jinja2.exceptions.UndefinedError: 'form' is undefined error in python, Flask

Question

I'm new to Flask and have been programming for 3 weeks now.

I am getting a: "jinja2.exceptions.UndefinedError: 'form' is undefined error" and I don't know how to solve it.

My main app.py: This is just the relevant section

 @app.route("/form", methods=["GET", "POST"])
def forms():
    global reverse_exchange_rate, exchange_rate
    form = CurrencyForm
    if request.method == 'GET':
        return render_template("form.html")

and my form.html:

{% extends "layouts/main.html" %}
{% from "formhelper.html" import render_field %}

{% block content %}

    <form method="GET" action="/form">
        <div style="text-align: center">
            <h1>Currency Exchange</h1>
            {{ form.csrf_token }}
            {{ render_field(form.currency_1) }}
            {{ render_field(form.currency_2) }}
            {{ render_field(form.exchange_rate) }}
            {{ render_field(form.submit) }}
        </div>
    </form>

{% endblock %}

I am getting this error:

File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\app.py", line 2464, in __call__
return self.wsgi_app(environ, start_response)
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\app.py", line 2450, in wsgi_app
response = self.handle_exception(e)
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\app.py", line 1867, in handle_exception
reraise(exc_type, exc_value, tb)
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\_compat.py", line 39, in reraise
raise value
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\app.py", line 2447, in wsgi_app
response = self.full_dispatch_request()
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\app.py", line 1952, in full_dispatch_request
rv = self.handle_user_exception(e)
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\app.py", line 1821, in handle_user_exception
reraise(exc_type, exc_value, tb)
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\_compat.py", line 39, in reraise
raise value
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\app.py", line 1950, in full_dispatch_request
rv = self.dispatch_request()
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\app.py", line 1936, in dispatch_request
return self.view_functions[rule.endpoint](**req.view_args)
File "C:\Users\sport\Documents\Flask Attempt 2\app.py", line 21, in forms
return render_template("form.html")
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\templating.py", line 140, in render_template
ctx.app,
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\flask\templating.py", line 120, in _render
rv = template.render(context)
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\jinja2\environment.py", line 1090, in render
self.environment.handle_exception()
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\jinja2\environment.py", line 832, in handle_exception
reraise(*rewrite_traceback_stack(source=source))
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\jinja2\_compat.py", line 28, in reraise
raise value.with_traceback(tb)
File "C:\Users\sport\Documents\Flask Attempt 2\templates\form.html", line 2, in top-level template code
{% from "formhelper.html" import render_field %}
File "C:\Users\sport\Documents\Flask Attempt 2\templates\layouts\main.html", line 26, in top-level template code
Open an interactive python shell in this frame{% block content %}{% endblock %}
File "C:\Users\sport\Documents\Flask Attempt 2\templates\form.html", line 9, in block "content"
{{ form.csrf_token }}
File "c:\users\sport\documents\flask attempt 2\venv\lib\site-packages\jinja2\environment.py", line 471, in getattr
return getattr(obj, attribute)
jinja2.exceptions.UndefinedError: 'form' is undefined

I have tried removing that section of python code and everything runs smoothly. But if I put it back in I get that error.

I have also tried changing the name of form.html to form1.html although the error still stays the same and says form is undefined. I don't know what my problem is.

The form.html is in the templates folder and the structure of files is correct.

Any help is appreciated. Please remember that I have been coding for about 3 weeks.

Answer 1

As you can see in the bottom of the stack trace, the UndefinedError comes from the template line {{ form.csrf_token }}, where the template expects a variable named form but no such variable was passed to it. The way to approach this in Flask is by explicitly passing all the variables the template would need as keyword arguments in the render_template function, e.g. render_template("form.html", form=form).

See the relevant section of the offficial quickstart tutorial for more detail.