Database connection lifecycle

Database connection lifecycle is the focus of this chapter in the Flask learning path. Before writing a large program, practice this one idea with a small input and a visible result. This chapter connects the idea to the code patterns used in Flask applications.

A function packages a calculation behind a descriptive name. You send it an argument and receive a return value. A function makes an operation reusable and testable; rounding helps display money-like values.

• Database connection lifecycle
• Key vocabulary and inputs
• Reading the output step by step
• Predicting edge cases
• Applying this skill in a small project

The important learning habit is to connect a technical word with an observable behavior, not simply memorize its definition. Make one prediction before pressing Run.

Identify the input, multiplication, return expression and printed result in that order.

1. Line 1 def add_tax(amount): Read this instruction before running the next line. Watch what it assigns, checks or prints.
2. Line 2 return round(amount * 1.13, 2) Read this instruction before running the next line. Watch what it assigns, checks or prints.
3. Line 3 subtotal = 28 Read this instruction before running the next line. Watch what it assigns, checks or prints.
4. Line 4 print(f"Subtotal: {subtotal}") Read this instruction before running the next line. Watch what it assigns, checks or prints.
5. Line 5 print(f"With tax: {add_tax(subtotal)}") Read this instruction before running the next line. Watch what it assigns, checks or prints.

Try it: Change the numeric input in line one or two. Say aloud which output line must change, and why, before you rerun.

Learning sequence note: the executable examples reinforce core programming patterns. The named advanced topic also needs a complete integration project, which is outside this demonstration.

Example 1: Database connection lifecycle — experiment 1

def add_tax(amount):
    return round(amount * 1.13, 2)
subtotal = 28
print(f"Subtotal: {subtotal}")
print(f"With tax: {add_tax(subtotal)}")

Subtotal: 28
With tax: 31.64

Why this output? A function packages a calculation behind a descriptive name. You send it an argument and receive a return value. Identify the input, multiplication, return expression and printed result in that order. Change one input value, rerun the program and compare the result before moving on.

Example 2: Database connection lifecycle — experiment 2

def add_tax(amount):
    return round(amount * 1.13, 2)
subtotal = 35
print(f"Subtotal: {subtotal}")
print(f"With tax: {add_tax(subtotal)}")

Subtotal: 35
With tax: 39.55

Why this output? A function packages a calculation behind a descriptive name. You send it an argument and receive a return value. Identify the input, multiplication, return expression and printed result in that order. Change one input value, rerun the program and compare the result before moving on.

Example 3: Database connection lifecycle — experiment 3

def add_tax(amount):
    return round(amount * 1.13, 2)
subtotal = 42
print(f"Subtotal: {subtotal}")
print(f"With tax: {add_tax(subtotal)}")

Subtotal: 42
With tax: 47.46

Why this output? A function packages a calculation behind a descriptive name. You send it an argument and receive a return value. Identify the input, multiplication, return expression and printed result in that order. Change one input value, rerun the program and compare the result before moving on.

1. Predict: What is printed when you run the first example without changing anything? Compare every character against the output panel.
2. Modify: Change the input number. Does the program compute or display a different value? Explain why.
3. Explain: Which statement produces the visible output, and which statement stores or transforms the input?
4. Debug: Intentionally remove a bracket or quote. Read the error and restore the missing character.

Worked answer

The final example is a complete, runnable model solution for this level. Read the input, execute the operation and check the output: no line should appear by accident.

def add_tax(amount):
    return round(amount * 1.13, 2)
subtotal = 42
print(f"Subtotal: {subtotal}")
print(f"With tax: {add_tax(subtotal)}")

Subtotal: 42
With tax: 47.46

Mini-project: add the core idea from Database connection lifecycle to a small Flask learning application. Name the input, validate it, transform it, and print a result. Keep the behavior simple enough to test by hand.

For framework-specific production examples and prerequisite setup, see the course overview and bundled starter project. The locally run examples above demonstrate underlying programming concepts, not every advanced feature named by this chapter.