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

But what about strings? They are immutable. We are not allowed to change a string directly, only with special operations. We won't know if we don't try; therefore, take a look at this example:


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

string[0] = 'k'

Output:
TypeError: 'str' object does not support item assignment
 ``` 


But it is hard not to agree with the statement that changing strings is necessary; everyone can change their opinion, so let's allow the string to do the same.

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


 ``` 
Input:
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)

Output:
I would like to order seafood
I would like to order fish
 ```


Numeric data types are crucial in Python, so it is important to start with them. Here you will work with simple math operations and find out the purposes of learning different types of numerical data.

The main goal of this section is to clarify the boolean data types; it's a sub type of numeric data, but indeed it is more challenging. Since it is always better to excel in the more difficult topics, this section is a valuable part of your curriculum.

Strings are a way for us to implement your phrases in Python, that is, to give instructions to users who are the target audience of your program. Indeed, it is always better to be able to work with strings because they should be used in your code. You'll understand why soon.

This chapter consists of practical tasks that combine all the examined topics together with a bit of theory.