Performing Operations on Variables

Seeing Stored Values

When a program does not behave as expected, one of the first questions to ask is: what value does this variable hold right now? Python's print() function gives you a direct view of a variable's value. If n holds the integer 17, print(n) displays 17 on the console. If pi holds 3.141592653589793, print(pi) displays that decimal value.

n = 17
pi = 3.141592653589793

print(n)
print(pi)

17
3.141592653589793

Value and Type

A variable's value and its type describe different aspects of what the variable holds. The value is the data you can display, such as 17, 3.141592653589793, or the text stored in message. The type classifies that value. Python uses classifications such as int for an integer, str for a string, and float for a floating-point number.

Two values can look similar while having different types. The integer 5 and the text '5' are fundamentally different: one is an int and the other is a str. This distinction matters when a program performs operations. Two integers can be added together, but an integer and a string cannot be added in the same way.

Inspecting Types

Use type(variable) to check the data type of a variable. The type() function takes a variable as input and returns the type classification of the value it holds. This lets you confirm whether a value is an int, str, float, or another type supported by Python.

message = 'Hello'
n = 17
pi = 3.141592653589793

print(type(message))
print(type(n))
print(type(pi))

<class 'str'>
<class 'int'>
<class 'float'>

The type() call produces the type information. In this example, print() is used around each type() call so that the returned type information is displayed on the console. The angle brackets and the word class are Python's formal notation for identifying a type.

Display or Inspect

Suppose a variable contains something that looks like the number 5. print(variable) helps you see the value. type(variable), often displayed with print(type(variable)), helps you determine whether Python treats that value as an integer or as text. The first operation checks what the variable contains; the second checks how Python classifies what it contains.

number = 5
text = '5'

print(number)
print(text)
print(type(number))
print(type(text))

5
5
<class 'int'>
<class 'str'>

Debugging Checks

Use both operations when debugging a program. First, display a variable's value to verify what the program currently holds. Then inspect its type when you need to understand which operations Python can perform on that value or why the program is behaving unexpectedly.

• Using print(variable) when you need to know the type

  The displayed value does not identify whether Python classifies it as int or str.

  Fix: Use type(variable), and use print(type(variable)) when you want the type information displayed on the console.

• Assuming similar-looking values have the same type

  The first is an integer and the second is a string, so Python does not treat them the same way in operations.

  Fix: Inspect the type whenever the intended operation depends on whether a value is numeric or textual.

• Ignoring type information during debugging

  The value may look reasonable even when its type is not the type you expected.

  Fix: Display the value and check its type as separate verification steps.

Practice Check

Key Takeaways

1. Use print(variable) to display the value held by a variable on the console.
2. Use type(variable) to inspect the type classification of the value.
3. A variable's type affects how Python interprets its value and what operations can be performed on it.
4. Values that look similar, such as 5 and '5', can have different types and different behavior.
5. Displaying values and checking types are fundamental tools for debugging and verifying a program.