Refactoring & Decomposition

Refactoring is improving code without changing what it does. The most common beginner refactor: copy-pasted code → functions.

Before:

print("Welcome Minh, your balance is 120")
print("Welcome Lan, your balance is 90")
print("Welcome Bo, your balance is 0")

After:

def welcome(name, balance):
    print(f"Welcome {name}, your balance is {balance}")

for who, amount in [("Minh", 120), ("Lan", 90), ("Bo", 0)]:
    welcome(who, amount)

Same output — but now one place to change the message, and a name that says what the line does.

How to spot refactor targets

• The same 2+ lines appear more than once → extract a function.
• A block does one identifiable job → give that job a name.
• A condition so long it needs a comment → name it (is_valid_order(order)).

Decomposition in practice

Build programs as pipelines of small functions:

def read_input(): ...
def process(data): ...
def format_output(result): ...
def main():
    data = read_input()
    print(format_output(process(data)))

Each function is testable alone; main reads like the problem statement. That is the shape every remaining project in this course uses.