1 The Problem
We want a habit tracker: define habits, mark them done on a given day, and compute the current streak — how many consecutive days you have kept it up. It teaches working with dates and the satisfying-but-tricky logic of finding consecutive runs.
Where this shows up: habit and fitness apps, attendance systems, subscription renewals, anything that reasons about dates and consecutive periods. Date logic is famously error-prone, so learning it carefully pays off.
2 How to Think About It
Think about what a streak means, before any code:
The plan — in plain English
1. Each habit stores the set of dates it was completed. → 2. Mark done adds today’s date. → 3. The streak is the count of consecutive days ending today: check today, yesterday, the day before, until you hit a gap. → 4. Persist to a file.
3 The Build — explained part by part
Here is the complete habit tracker. The streak logic uses real dates. Each part is explained below.
Pythonhabits.py
from datetime import date, timedelta
def mark_done(completed_dates, day=None):
# Record a habit as done on a day (default today).
day = day or date.today()
completed_dates.add(day.isoformat())
return completed_dates
def current_streak(completed_dates, today=None):
today = today or date.today()
streak = 0
day = today
# Step backwards one day at a time while each is completed.
while day.isoformat() in completed_dates:
streak += 1
day = day - timedelta(days=1)
return streak
if __name__ == "__main__":
done = set()
mark_done(done, date(2026, 6, 22))
mark_done(done, date(2026, 6, 23))
mark_done(done, date(2026, 6, 24))
print("Streak:", current_streak(done, date(2026, 6, 24)))
What each part does — in plain words
from datetime import date, timedelta — date represents a calendar day; timedelta represents a span (like “one day”) we can subtract.completed_dates — a set of date strings (ISO format like
"2026-06-24"). A set gives instant “was this day done?” checks and ignores duplicate marks.day.isoformat() — turn a date into its text form for storing and comparing.
the while loop in current_streak — the heart of it: start at today, and while that day is in the completed set, count it and step back one day with
day - timedelta(days=1). The moment a day is missing, the streak ends.day = day - timedelta(days=1) — proper date maths: subtracting a timedelta correctly handles month and year boundaries, which manual arithmetic would get wrong.
Common mistakes — and how to avoid them
✗ Doing date maths by hand (adding/subtracting day numbers) — it breaks at month boundaries.
✓ Use
timedelta(days=1) so month and year rollovers are handled.✗ Storing dates as inconsistent strings — comparisons fail.
✓ Always store with
.isoformat() for a consistent format.✗ Using a list instead of a set for dates — slow lookups and duplicate marks.
✓ A set gives instant membership checks and ignores duplicates.
4 Test & Prove Each Part
We test the streak logic with fixed dates, so we can verify the consecutive-day counting exactly.
Three consecutive days give a streak of 3
A gap breaks the streak
No completions gives a streak of 0
Pythontest_habits.py
from datetime import date, timedelta
def current_streak(completed_dates, today):
streak = 0
day = today
while day.isoformat() in completed_dates:
streak += 1
day = day - timedelta(days=1)
return streak
def test_three_day_streak():
"""Three consecutive days give a streak of 3."""
dates = {"2026-06-22", "2026-06-23", "2026-06-24"}
assert current_streak(dates, date(2026, 6, 24)) == 3
def test_gap_breaks_streak():
"""A missing day stops the streak."""
dates = {"2026-06-22", "2026-06-24"} # 23rd missing
assert current_streak(dates, date(2026, 6, 24)) == 1
def test_no_completions():
"""No completions means a streak of 0."""
assert current_streak(set(), date(2026, 6, 24)) == 0
Run with pytest -v. By passing a fixed today, the tests are deterministic — they do not depend on the real date. The gap test proves a missing day correctly breaks the streak.
5 The Interface
markmark_done(habit, day)record a completion
streakcurrent_streak(habit)consecutive days
Returns
Streak: 36 Run It & Automate It
Run it locally
python3 habits.pySee a 3-day streak computed. Mark habits done each day to build real streaks.
Jenkins runs the tests automatically — each line explained below.
What you should see when it works
Terminala real run
Streak: 3If it breaks — how to fix it
🚨 The streak is always 0.
Your stored date format does not match what you compare against. Use
.isoformat() consistently.🚨 Streak counts non-consecutive days.
Make sure you step back exactly one day each loop and stop at the first gap.
GroovyJenkinsfile
// Jenkinsfile — automated tests on every change.
pipeline {
agent any
stages {
stage('Get the code') { steps { checkout scm } } // download the code
stage('Install') { steps { sh 'python3 -m venv venv && . venv/bin/activate && pip install pytest' } } // test tool
stage('Test') { steps { sh '. venv/bin/activate && pytest -v' } } // run every test
}
post {
success { echo 'All tests passed.' }
failure { echo 'A test failed — look above.' }
}
}
🎯 Try this next — make it yours
- Longest streak. Track the best streak ever, not just current. (Teaches: scanning all dates.)
- Multiple habits. Track several habits at once. (Teaches: a dict of habits.)
- Weekly view. Show a calendar grid of done days. (Teaches: date formatting.)
What you learned
You learned to work with real dates using date and timedelta, and to compute consecutive runs by stepping backward day by day. Proper date maths avoids the bugs that manual day-counting causes. Related: datetime, Control Flow.