Python Intermediate Lesson 2: Comprehensions, Generators and Sorting

📎 This article includes 3 downloadable practice files ↓

📘 Python Intermediate Course · Lesson 2 of 10

Most loops that build a list or count something can be one readable line.

reps = {r["rep"] for r in rows}                                  # set: unique values
by_rep = {rep: sum(1 for r in rows if r["rep"] == rep) for rep in sorted(reps)}   # dict
qty = sum(int(r["qty"]) for r in rows if r["qty"])              # generator: no list in memory
top = sorted(by_rep.items(), key=lambda kv: kv[1], reverse=True)[:2]
for i, (rep, n) in enumerate(top, start=1):
    print(f"{i}. {rep:<7} {n:>3}")

Generators: one item at a time

A comprehension in round brackets, or a function with yield, produces values on demand. Summing a 2 GB log file line by line uses almost no memory:

def big_amounts(path):
    for line in open(path):
        amt = float(line.split(",")[3])
        if amt > 100000:
            yield amt
print(sum(big_amounts("ledger.csv")))

Common mistakes

• Nested comprehensions three levels deep: write a normal loop; readability wins.
• Reusing a generator: it’s empty after one pass. Make a list if you need it twice.

Practice

Unzip the practice data into a folder, put this lesson’s script in the same folder and run it (python pyi-NN.py). Compare with the expected output file. Then change one thing (a filter, a column, a format) and predict the new output before you run it.