Adding a column with a default value requires a full table rewrite, and therefore it is often suggested to avoid the default value. However, it is possible to add the default value without having PostgreSQL perform the full table rewrite.
Today a friend of mine asked me for a trouble between PostgreSQL 10 and Python. Since I’m not a pythonist, my quick answer was that PostgreSQL 10 does support Python 3. But it turned out it was not so simple (at least, not so simple as to get Perl 5 working!).