Use datetime.strptime(text, format) when the string follows a known custom pattern, and datetime.fromisoformat(text) when it is an ISO 8601 timestamp. Both return a datetime object, and both raise ValueError if the input is malformed or doesn’t match. This guide covers when to pick each, the format codes you’ll actually need, and the traps (missing years, timezones, ambiguous strings) that produce wrong dates without any error. Details follow the Python 3.14 datetime documentation.
The two calls side by side
Custom format:
from datetime import datetime
value = datetime.strptime("2026-10-05 14:30", "%Y-%m-%d %H:%M")ISO timestamp with an offset:
from datetime import datetime
value = datetime.fromisoformat("2026-10-05T14:30:00+00:00")The first returns a naive datetime (no timezone); the second returns a timezone-aware one because the string carries an offset.
Choosing the parser
| Question | Use strptime() |
Use fromisoformat() |
|---|---|---|
| Input shape | Any known custom pattern, e.g. 05/10/2026 2:30 PM |
ISO 8601 forms Python supports |
| Needs a format string | Yes | No |
| Python version sensitivity | Low | High: before 3.11 it accepted only output of isoformat(); 3.11 broadened it |
| Timezone | Only if you include %z |
Aware if the string has Z or a numeric offset |
When to use strptime()
Each directive in the format string describes one component of the input, and the format must match the input’s field order and separators. If it can’t, you get a ValueError.
When to use fromisoformat()
The 3.14 docs show it handling date-only strings, compact dates, week dates, a T or space separator, a trailing Z, and numeric UTC offsets. Because acceptance widened in Python 3.11, code that relies on newer forms such as a trailing Z should state a minimum Python version.
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“ISO” does not mean every ISO 8601 form
The documented exceptions are reduced-precision dates (YYYY-MM and YYYY), extended signed-year forms, ordinal dates (YYYY-OOO), and fractional hours or minutes. For those, use strptime() with an explicit format or handle the string yourself.
Date-only or time-only data
If there is no time component, parse to a date; if there is no date, parse to a time. The standard library has separate parsing methods for date, datetime and time, so you don’t invent a midnight that wasn’t in the data.
Format codes you’ll use most
| Code | Meaning |
|---|---|
%Y |
Year with century (normally four digits) |
%y |
Year without century |
%m |
Month number |
%d |
Day of month |
%H:%M:%S |
24-hour time |
%I:%M %p |
12-hour time with AM/PM; %p affects the hour only when %I is used |
%f |
Fractional seconds as microseconds; strptime() accepts one to six digits and zero-pads on the right |
%z |
UTC offset; in strptime() a colon may separate offset parts, and Z is equivalent to +00:00 |
strptime() parses a string into an object; strftime() goes the other way and formats an object as a string. They are not interchangeable.
ISO week dates
%G is the ISO week-year, %V the ISO week number, and %u the ISO weekday. The ISO week-year doesn’t always equal the calendar year, so don’t swap %G for %Y. Incomplete or ambiguous combinations of these directives can raise ValueError.
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Pitfalls that produce wrong or failing results
1. Omitted fields take defaults
For datetime.strptime() and date.strptime(), anything you leave out comes from 1900-01-01T00:00:00.000. A format with no year therefore silently uses 1900.
2. February 29 without a year
1900 is not a leap year, so “02-29” fails to parse. Include a year. If you must parse partial month/day values that could be February 29, append a known leap year for parsing, then handle the result according to what your application means by that date. The 3.14 docs add that a day-of-month format without a year raises a DeprecationWarning, and such values may error as of Python 3.15.
3. Compact strings with no separators
The docs give %Y%m%d against 2026111 as an example that could be read as November 1 or January 11. Use separators, or fixed-width zero-padded input, so there’s only one reading.
4. Timezone meaning
A numeric offset or Z yields an aware datetime; a string without one yields a naive datetime, which doesn’t identify an absolute moment. Keep or assign timezone meaning deliberately based on what your data source promises. Don’t silently treat local time as UTC.
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1Scan for outdated or missing drivers - takes under a minute2Clear out junk files and repair common Windows errors3Fix the driver behind crashes, sound loss and screen glitches5. Platform and locale variation
Python relies on the platform C library, so the set of supported format codes can differ between systems, and locale-dependent month and day names can vary. Don’t assume a localized name like a month abbreviation parses identically everywhere.
6. Error handling
Treat ValueError as an expected outcome for malformed, impossible, or mismatched input. Catch it where untrusted data enters your program:
Quick Recap
from datetime import datetime
def parse_timestamp(text):
try:
return datetime.fromisoformat(text)
except ValueError:
return None # or report the bad inputQuick decision guide
- Input is ISO 8601 and your Python is 3.11+:
fromisoformat(). - Input is ISO-like but on an older Python, or uses an unsupported ISO form:
strptime()with an explicit format. - Input is any other known pattern:
strptime(). - Input has no timezone information: you get a naive datetime, so decide what zone it represents.
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