Check Yourself
Conceptual
What's the difference between GET and POST?
GET asks the server to show something — used when you visit a URL or click a link. POST sends data to the server — used when you submit a form. GET requests don't carry form data in the request body. POST does.
What does the name= attribute on a form input do?
It becomes the key in request.form. When the form is submitted, the browser labels each field's value with its name. Flask reads those labels. name="title" → request.form["title"]. Without name, Flask can't find that field's value at all.
Why redirect after POST instead of using render_template?
If you render a page after a POST, the browser's last request was a POST. When the user refreshes, the browser resends the POST — submitting the form again and creating a duplicate row. Redirecting after POST sends the browser to a GET request. Refreshing that just re-runs the GET, not the POST.
:::collapsible{title="Why use "a" mode and not "w" when writing the CSV?"}
"w" (write) truncates the file to zero before writing anything. Every form submission deletes all previous rows and replaces them with just the new one. "a" (append) positions the write cursor at the end of the file, leaving existing rows untouched.
:::
:::collapsible{title="What does url_for("display") do that redirect("/display") doesn't?"}
url_for generates the URL from the function name, not the path. If you ever change the route from /display to something else, url_for("display") still works — it finds the function and generates the new path. A hardcoded "/display" would silently break.
:::
What does csv.DictReader give you, and how does it know the column names?
DictReader reads each row of the CSV as a dictionary. It uses the first row of the file (the header row) as the dictionary keys. If your CSV has datetime,project_title,title,... as its first line, each row becomes {"datetime": "...", "project_title": "...", "title": "...", ...} — exactly the shape Jinja expects for {{ task.title }}.
Practical
- Build an HTML form with the correct
action,method, andnameattributes? - Read a submitted field with
request.form["field_name"]? - Handle GET and POST in the same route using
request.method? - Append a new row to a CSV with
csv.DictWriterin"a"mode? - Read all rows from a CSV with
csv.DictReader? - Redirect to a different route after a successful POST?
- Use
url_for("function_name")to generate a route URL? - Submit a task and confirm it persists after a server restart?
@app.route("/input", methods=["GET", "POST"])
def input_task():
if request.method == "POST":
title = request.form["title"]
with open("tasks.csv", "a", newline="") as f:
writer = csv.DictWriter(f, fieldnames=["title"])
writer.writerow({"title": title})
return redirect(url_for("display"))
return render_template("input.html")What happens if you submit this form and then immediately press refresh in the browser?
Answer
The page refreshes by re-running a GET request to whatever URL you were redirected to — in this case /display. It does not re-submit the form. The POST-redirect-GET pattern breaks the refresh cycle: the last request the browser made was a GET (the redirect), so refreshing repeats the GET, not the POST.
If the code had used render_template instead of redirect, refreshing would have re-submitted the POST and added a duplicate row.
What's coming next
Your task tracker records entries, but it can't edit or delete them. That changes in Week 5. First, Week 4 takes a detour: a Birthdays app that reinforces everything you know so far with a different dataset and slightly more complexity. Same patterns, different context — exactly the kind of repetition that makes the skills stick.