django - change list in POST request - html

Question

I have a tuple list and wanted to delete or add tuples in it depending on what button has been pressed. Adding tubles is functioning fine but my problem is, that for some reason if Im clicking on the button to delete a tuple, the list resets back the time to the state before the delete happened.
For example I have a list:
ctestformat = [('sung', 4, 15), ('ren', 3, 27), ('lexe', 4, 39)]
after deleting the number 15 I get:
ctestformat = [('ren', 3, 27), ('lexe', 4, 39)]
But after getting another POST request to delete or add, the list resets to the first state as if nothing got deleted
Here is my view to add and delete tuple depending on which button was clicked:
def editorstart(request, ctestformat=[]):
if request.method == 'POST':
"""If clicked on create gap button, create a new gap and put it in ctestformat"""
if 'create_gap' in request.POST:
selectedgap = request.POST['sendgap']
startindex = int(request.POST['startpoint'])-13
ctestformat.append((selectedgap, len(selectedgap), startindex))
ctestformat.sort(key=operator.itemgetter(2))
"""if clicked on deletegap, delete the gap from ctestformat"""
elif 'deletegap' in request.POST:
deleteindex = request.POST['deletegap']
test = [t for t in ctestformat if t[2] != int(deleteindex)]
ctestformat = test
# This function doesnt change anything to ctestformat
modifiedtext = createmodifiedtext(ctestformat)
return render(request, 'editor_gapcreate.html', {"text": modifiedtext, 'ctestformat': ctestformat})
If you have any other questions, just ask :)
EDIT:
added return in my view
my template:
{% extends "base_generic2.html" %}
{% block content %}
<form action="editorgapcreate" id=create method="POST">
<input type="hidden" name="sendgap" id="sendgap">
<input type="hidden" name="startpoint" id="startpoint">
<script src="../static/textselector.js"></script>
<div id="thetext" onmouseup="getSelectionText()">
<h1>{{ text|safe }}</h1>
</div>
{% csrf_token %}
<p></p>
<b>Your current selected gap:</b>
<p id="currentgap"></p>
<input type="hidden" name="text" id="text" value="{{ text }}">
<button type="submit" name="create_gap" id="gapcreate">Create gap</button>
</form>
{% endblock %}

Answer 1

Using a mutable value for a default argument in Python (a list in this case) is not normally a good idea. The list is created once when the function is defined, which means any changes you make to it are visible in subsequent function invocations. However, it seems as though this may be intended in your case.
The reason why you're not seeing the list change, is that the assignment you're making ctestformat = test after filtering out an item has no effect. You need to mutate the original list rather than reassigning, by first finding the index of the item within that list, and then using pop() to remove it. For example:
elif 'deletegap' in request.POST:
deleteindex = request.POST['deletegap']
for i, t in enumerate(ctestformat):
if t[2] == int(deleteindex):
ctestformat.pop(i) # Modify original list
break
...
I would still recommend not using a mutable default argument to achieve this. If you need to share data across requests, you'd be better to use a cache or a database, or possibly session state, depending upon your application requirements.