Models must have a __format__ method

[jace]

We have constructs like this in various places: _("Something related to {entity}").format(entity=entity.title)

This is error prone because we may accidentally pass the object instead of the attribute, requiring careful audit. This can be avoided if the model implements __format__ that reflexively returns self.title, or something else as per the format spec.

Bonus: This will also make TemplateVarMixin for SMS templates obsolete, as the template can instead use something like "{project:sms} has an update: {project:shorturl}" and the __format__ method can return a truncated title or short URL based on the format spec.

[jace]

Python has an elaborate format spec mini-language, so we shouldn't be adding arbitrary keywords like this. Safer to start with just the standard self.title type return value, deferring to str.__format__ for any other format spec.

[jace]

The mini language suggests <align><width> as a valid syntax, and the existing alignment characters are <, >, = and ^. We can add an additional option with . or … (Unicode) to indicate truncation to the given width. Syntax:

1. {var:.30} truncates to 30 using three dots ... as suffix (30 inclusive, so 27 from the var)
2. {var:…30} truncates to 30 using ellipsis … as suffix (30 inclusive, so 29 from the var)

The problem is that SMS template strings will now have that number 30 appearing everywhere, instead of reading it once from self.var_max_length. The __format__ method of the embedded object (say Project or Update) gets no context for where it's being called from, so it can't assume 30 chars.

{var:…{var_max_length}} is an option, but given the verbosity it can only be used in templates shared across transports with only length differences (eg: SMS DLT and WhatsApp). This is unlikely.

[jace]

Argument against using format_spec: i18n strings introduce a risk of translator error. We are less likely to have a typo in translating {project} than in {project:.} or {project:…}, so it's better for the code to compensate by calling .format(project=project.title) explicitly.

[jace]

The base classes got default __format__ implementations in hasgeek/coaster#396.

[jace]

Custom __format__ methods were added in #1697, #1793, #1812 and #1842.