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📘 Python Beginner Course · Lesson 5 of 12

In Excel you fill a formula down 500 rows. In Python you write a loop: “for each row, do this”. Loops are where Python starts saving real time.

for loops

marks = {"Aarav": 78, "Diya": 91, "Kabir": 45, "Meera": 66, "Rohan": 33}
passed = 0
for name, m in marks.items():
    result = "Pass" if m >= 40 else "Fail"
    if result == "Pass":
        passed += 1                # same as passed = passed + 1
    print(f"{name:<6} {m:>3}  {result}")
print("Passed:", passed, "of", len(marks))

The indented block runs once per item. {name:<6} pads text to 6 characters so columns line up.

range()

for month in range(1, 13):      # 1 to 12 (the end is excluded)
    print(month)

List comprehensions

squares = [n * n for n in range(1, 6)]                 # [1, 4, 9, 16, 25]
big = [m for m in marks.values() if m >= 75]           # filter
names = [n.upper() for n in marks]                      # transform

One line that builds a new list, like an Excel spill formula. Use it for simple transformations; switch to a normal loop when the logic grows.

while loops

balance, month = 100000, 0
while balance > 0:
    balance -= 9000
    month += 1

Repeats while the condition is true. Make sure something inside changes the condition, or it never stops (press Ctrl+C to break out).

enumerate and zip

for i, region in enumerate(["North", "South"], start=1):
    print(i, region)                         # 1 North, 2 South
for name, m in zip(["A", "B"], [70, 80]):    # walk two lists together
    print(name, m)

Where beginners go wrong

Practice

Download the script (and data file, if listed) below into one folder. Open a terminal in that folder and run python lesson-NN.py. Compare with the expected output, then change something and run it again: that’s how Python is learned.