Please Help me with connecting my PostgreSQL

It's the first time that I can't find the answer about some tech problems
Here's my problems:

>>conn=psycopg2.connect(database="mydb", user="postgres",
password="123",port=5433)

Traceback (most recent call last):
File "<stdin>", line 1, in <module>
psycopg2.OperationalError: could not connect to server: No such file or
directory
Is the server running locally and accepting
connections on Unix domain socket "/var/run/postgresql/.s.PGSQL.5432"?

1.my postgreSQL is running
2.my listeningport is 5432 for sure
3.
root(at)lanston-laptop:~# psql -l
Password:
List of databases
Name | Owner | Encoding | Collation | Ctype | Access
privileges
---------------+----------+----------+------------+------------+-----------------------
checkdatabase | postgres | UTF8 | en_US.utf8 | en_US.utf8 |
mydb | postgres | UTF8 | en_US.utf8 | en_US.utf8 |
postgres | postgres | UTF8 | en_US.utf8 | en_US.utf8 |
template0 | postgres | UTF8 | en_US.utf8 | en_US.utf8 | =c/postgres
+
| | | | |
postgres=CTc/postgres
template1 | postgres | UTF8 | en_US.utf8 | en_US.utf8 | =c/postgres
+
| | | | |
postgres=CTc/postgres
(5 rows)

*This is the problem that so many people asked,but there's no answers.*
*Thanks a lot*