Files and Data Formats Checkpoint

Optional

25 min · Beginner

A review of this module. Skip it if you feel confident; the next module does not need it.

Fix the mistakes

The slips that cost marks in this module. Spot each one, then check the fix.

Test what you know about text files, paths, CSV and JSON. Every program creates the file it reads, so each one runs as a single file.

Pick the right tool

Plain text suits notes and logs. CSV suits flat tables. JSON suits nested data. Use with for every file, and name the encoding.

Python

1import csv2import json3
4with open("notes.txt", "w", encoding="utf-8") as f:5    f.write("first\nsecond\n")6with open("t.csv", "w", newline="") as f:7    csv.writer(f).writerow(["Asha", 82])8with open("d.json", "w") as f:9    json.dump({"name": "Asha"}, f)10with open("notes.txt", encoding="utf-8") as f:11    print(f.read().splitlines())12with open("t.csv", newline="") as f:13    print(next(csv.reader(f)))14with open("d.json") as f:15    print(json.load(f))

Example 1 · Text, CSV and JSON side by side

Python

1import csv2import json3
4with open("t.csv", "w", newline="") as f:5    csv.writer(f).writerow(["Asha", 82])6with open("t.csv", newline="") as f:7    row = next(csv.reader(f))8print(row)9print(json.dumps({"name": row[0], "mark": int(row[1])}))

Output

['Asha', '82']
{"name": "Asha", "mark": 82}

What this code does

The CSV value '82' is text, so int converts it.
The JSON text shows the dictionary built from the row.

Each format gives a different Python type back: a list of lines, a list of strings for a CSV row, and a dictionary for JSON. Notice the CSV mark comes back as the string '82'.

Read, change, write back

A common job is to load data, change it and save it. Read the whole file first, then open it again for writing, so a crash in between cannot leave a half-written file.

Python

1import json2
3with open("cart.json", "w") as f:4    json.dump({"items": ["pen"]}, f)5
6with open("cart.json") as f:7    cart = json.load(f)8cart["items"].append("book")9with open("cart.json", "w") as f:10    json.dump(cart, f)11
12with open("cart.json") as f:13    print(json.load(f)["items"])

Example 2 · Update a JSON file

Python

1import json2
3with open("cart.json", "w") as f:4    json.dump({"items": ["pen"]}, f)5with open("cart.json") as f:6    cart = json.load(f)7cart["items"].append("book")8with open("cart.json", "w") as f:9    json.dump(cart, f)10with open("cart.json") as f:11    print(json.load(f))

Output

{'items': ['pen', 'book']}

What this code does

The file is read fully before it is opened again for writing.
The saved list now holds two items.

The first open for reading finished before the second open erased the file with "w". Doing both at once with "w" would delete the data before it was read.

A missing or broken file

Files can be missing and text can be invalid. Catch FileNotFoundError and json.JSONDecodeError separately, so each problem gets its own clear answer.

Python

1import json2
3def load(name):4    try:5        with open(name) as f:6            return json.load(f)7    except FileNotFoundError:8        return "missing"9    except json.JSONDecodeError:10        return "broken"11
12with open("bad.json", "w") as f:13    f.write("{oops")14print(load("none.json"))15print(load("bad.json"))

Example 3 · Friendly answers for bad files

Python

1import json2
3def load(name):4    try:5        with open(name) as f:6            return json.load(f)7    except FileNotFoundError:8        return "missing"9    except json.JSONDecodeError:10        return "broken"11
12with open("bad.json", "w") as f:13    f.write("{oops")14print(load("none.json"))15print(load("bad.json"))

Output

missing
broken

What this code does

A missing file and a broken file raise different errors.
Each error gets its own answer.

Each except handles one kind of failure. The first call has no file. The second finds a file, but its text is not valid JSON.

Finish up

Mark complete & continue

Next: Must-do problems: Files and Data Formats

1import csv2import json3 4with open("notes.txt", "w", encoding="utf-8") as f:5 f.write("first\nsecond\n")6with open("t.csv", "w", newline="") as f:7 csv.writer(f).writerow(["Asha", 82])8with open("d.json", "w") as f:9 json.dump({"name": "Asha"}, f)10with open("notes.txt", encoding="utf-8") as f:11 print(f.read().splitlines())12with open("t.csv", newline="") as f:13 print(next(csv.reader(f)))14with open("d.json") as f:15 print(json.load(f))

1import csv2import json3 4with open("t.csv", "w", newline="") as f:5 csv.writer(f).writerow(["Asha", 82])6with open("t.csv", newline="") as f:7 row = next(csv.reader(f))8print(row)9print(json.dumps({"name": row[0], "mark": int(row[1])}))

1import json2 3with open("cart.json", "w") as f:4 json.dump({"items": ["pen"]}, f)5 6with open("cart.json") as f:7 cart = json.load(f)8cart["items"].append("book")9with open("cart.json", "w") as f:10 json.dump(cart, f)11 12with open("cart.json") as f:13 print(json.load(f)["items"])

1import json2 3with open("cart.json", "w") as f:4 json.dump({"items": ["pen"]}, f)5with open("cart.json") as f:6 cart = json.load(f)7cart["items"].append("book")8with open("cart.json", "w") as f:9 json.dump(cart, f)10with open("cart.json") as f:11 print(json.load(f))

1import json2 3def load(name):4 try:5 with open(name) as f:6 return json.load(f)7 except FileNotFoundError:8 return "missing"9 except json.JSONDecodeError:10 return "broken"11 12with open("bad.json", "w") as f:13 f.write("{oops")14print(load("none.json"))15print(load("bad.json"))