Add column with default value
On large table adding a column with a default value can be expensive because it has to write on all the rows.
But since PostgreSQL 11, adding a column with a default value does not write on each row. So we could add the column with the default python value as SQL default value and then drop the default value (because we do not want to store in the schema volatile value).
I tested that it is still the original default value (when the column was added) that is used on the first access (even if the schema has been modified after).