Python: Numbers, Dates & Times

What is the difference between strftime and strptime?

strftime (string format time) converts a datetime object to a formatted string: dt.strftime("%Y-%m-%d") produces "2026-03-25". strptime (string parse time) does the reverse and parses a string into a datetime: datetime.strptime("2026-03-25", "%Y-%m-%d"). Remember: f = format (to string), p = parse (from string).

Why does 0.1 + 0.2 not equal 0.3 in Python?

Floating-point numbers use binary (base-2) representation, and 0.1 cannot be exactly represented in binary, similar to how 1/3 cannot be exactly written in decimal. The result 0.30000000000000004 is the closest representable binary fraction. Use math.isclose() for comparisons, Decimal for exact arithmetic (especially money), or round() for display.

Should I use datetime.now() or datetime.utcnow()?

Neither! datetime.utcnow() is deprecated in Python 3.12+. Use datetime.now(timezone.utc) for timezone-aware UTC. For local time with timezone info, use datetime.now(ZoneInfo("America/New_York")). Always store and compute in UTC, convert to local time only for display.

How do I handle timezones in Python?

Use the zoneinfo module (Python 3.9+) with IANA timezone names. Create timezone-aware datetimes: datetime(2026, 1, 1, tzinfo=ZoneInfo("US/Eastern")). Convert with .astimezone(). Never mix naive (no timezone) and aware datetimes because it raises a TypeError.

What is the difference between round(), math.floor(), and math.ceil()?

round() uses banker's rounding (round-half-to-even): round(0.5)=0, round(1.5)=2. math.floor() always rounds down: floor(3.9)=3, floor(-3.1)=-4. math.ceil() always rounds up: ceil(3.1)=4, ceil(-3.9)=-3. int() truncates toward zero: int(3.9)=3, int(-3.9)=-3.