Day 6: Files, CSV, JSON, Errors & Modules

with open("notes.txt", "w", encoding="utf-8") as f:     # w = write (new file)
    f.write("Day 6 – files and errors\n")
    f.write("Data science needs clean files.\n")
with open("notes.txt", "a", encoding="utf-8") as f:     # a = append
    f.write("Line added later.\n")
with open("notes.txt", encoding="utf-8") as f:          # default r = read
    for n, line in enumerate(f, 1):
        print(n, line.strip())

import csv
rows = [["bill_no", "item", "qty", "price"],
        [1, "Tea 1kg", 2, 520], [2, "Atta 10kg", 1, 459], [3, "Earbuds", 1, "1,499"],
        [4, "Pen Set", "two", 150], [5, "Bedsheet", 3, 699]]
with open("bills.csv", "w", newline="", encoding="utf-8") as f:
    csv.writer(f).writerows(rows)
print(open("bills.csv", encoding="utf-8").read())

import csv
total, bad = 0, []
with open("bills.csv", encoding="utf-8") as f:
    for row in csv.DictReader(f):                 # each row is a dict
        try:
            qty = int(row["qty"])
            price = float(row["price"].replace(",", ""))
            total += qty * price
        except ValueError:
            bad.append(row["bill_no"])
print("Total sales: ₹", total)
print("Rows skipped (bad data):", bad)

import json
payments = [{"txn": "T1", "mode": "UPI", "amount": 520.0, "ok": True},
            {"txn": "T2", "mode": "Card", "amount": 1499.0, "ok": False},
            {"txn": "T3", "mode": "UPI", "amount": 459.0, "ok": True}]
with open("payments.json", "w", encoding="utf-8") as f:
    json.dump(payments, f, indent=2)
with open("payments.json", encoding="utf-8") as f:
    data = json.load(f)
success = [p for p in data if p["ok"]]
print("Successful:", len(success), "| Amount: ₹", sum(p["amount"] for p in success))
print(json.dumps(data[1]))                 # dict -> JSON text

def safe_average(values):
    try:
        avg = sum(values) / len(values)
    except ZeroDivisionError:
        print("No values – returning 0")
        return 0
    except TypeError as e:
        print("Bad value in list:", e)
        return None
    else:
        return round(avg, 2)          # runs only if no error
    finally:
        print("checked", values)      # always runs

print(safe_average([10, 20, 35]))
print(safe_average([]))
print(safe_average([10, "x"]))

def check_age(age):
    if not 0 < age < 120:
        raise ValueError(f"Impossible age: {age}")
    return age

for a in [25, 150]:
    try:
        print("OK", check_age(a))
    except ValueError as e:
        print("Rejected ->", e)

import math, statistics as st, random
from datetime import date, datetime, timedelta
print(math.sqrt(144), math.ceil(4.2), math.floor(4.8), round(math.pi, 4))
marks = [56, 78, 91, 45, 88, 67, 78]
print("mean", round(st.mean(marks), 2), "| median", st.median(marks), "| mode", st.mode(marks))
random.seed(1)                                # same "random" result every run
print("Lucky draw:", random.choice(["Aman", "Priya", "Rahul", "Sneha"]), random.sample(range(1, 50), 3))
d = date(2024, 10, 31)
print(d.strftime("%d %b %Y, %A"), "| +45 days:", d + timedelta(days=45))
print(datetime.strptime("15/08/2024", "%d/%m/%Y").year)

import os
print("Current folder has CSV files:", sorted(f for f in os.listdir(".") if f.endswith(".csv"))[:4])
print("bills.csv exists:", os.path.exists("bills.csv"), "| size:", os.path.getsize("bills.csv"), "bytes")
os.makedirs("reports", exist_ok=True)
print(os.path.join("reports", "summary.txt"))

code = 'def gst(amount, rate=18):\n    return round(amount * rate / 100, 2)\n'
with open("mytools.py", "w") as f:       # normally you create this file in an editor
    f.write(code)
import mytools
print(mytools.gst(1499), mytools.gst(459, rate=5))

import csv, json
from datetime import date
summary = {"date": str(date(2024, 11, 1)), "bills": 0, "revenue": 0.0, "skipped": 0}
with open("bills.csv", encoding="utf-8") as f:
    for row in csv.DictReader(f):
        try:
            summary["revenue"] += int(row["qty"]) * float(row["price"].replace(",", ""))
            summary["bills"] += 1
        except ValueError:
            summary["skipped"] += 1
with open("reports/daily_summary.json", "w") as f:
    json.dump(summary, f, indent=2)
print(open("reports/daily_summary.json").read())