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# Keep the sign-ups
Web apps with Flask · Forms and saving
---
## Today
**We are learning to** save each new member to a CSV file, and read the file to show the list, so sign-ups last after the app stops.
- I can add a row to a CSV file and read the file to show its rows.
- I can explain why a web app keeps data in a file, not a variable.
- I can change one row by reading every row, changing it, and writing them all back.
---
## Why it matters
> A web app that forgets everyone when it restarts is no use. Real apps keep their data in files and databases.
---
💬 Discussion
## Predict
```python
with open("log.txt", "w") as f:
f.write("Ada\n")
with open("log.txt", "a") as f:
f.write("Sam\n")
with open("log.txt") as f:
print(f.read())
```
**What does it print?**
- An error: log.txt already exists
- Ada, then Sam on the next line
- Ada
- Sam
---
✅ Check
## What happened?
* Ada, then Sam. "w" makes the file and writes Ada; "a" adds Sam to the end instead of emptying the file.
---
✍️ Activity
## Run it
Open the lesson at this step and try it together.
[studio.learningwaypoints.com/tutorials/code/flask/flask-save/?full=step-2#step-2](https://studio.learningwaypoints.com/tutorials/code/flask/flask-save/?full=step-2#step-2)
---
💬 Discussion
## Investigate
_The program is 53 lines long: open it in the tutorial and read it together._
[Open this step](https://studio.learningwaypoints.com/tutorials/code/flask/flask-save/?full=step-3#step-3)
---
✅ Check
## Read every time
**Why does home() call load_members() every time, instead of reading the file once at the start?**
- Flask needs it
- To check the file is still there
- So the page always shows what is in the file now, including members who just joined
- Reading files is free
* Answer: So the page always shows what is in the file now, including members who just joined
---
✅ Check
## A quirk of csv
**Why is newline="" there when the file is opened to add a row?**
- It empties the file
- It stops csv writing blank lines between rows on some computers
- It makes the file a CSV
- It adds a new line
* Answer: It stops csv writing blank lines between rows on some computers
---
✅ Check
## Spot the bug
**After saving Zoe, the file reads back as an empty list, and Ada and Sam have gone. Click the line that is wrong.**
```
1 import csv
2
3
4 def save_member(name, group):
5 with open("members.csv", "w", newline="") as f:
6 csv.writer(f).writerow([name, group])
7
8
9 save_member("Zoe", "Players")
10 with open("members.csv") as f:
11 print(list(csv.DictReader(f)))
```
* Answer: line 5
---
✍️ Activity
## Modify
- **No repeats:** Refuse a name that is already in the file, whatever its capitals, with a message saying it is already a member.
- **How many members:** Show the number of members in the **Members** heading.
[Open this step](https://studio.learningwaypoints.com/tutorials/code/flask/flask-save/?full=step-4#step-4)
---
✍️ Activity
## Make: results that last
The Add a win form is there, but `win()` does nothing yet. Make it add 1 to the chosen player's wins and **save the whole table** back to `results.csv`, header first, so the wins last after a Run. (`"a"` only adds rows; changing a row means writing the file again.)
[Open this step](https://studio.learningwaypoints.com/tutorials/code/flask/flask-save/?full=step-5#step-5)
---
## Where this leads
On a computer, `members.csv` would sit next to `app.py`. Next: share the members with other programs as JSON.
---
✅ Check
## Exit ticket
**Why does the app read members.csv for every page?**
* The file changes when people join, so reading it each time means the page always shows the current members.