Admin.display decorator functions do not output correct-format for 'modern' Python string-formatting

[Stephen Skett]

I am trying to write an admin display function (using the ​admin.display decorator) to output the values of a float-field to 3 decimal places.

Seemed like this should be pretty simple, and indeed it is, IF you use ​'old-style' Python format-strings, e.g.

class ModelFoo_Admin(admin.ModelAdmin):
    @admin.display
    def get_value_to_3dp(self, obj):
        return "%.3f" % obj.bar

However, if you use either of the more 'modern'-style Python string-formatting methods (i.e. ​str.format or ​formatted string-literals), the specified string is not displayed using the specified formatting, i.e. this:

class ModelFoo_Admin(admin.ModelAdmin):
    @admin.display
    def get_value_to_3dp(self, obj):
        return "{0:3f}".format(obj.bar)

or this:

class ModelFoo_Admin(admin.ModelAdmin):
    @admin.display
    def get_value_to_3dp(self, obj):
        return f'{obj.bar:3f}'

display the output string to the default number of decimal-places (in my case, 6), with trailing zeroes, rather than to 3 decimal places as expected.

I don't understand why there would be any difference between these approaches, so I am assuming this is a bug rather than the intended behaviour.