying string data using built-in methods.

But what about strings? They are immutable. We cannot change a string directly, only with special operations. We will not have knowledge of the outcome unless we make an attempt. Therefore, let us examine this example.

string = "What does it really take to be first past the post?"

string[0] = 'k'

It is difficult to disagree with the assertion that modifying strings is essential. Just as individuals can alter their viewpoints, we should permit strings to undergo similar changes.

The `replace()` method was implemented to solve this problem. It has the following syntax: 

```python
string.replace(old_word, new_word)
```
By the way, each method that can be implemented for string just creates new string because the current one cannot be modified. Look at the example in which I want to **change seafood to fish**:

string = "I would like to order seafood"
# I changed my mind and want to order fish now
new_string = string.replace('seafood','fish')

# The previous order
print(string)
# The current order
print(new_string)

import unittest


class TestUserCode(unittest.TestCase):
    def _test_variable_equals_string(self, var_name, expected):
        import user_code
        if not hasattr(user_code, var_name):
            return (False, f"Expected '{var_name}' variable to be defined.")
        var = getattr(user_code, var_name)
        if var != expected:
            return (False, f"Expected '{var_name}' to equal '{expected}', but got '{var}'.")
        return (True, f'The variable {var_name} equals "{expected}"')

    def test_new_string1(self):
        passed, result_text = self._test_variable_equals_string(
            'new_string1', 'Paris is the capital of France')
        self._testMethodName = result_text
        self.assertTrue(passed, result_text)

    def test_new_string2(self):
        passed, result_text = self._test_variable_equals_string(
            'new_string2', 'Brasilia is located in South America')
        self._testMethodName = result_text
        self.assertTrue(passed, result_text)

    def test_new_string3(self):
        passed, result_text = self._test_variable_equals_string(
            'new_string3', 'Monkeys eat bananas')
        self._testMethodName = result_text
        self.assertTrue(passed, result_text)


if __name__ == '__main__':
    unittest.main()


test_main.py

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