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📘 Python Beginner Course · Lesson 4 of 12

One variable holds one value. Real data has many: all regions, every product’s price. Python’s two workhorses for this are lists (an ordered column of values) and dictionaries (a lookup table of key → value).

Lists

regions = ["North", "South", "East", "West"]
regions[0]          # 'North'  (counting starts at 0)
regions[-1]         # 'West'   (negative counts from the end)
regions[1:3]        # ['South', 'East']  (slice: from 1 up to, not including, 3)
len(regions)        # 4
regions.append("Central")
sorted(regions)     # a sorted copy; regions.sort() sorts in place
"East" in regions   # True

Dictionaries

prices = {"Laptop": 42000, "Monitor": 12800, "Printer": 9500}
prices["Monitor"]                 # 12800
prices["Router"] = 3400           # add or change
prices.get("Mouse", "not in list")  # safe lookup with a default
for name, price in prices.items():
    print(name, price)

A dictionary is VLOOKUP built in: prices[code] instead of =VLOOKUP(code, table, 2, FALSE). prices["Mouse"] raises a KeyError when missing; .get() doesn’t.

Nesting

order = {"id": "SO-1001", "customer": "Iyer Stores",
         "items": [("Laptop", 2), ("Router", 3)]}
total = sum(prices[p] * q for p, q in order["items"])

Lists of dictionaries are how JSON data from websites and APIs looks.

Tuples and sets, briefly

• Tuple ("Laptop", 2): like a list, but can’t be changed. Good for fixed pairs.
• Set {"North", "South"}: unique values, no order. set(list_with_repeats) removes duplicates.

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.