PART 1 · PYTHON FROM ZERO · LESSON 18

Files: read and write

FreeLesson 20 of 78

Data engineering starts with files. Reading, writing and appending text files needs only open() and a with block.

READING A FILEPYTHON 3.12

with open("orders.csv") as f:          # open for reading
    header = f.readline()               # first line
    first_row = f.readline()
print(repr(header))
print(first_row.strip().split(","))

with open("orders.csv") as f:
    lines = f.read().splitlines()       # the whole file
print(len(lines), "lines; last:", lines[-1])

Output

'order_id,customer_id,order_date,status,amount\n'
['101', '1', '2025-01-10', 'delivered', '1198.00']
16 lines; last: 115,5,2025-04-28,delivered,1598.00

LOOP OVER A FILE LINE BY LINEPYTHON 3.12

with open("orders.csv") as f:
    next(f)                              # skip the header
    delivered = 0
    for line in f:                       # one line at a time
        cols = line.strip().split(",")
        status, amount = cols[3], cols[4]
        if status == "delivered":
            delivered += float(amount)
print(f"delivered revenue: {delivered:,.2f}")

Output

delivered revenue: 18,972.00

Looping over f reads one line at a time, so this works on a file of any size. split(",") is fine for simple files; real CSVs with quoted commas need the csv module (page 33).

WRITING AND APPENDINGPYTHON 3.12

with open("report.txt", "w") as f:   # write (replace)
    f.write("date,revenue\n")
    f.write("2025-04-20,2397\n")

with open("report.txt", "a") as f:   # append
    f.write("2025-04-28,1598\n")

print(open("report.txt").read())

Output

date,revenue
2025-04-20,2397
2025-04-28,1598

Mode	Does	If the file exists	If it does not
`"r"`	Read (the default)	Reads it	FileNotFoundError
`"w"`	Write	Empties it first	Creates it
`"a"`	Append	Adds to the end	Creates it
`"x"`	Create	FileExistsError	Creates it

PATHS AND FOLDERS WITH PATHLIBPYTHON 3.12

from pathlib import Path

out_dir = Path("reports") / "2025-04"          # / joins path parts
out_dir.mkdir(parents=True, exist_ok=True)      # make folders
(out_dir / "summary.txt").write_text("orders,15\n")

print(Path("orders.csv").exists(), Path("missing.csv").exists())
print(sorted(p.name for p in Path(".").glob("*.json*")))
print([str(p) for p in Path("reports").rglob("*.txt")])

Output

True False
['customers.json', 'events.jsonl']
['reports/2025-04/summary.txt']

pathlib builds paths that work on Windows, Mac and Linux, so you never glue folder names with "/" or "\\" by hand. glob finds files by pattern; rglob searches sub-folders too.

Try it yourself before moving on

Count the lines in orders.csv without the header.
Write every cancelled order into cancelled.txt.
Append a line with today's total to report.txt without deleting what is there.