Writing a connection string when password contains special characters

You need to URL-encode the password portion of the connect string:

from urllib.parse import quote_plus
from sqlalchemy.engine import create_engine
engine = create_engine("postgres://user:%s@host/database" % quote_plus("p@ss"))

If you look at the implementation of the class used in SQLAlchemy to represent database connection URLs (in sqlalchemy/engine/url.py), you can see that they use the same method to escape passwords when converting the URL instances into strings.