Django 5 breaks value of Date field rendering correctly in template

[Michael]

This used to work correctly in Django 4. Django 5 removed USE_L10N setting from settings.py

When rendering an input field that is of type date:

  /home/michael/.venv/project/lib/python3.11/site-packages/django/forms/boundfield.py(108)as_widget()
-> return widget.render(
  /home/michael/.venv/project/lib/python3.11/site-packages/django/forms/widgets.py(279)render()
-> context = self.get_context(name, value, attrs)
  /home/michael/.venv/project/lib/python3.11/site-packages/django/forms/widgets.py(332)get_context()
-> context = super().get_context(name, value, attrs)
  /home/michael/.venv/project/lib/python3.11/site-packages/django/forms/widgets.py(271)get_context()
-> "value": self.format_value(value),
  /home/michael/.venv/project/lib/python3.11/site-packages/django/forms/widgets.py(555)format_value()
-> value, self.format or formats.get_format(self.format_key)[0]

We see that formats.get_format(self.format_key) only passes in 1 argument to:

# django/utils/formats.py(109)get_format()

def get_format(format_type, lang=None, use_l10n=None):
    """
    For a specific format type, return the format for the current
    language (locale). Default to the format in the settings.
    format_type is the name of the format, e.g. 'DATE_FORMAT'.

    If use_l10n is provided and is not None, it forces the value to
    be localized (or not), otherwise it's always localized.
    """
    if use_l10n is None:
        use_l10n = True
    if use_l10n and lang is None:
        lang = get_language()
    format_type = str(format_type)  # format_type may be lazy.
    cache_key = (format_type, lang)
    try:
        return _format_cache[cache_key]
    except KeyError:
        pass

this means use_l10n initial value is None, which means it will be set to True via the logic:

    if use_l10n is None:
        use_l10n = True

So then it sets a language, in my case en-gb, which makes results in a cache_key of ('DATE_INPUT_FORMATS', 'en-gb')

Which means the get_format method returns ['%d/%m/%Y', '%d/%m/%y', '%Y-%m-%d']
Which is really bad when format_type = 'DATE_INPUT_FORMATS', because HTML inputs must always be in the format YYYY/MM/DD, and hence selecting the first format of %d/%m/%Y results in an error in your HTML template when your input is rendered like <input type="date" value="31/12/2024"> instead of <input type="date" value="2024/12/31">

Not that it changes anything, but here is my DATE_INPUT_FORMATS from settings.py (it has no USE_L10N setting as unfortunately that has been removed):

DATE_INPUT_FORMATS = [
    '%Y-%m-%d',
]