In Python, a variable is a case-sensitive name that can hold a value. A variable can hold all kinds of values such as numbers, strings, etc. Below is an example of a variable storing a number.

```
>>> price = 200
>>> print(price)
200
```

The above code creates a variable called `price`

and assigns the integer value `200`

to it. We can ask Python to tell us what is stored in the variable `price`

using the`print`

statement and it will return the number `200`

.

We can change the value stored in the variable by just reassigning a new value to it.

```
>>> price = 200
>>> print(price)
200
>>> price = 300
>>> print(price)
300
```

### Variables in Calculations

We can use variables in calculations instead of directly using values. Let’s take an example. Let’s say you have $100 in savings and you put it in a bank account that pays a simple interest of 6% per year. We can create a formula in Python that can help us calculate the simple interest you will earn on your savings.

```
p = 100
r = 6
t = 2
I = p *(r/100) * t
print(I)
```

- p is the principal amount, $100.00.
- r is the interest rate, 6% per year, or in decimal form, 6/100=0.06.
- t is the time involved, 2 years time periods.

Save this script in your folder with the name `simpleinterest.py`

. Run the script on the command prompt using `python simpleinterest.py`

It will produce the following result:

```
C:\pythondatascience>python simpleinterest.py
12.0
```

## Common Data Types

The most common data types in Python are:

`int`

, or integer. These are positive or negative whole numbers with no decimal point`float`

, or floating point. They are real numbers and are written with a decimal point between the integer and fractional parts.`str`

, or string. A string holds any combination of letters and numbers you would like it to hold. Use double or single quotes to build strings.`bool`

, or boolen. Python provides the boolean type that can be either set to False or True.

Below are a few examples:

```
>>> course = "Financial Data Science" # String example
>>> print (course)
Financial Data Science
>>> age = 7.5 # Float example
>>> print(age)
7.5
>>> score = 103 # Integer example
>>> print(score)
103
>>> valuable = False # Boolen example (True or False)
>>> print(valuable)
False
```

You can check that these variables have the correct data type using the `type()`

function.

```
>>> type(course)
<class 'str'>
>>> type(age)
<class 'float'>
>>> type(score)
<class 'int'>
>>> type(valuable)
<class 'bool'>
>>>
```

## Operations on Data Types

We have already seen that we can perform math operations on integer variable. Python behaves differently when you perform these operations on other data types.

**Adding Two Strings**

If you try to add two strings, Python will concatenate the two strings as shown below:

```
>>> the_text = "The " + "Quick " + "Brown " + "Fox"
>>> print(the_text)
The Quick Brown Fox
```

**Adding Two Boolens**

Internally, Python treats `True`

as 1 and `False`

as 0. So, if you perform any mathematical calculation on boolens, it will do the operation on their numeric counterpart.

```
>>>True + True # Think 1 + 1
2
>>>True + False # Think 1 + 0
1
>>>False + False # Think 0 + 0
0
```

For Boolen variables, only the words `True`

and `False`

are recognized. Other variations, such as `TRUE`

, `true`

, or `T`

are incorrect.

**Adding Variables of Different Types**

What if we try to add variables of different types, for example, string and integer?

```
>>> text = "My total portfolio value is: "
>>> value = 5000
print(text+value)
```

As you can guess, this is not possible and will throw an error. If we want to add these two things, we can convert `value`

to string type using the `str()`

function as shown below:

```
>>> text+str(value)
'My total portfolio value is: 5000'
>>>
```

You can use the functions str(), int(), float() and bool() to convert Python values into any type.

## Exercise

- Create the simple interest formula as we did in the above example, and present the results in a nicely formatted way, e.g., “I will earn an interest of 12”.
- Create a
`boolen`

variable with value`False`

. Then convert it into a string, integer, and float and print the results. Notice the difference in the formats.