Contents

• Overview
• General Form
• Casting
• Casting Examples
• Data Validation

Overview

So far, the variables and data values we have seen have been hardcoded, that is, we've written the data values directly in our code. This is not a very common approach in programming. More often we acquire data from outside of our programs, for example, by reading a file, querying a database, or prompting the user to type in values. Prompting a user is called user input. In Python, to prompt a user for a value, we use the input() function.

The general form of the input() function is:

Here is a code example:

age = input("Plese enter your age: ")

The user would see Please enter your age: on the screen and the cursor would be positioned to the right of the prompt on the same line. When they type in a value and press Enter, their entry is assigned to our variable, age in this example.

It is important to be aware that anything the user types will be read by the input() function as a string (str) data type, even if they enter numbers. Consider the following code example:

age = input("Plese enter your age: ")
print(age)
print(type(age))

The following is an example run of this program and the output after a user enters a value:

Notice that the user entered 25 but when we print(type(age)) it informs us that the data type is a string. Remember that the digits 0 to 9 can be of string data type if surrounded by quotes or, in this case, if entered by a user through the input() function. The more likely intent is that we want the user's age as an integer, so we need to cast the entered string value into an integer value.

In programming we often need to convert values of one data type into another data type, this process is called casting. There are two types of casting: implicit and explicit. Implicit casting is automatically handled by the Python interpreter when needed, for example, when dividing two integers and the resulting value is a floating-point number. Explicit casting is when the programmer uses built-in functions to intentionally cast one data type to another.

Let's take a look at examples of implicit and explicit casting with some code.

Implicit Casting Example

Consider the following code:

x = 10
y = 3
z = x / y
print(z)
print(type(z))

We have not yet learned how to do math with Python (we'll get to that over the next few pages), but simple division is accomplished using the division symbol in Python (/). From simple mathematics though, we know that dividing 10 by 3 will not produce a whole number (integer), rather it will result in a decimal number (float). Python automatically sets the appropriate data type of our variable z because the result of the division is a floating-point number rather than an integer and this is implicit casting.

3.3333333333333335
class 'float'

Explicit Casting

In Python, we can explicitly cast using built-in functions such as int(), float() and str(). Each of these functions are used in the following general form:

Where expression is a value or operation that results in a value of a particular data type that you want to convert (cast) to another type.

Consider the following code example:

x = "999"
print(x)
print(type(x))
x = int(x)
print(x)
print(type(x))

And the output:

999
class 'str'
999
class 'int'

Based on the example above, answer the following question...

We will use casting frequently throughout our work in this book.

One possible problem we have with user input is, what if the user types in something we do not expect?

For example, reusing our input function code above:

age = input("Plese enter your age: ")
print(age)
print(type(age))

Say a user runs this program and enters something like this:

Please enter your age: blahblahblah

blahblahblah

class 'str'

That is an invalid entry, what do we do about it? For now, not much. We need to learn a few more basics of Python and then we will revisit this problem in detail. For now, just be aware that any time we collect data from outside of our program we will need to take steps to validate the data we are receiving is valid.