Reading and writing files

Data engineers move files all day: CSV exports from vendors, JSON from APIs. Open files with with open(...) so they're closed automatically, even if something fails.

python

import csv

with open("orders.csv") as f:

    reader = csv.DictReader(f)   # each row becomes a dict

    rows = list(reader)

print(rows[0])

OUTPUT

{'id': '1042', 'amount': '499', 'city': 'Pune'}

Every value comes back as a string. Convert types (section 1.2.2) before doing maths.

python

import json

with open("orders.json", "w") as f:

    json.dump(rows, f, indent=2)

with open("orders.json") as f:

    data = json.load(f)

Mode	Meaning
`"r"`	Read (the default)
`"w"`	Write, replacing what was there
`"a"`	Append to the end