Python Flask-WTF - use same form template for add and edit operations

The simplest way I tried to overcome this issue was to not include the SubmitField in the form (BookForm). We can pass the form submit value as variable to the form, e.g submit='Update' in the edit_book route and submit='Add Book'.

In the route, populate the fields with the details related to the book if the post method is 'GET'

There is absolutely no reason to have separate templates for adding / editing different kinds of things even. Consider:

{# data.html #}
<!-- ... snip ... -->
{% block form %}
<section>
<h1>{{ action }} {{ data_type }}</h1>
<form action="{{ form_action }}" method="{{ method | d("POST") }}">
{% render_form(form) %}
</form>
</section>
{% endblock form %}

Ignore the macro render_form works (there's an example one in WTForms' documentation) - it just takes a WTForms-type object and renders the form in an unordered list. You can then do this:

@app.route("/books/")
def add_book():
    form = BookForm()
    # ... snip ...
    return render_template("data.html", action="Add", data_type="a book", form=form)

@app.route("/books/<int:book_id>")
def edit_book(book_id):
    book = lookup_book_by_id(book_id)
    form = BookForm(obj=book)
    # ... snip ...
    return render_template("data.html", data_type=book.title, action="Edit", form=form)

But you don't need to limit yourself to just books:

@app.route("/a-resource/")
def add_resource():
    # ... snip ...
    return render_template("data.html", data_type="a resource" ...)

# ... etc. ...