datetime, os & sys

The standard library and import

Which import style should I use?

• import os: you always write os.something, so it is clear where every tool comes from. Good for os and sys.
• from datetime import datetime, date: shorter to use, and avoids the datetime.datetime confusion. Good for this module.
• import datetime as dt: a short nickname. Use it when a name is long and you use it a lot.

Don't name your file after a module. If you save your program as datetime.py or random.py, Python finds your file first when you import, and the real module's tools seem to be missing. Pick a different file name.

Reading the clock: now() and today()

from datetime import datetime, date

now = datetime.now()
print(now)         # 2026-10-09 14:05:03.123456
print(now.year, now.month, now.day)
print(now.hour, now.minute, now.second)

today = date.today()
print(today)       # 2026-10-09

Using the attributes

Because now.hour and the others are ints, you can make decisions with them:

from datetime import datetime
now = datetime.now()
if now.hour < 12:
    print("Good morning!")
elif now.hour < 18:
    print("Good afternoon!")
else:
    print("Good evening!")

No parentheses on attributes: now.year is a stored value, not a function, so now.year() is a TypeError. But now(), today(), and weekday() are functions and need them.

Dates compare like numbers: date(2026, 10, 9) < date(2026, 12, 25) is True, because October 9 comes first.

Formatting dates with strftime()

Build a format one piece at a time

To show Friday, October 09, 2026, list each part and its code, then copy the punctuation between them exactly:

Friday   ,   October   09   ,   2026
  %A     ,     %B      %d   ,    %Y      →  "%A, %B %d, %Y"

Anything that is not a % code (commas, spaces, slashes, words like "at") appears in the result exactly as typed.

Going the other way: strptime()

strftime turns a date into text (f for format). strptime reads text into a date (p for parse), using the same codes:

d = datetime.strptime("10/09/2026", "%m/%d/%Y")
print(d)     # 2026-10-09 00:00:00

What day of the week? weekday()

days = ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"]
d = date(2026, 10, 9)
print(days[d.weekday()])    # Fri
print(d.strftime("%A"))     # Friday

Why Monday is 0

Python follows the international standard (ISO 8601) where the week starts on Monday. If you are used to calendars that start on Sunday, write a list that starts with Monday and you can never be off by one.

Using the number

if d.weekday() >= 5:        # 5 = Saturday, 6 = Sunday
    print("Weekend!")
else:
    print("School day.")

Python checks real calendars: date(2024, 2, 29) works because 2024 is a leap year, but date(2026, 2, 29) is a ValueError because 2026 is not.

Date math with timedelta

from datetime import date, timedelta
today = date(2026, 10, 9)
trip = date(2026, 12, 21)
print(trip - today)          # 73 days, 0:00:00
print((trip - today).days)   # 73
print(today + timedelta(days=14))   # 2026-10-23

A countdown program

from datetime import date
today = date.today()
break_starts = date(2026, 12, 21)
days_left = (break_starts - today).days
print(f"{days_left} days until winter break!")

Subtracting two dates gives a timedelta; .days turns it into a plain int you can print or compare.

Working out someone's age

born = date(2010, 5, 17)
today = date(2026, 10, 9)
age = today.year - born.year
if (today.month, today.day) < (born.month, born.day):
    age -= 1        # birthday hasn't happened yet this year
print(age)          # 16

The tuple comparison checks the month first, then the day, just like the tuples lesson.

Format challenge

Type the format string (the part inside the quotes) that turns d into each target, then click Check. Practice only; nothing is recorded.

Codes people mix up most

A pattern to remember: for names, lowercase is short and uppercase is full.

The os module: where am I?

Why the current folder matters

When your program opens "notes.txt" with no folder in front, Python looks in the current working directory. If you run the program from a different folder, the same line may not find the file. Printing os.getcwd() is the first thing to try when a file "isn't there" but you can see it.

Listing only some files

for name in sorted(os.listdir()):
    if name.endswith(".txt"):
        print(name)

listdir gives plain strings, so all your string tools (endswith, startswith, in) work on the names.

Checking files: exists, isfile, isdir

import os
if os.path.exists("notes.txt"):
    print("Found it!")
else:
    print("No notes yet.")

exists vs. isfile vs. isdir

Use the most specific check for the job. If you are about to read a file, isfile is better than exists, because exists would also be True for a folder with that name.

Building and splitting paths

Why not just use +?

"data" + "/" + "day1.txt" works on Mac and Linux but uses the wrong separator on Windows, and it is easy to end up with doubled or missing slashes. os.path.join always gets the separator right for whatever computer runs the program.

