Unexpected auto-formatting of datetime object

[maximesunnen]

forms.py:

from flask_wtf import FlaskForm
from wtforms import DateField

class testForm(FlaskForm):
     birthday = DateField()

app.py:

     from forms.py import testForm
     from flask import render_template

    @app.route("/issue", methods=["GET", "POST"]):

      # Form
      test_form = testForm()

      # Extract form data
      form_data = form.data
      return render_template("issue.html", form_data=form_data)

test.html:

    {% if form_data["birthday"] %}
           <input class="form-control" id="birthday" name="birthday" type="date" value={{form_data["birthday"]}}>
    {% else %}
           {{ test_form.birthday() }}
    {% endif %}

Issue description

• If I print form_data["birthday"] from my app.py code, I get the following output.

'birthday': datetime.date(1111, 11, 3)

So birthday is a datetime.date object.

• If I print form_data["birthday"] from my test.html code using Jinja syntax:

{{ test_data["birthday"] }}

I see the following inside my app:

"Sun, 11 Nov 0001 00:00:00 GMT"

And in the developer tools:

<input class="form-control" id="birthday" name="birthday" type="date" value=Sun, 11 Nov 0001 00:00:00 GMT> The specified value "Sun," does not conform to the required format, "yyyy-MM-dd".

I do not understand why form_data["birthday"] is not considered a datetime.date object anymore? Why is the time added? Why is it converted to a string? This is causing the problem that I'm not able to maintain the birthday input from the user across multiple requests (i'm saving form data temporarily inside session).

[maximesunnen]

The following line solves the issue:

session["form_user_data"]["birthday"].strftime("%Y-%m-%d")

Still not satisfying because the "%Y-%m-%d"format should be the default and is not changed anywhere.