Postgres: INSERT if does not exist already

Question

I'm using Python to write to a postgres database:

sql_string = "INSERT INTO hundred (name,name_slug,status) VALUES ("
sql_string += hundred + ", '" + hundred_slug + "', " + status + ");"
cursor.execute(sql_string)

But because some of my rows are identical, I get the following error:

psycopg2.IntegrityError: duplicate key value  
  violates unique constraint "hundred_pkey"

How can I write an 'INSERT unless this row already exists' SQL statement?

I've seen complex statements like this recommended:

IF EXISTS (SELECT * FROM invoices WHERE invoiceid = '12345')
UPDATE invoices SET billed = 'TRUE' WHERE invoiceid = '12345'
ELSE
INSERT INTO invoices (invoiceid, billed) VALUES ('12345', 'TRUE')
END IF

But firstly, is this overkill for what I need, and secondly, how can I execute one of those as a simple string?

Answer 1

Postgres 9.5 (released since 2016-01-07) offers an "upsert" command, also known as an ON CONFLICT clause to INSERT:

INSERT ... ON CONFLICT DO NOTHING/UPDATE

It solves many of the subtle problems you can run into when using concurrent operation, which some other answers propose.