Model meta field "to_python" behaviour with decimal fields

[aiacamposm]

When calling to_python method of a model field I expect it returns the exact same value that will be stored in the database, i.e., when saved the instance and retrieved later, to have this same value as the corresponding attribute. However, it doesn't seem to behave correctly when the field is of type DecimalField.

I have a model MyModel that has a field foo with max_digits=10 and decimal_places=2, so when I save a value 3.4211 (passed as a common float) the value stored is Decimal("3.42"). However, if call MyModel._meta.get_field("foo").to_python(3.4211) the result is Decimal("3.421100000").

Inspecting the source code of this method to_python I guess the problem is that it uses decimal.Context with prec equal to the number of max_digits, which is something that only makes sense if the number passed has max_digits meaningful digits.

Should it be changed to return the decimal with the specified number of digits, or am I wrong assuming that to_python should return the exact same value that will be stored?

[Tim Graham]

Duplicate of #26459.