How to use "WHERE NOT EXISTS in INSERT STATEMENT"

Hello All,

I want to insert some data in my data base (postgrsql) using Python but i
want to check first that the row does not already exist.
The normal command is:

cur.execute("INSERT INTO COMPANYLIST (ID, NAME, AGE, ADDRESS, SALARY) \
VALUES (1, 'Paul', 32, 'California', 20000.00)");

I get the error below when running the code twice.

===================================================

*psycopg2.IntegrityError: duplicate key value violates unique
constraint "companylist_pkey"
DETAIL: Key (id)=(1) already exists.*

====================================================

I have tried to replace it by:

cur.execute("INSERT INTO COMPANYLIST (ID, NAME, AGE, ADDRESS, SALARY) \
WHERE NOT EXISTS (SELECT 1 FROM table WHERE id='1')\
VALUES (1, 'Paul', 32, 'California', 20000.00)");

It does not work. Can anybody please help?
Kind Regards,
Christian