PART 2 · PYTHON FOUNDATIONS · CHAPTER 2

Numbers, strings and dates that bite

FreeLesson 25 of 78

Money, text encodings and time zones break more pipelines than any algorithm. Each one has a single rule to remember.

FLOATS, DECIMAL, ROUNDINGPYTHON 3.12

from decimal import Decimal, ROUND_HALF_UP

print(0.1 + 0.2, 0.1 + 0.2 == 0.3)
print(Decimal("0.1") + Decimal("0.2"))
print(Decimal(0.1))             # from a float: too late

print(round(2.5), round(3.5))   # banker's rounding
price = Decimal("2.675")
print(price.quantize(Decimal("0.01"), ROUND_HALF_UP))

print(7 // 2, -7 // 2, int(-3.5))  # floor vs trunc

Output

0.30000000000000004 False
0.3
0.1000000000000000055511151231257827021181583404541015625
2 4
2.68
3 -4 -3

STRINGSPYTHON 3.12

status = "  DELIVERED \n"
print(repr(status.strip().lower()))

amount = 1198.5
print(f"{amount:,.2f} | {amount:>10.1f} | {0.256:.1%}")

parts = "101,1,2025-01-10,delivered,1198.00".split(",")
print(parts[3], "|".join(parts[:2]))

word = "café"
print(word, len(word), len(word.encode("utf-8")))

Output

'delivered'
1,198.50 |     1198.5 | 25.6%
delivered 101|1
café 4 5

DATES AND TIME ZONESPYTHON 3.12

from datetime import date, datetime, timedelta, timezone
from zoneinfo import ZoneInfo

d = datetime.strptime("2025-04-01 10:00", "%Y-%m-%d %H:%M")
print(d.isoformat(), d.tzinfo)                  # naive

ist = d.replace(tzinfo=ZoneInfo("Asia/Kolkata"))
print(ist.astimezone(timezone.utc).isoformat())

print(date(2025, 3, 31) + timedelta(days=1))
print((date(2025, 4, 28) - date(2025, 3, 12)).days)
print(d.strftime("%Y-%m"), d.weekday())         # Monday = 0

Output

2025-04-01T10:00:00 None
2025-04-01T04:30:00+00:00
2025-04-01
47
2025-04 1

Practice before you move on

Add up the amount column of orders.csv once as float and once as Decimal, and compare the two totals.
Parse every order_date into a date and print the weekday of the first and the last order.