Python: your first practical programs

A variable gives a value a readable name. Use = to assign: price = 4. Numbers can be added as numbers; text must be written in quotes. print() shows a value so you can inspect it. Python distinguishes uppercase and lowercase, so price and Price are different names.

Run one small line at a time. If Python displays an error, read the line number and message; an error is information about what needs fixing, not a verdict on your ability.

An if statement chooses a path when a condition is true. Indentation marks which instructions belong to that path. A function packages a small task under a name; return sends its result back to the place where it was called.

def line_total(quantity, price):
    if quantity < 0 or price < 0:
        return None
    return quantity * price

print(line_total(3, 4))  # 12

Clear names and one job per function make mistakes easier to find.

A list keeps several values in order: sales = [12, 10, 15]. A for loop works through each value once. Use an accumulator when you want a total, starting at zero and adding each value. An empty list should naturally produce a total of zero.

total = 0
for amount in sales:
    total = total + amount
print(total)  # 37

Test one normal case, an empty list, and a list containing zero. Those small tests help reveal assumptions.

Combine the skills into a tiny report. Store sales as a list, calculate the total with a function, and print a clear result. Separate the work into small steps: create data, calculate, format output, then test. Do not worry about building a full application yet.

def total_sales(values):
    total = 0
    for value in values:
        total += value
    return total

sales = [12, 10, 15]
print(f"Total sales: ${total_sales(sales)}")

A test is a specific input and an expected result. Check ordinary values, an empty list, and a value that should not be accepted.