These functions only work with text

basename, dirname, join, and splitext never look at the drive. os.path.join("nope", "fake.txt") happily gives 'nope/fake.txt' even though neither exists. Use exists if you need to know whether it is real.

splitext only splits off the last extension: os.path.splitext("a.tar.gz") gives ('a.tar', '.gz').

Creating, renaming, and deleting

Think before you change files

Programs that create, rename, or delete files can do a lot of damage quickly, especially inside a loop. Good habits:

• Test first with print: print what the program would delete or rename, check the list, and only then switch to the real os.remove or os.rename.
• Work in a test folder with copies of your files.
• Use exist_ok=True with makedirs when it is fine if the folder is already there.

Why two delete functions? Deleting a folder is riskier than deleting one file, so os.rmdir only removes empty folders. You have to remove the files inside on purpose first.

Two safe patterns: check first, or try

if os.path.exists("report.txt"):
    os.remove("report.txt")
else:
    print("Nothing to delete")

try:
    os.remove("report.txt")
except FileNotFoundError:
    print("Nothing to delete")

Which pattern should I use?

Both work for simple programs. The try version is a little safer: between your exists check and the remove, another program could delete or create the file. With try, you simply attempt the action and handle whatever actually happens.

Catch the specific error. except FileNotFoundError: handles the case you expected. A bare except: would also hide unrelated bugs, like a typo in a variable name.

Command-line arguments: sys.argv

Reading python greet.py Ava 16

The word python itself is not in the list; it is the program that runs your script.

Why use arguments instead of input()? The program can run without anyone typing answers, so it can be started by another program, a schedule, or a shortcut, with the information given up front.

Using arguments safely and sys.exit()

import sys
if len(sys.argv) < 3:
    sys.exit("Usage: python age.py NAME AGE")
name = sys.argv[1]
age = int(sys.argv[2])     # '16' becomes 16
print(f"Hi {name}! Next year you will be {age + 1}.")

Why check the length first?

You cannot control what people type. Checking len(sys.argv) before using sys.argv[2] means a missing argument gives a helpful "Usage" message instead of an IndexError.

A bad number still crashes: python age.py Ava sixteen passes the length check, but int("sixteen") is a ValueError. Wrap the conversion in try / except ValueError to handle it:

try:
    age = int(sys.argv[2])
except ValueError:
    sys.exit("AGE must be a whole number")

Other handy sys tools: sys.version shows which Python is running, and sys.platform tells you the operating system (like 'win32' or 'linux').

Common mistakes

Windows words some file errors differently. For example, a missing file shows [WinError 2] The system cannot find the file specified.

Sort the error by module

• datetime: AttributeError usually means module vs. class (datetime.datetime). A ValueError means an impossible date. A TypeError means you added a plain number instead of a timedelta.
• os: errors with [Errno ...] are file system errors. Read the error type (NotFound, Exists, IsADirectory) and the file name at the end.
• sys: IndexError means too few arguments; TypeError or ValueError means you forgot to convert a string.
• No error, wrong output: check format codes (%m vs. %M) and the weekday numbering.

Check yourself

The file questions use this page's pretend drive: notes.txt, scores.csv, photo.png, and a folder named data. Choose an answer, then click Check. Practice only; nothing is graded or recorded.

Cheat sheet

from datetime import datetime, date, timedelta
now = datetime.now()
today = date.today()
now.year   now.hour
now.strftime("%A, %B %d, %Y")
now.strftime("%I:%M %p")
today.weekday()      # Mon = 0
today + timedelta(days=7)

import os
os.getcwd()
sorted(os.listdir())
os.path.exists(p)  isfile  isdir
os.path.getsize(p)
os.path.join("data", "a.txt")
os.mkdir(d)   os.makedirs(d)
os.rename(old, new)
os.remove(f)  os.rmdir(d)

import sys
# python greet.py Ava 16
sys.argv      # ['greet.py', 'Ava', '16']
sys.argv[0]   # script name
len(sys.argv)
int(sys.argv[2])
sys.exit("Usage: ...")

Remember:

• import datetime gives the module; the class is datetime.datetime
• %m is month, %M is minute; weekday() starts at Monday = 0
• Check with os.path.exists or use try/except FileNotFoundError
• os.remove deletes for real, and only files; os.rmdir only removes empty folders
• Every item in sys.argv is a string, and argv[0] is the script name

Key words to know

module · standard library · import / from ... import / as · attribute · strftime / strptime · timedelta · current working directory · relative / absolute path · sys.argv · exit code