Form

from django import forms

# archivo forms.py
from django import forms

class RenewBookForm(forms.Form):
    renewal_date = forms.DateField(help_text="Introduce una fecha entre hoy y 4 semanas(default 3).")

class RenewBookForm(forms.Form):
    renewal_date = forms.DateField(help_text="Introduce una fecha entre hoy y 4 semanas(default 3).")
    def clean_renewal_date(self):
        data = self.cleaned_data['renewal_date']

        # Check if a date is not in the past.
        if data < datetime.date.today():
            raise ValidationError(_('Invalid date - renewal in past'))

        # Check if a date is in the allowed range (+4 weeks from today).
        if data > datetime.date.today() + datetime.timedelta(weeks=4):
            raise ValidationError(_('Invalid date - renewal more than 4 weeks ahead'))

        # Remember to always return the cleaned data.
        return data    

urlpatterns += [
    path('book/<uuid:pk>/renew/', views.renew_book_librarian, name='renew-book-librarian'),

import datetime

from django.shortcuts import render, get_object_or_404
from django.http import HttpResponseRedirect
from django.urls import reverse

from catalog.forms import RenewBookForm

def renew_book_librarian(request, pk):
    book_instance = get_object_or_404(BookInstance, pk=pk)

    # If this is a POST request then process the Form data
    if request.method == 'POST':

        # Create a form instance and populate it with data from the request (binding):
        form = RenewBookForm(request.POST)

        # Check if the form is valid:
        if form.is_valid():
            # process the data in form.cleaned_data as required (here we just write it to the model due_back field)
            book_instance.due_back = form.cleaned_data['renewal_date']
            book_instance.save()

            # redirect to a new URL:
            return HttpResponseRedirect(reverse('all-borrowed') )

    # If this is a GET (or any other method) create the default form.
    else:
        proposed_renewal_date = datetime.date.today() + datetime.timedelta(weeks=3)
        form = RenewBookForm(initial={'renewal_date': proposed_renewal_date})

    context = {
        'form': form,
        'book_instance': book_instance,
    }

    return render(request, 'catalog/book_renew_librarian.html', context)

{% extends "base_generic.html" %}

{% block content %}
  <h1>Renovar libro</h1>
  <form method="post">
    {% csrf_token %}
    {{ form.as_p }}
    <input type="submit" value="Renovar" />
  </form>
{% endblock %}

 {{ form.as_table }} 
 {{ form.as_ul }}) 
 {{ form.as_p }}

{% extends "base_generic.html" %}

{% block content %}

  <h1>Renovar libro para {{ book_instance.book.title }} </h1>

  <form method="post">
    {% csrf_token %}
    {{ form.as_p }}
    <input type="submit" value="Renovar" />
  </form>

{% endblock %}