discard on constraint violation

Is there a way to tell a table to discard an insert if it violates a
constraint (unique in my case) instead of giving an error? I don't want
the overhead of a trigger on each row inserted.

My situation is that I'm reading data from an external website with lots
of duplicate data. I am reading the data in a plpythonu function. I
never update, only insert. A trigger on the table that has to check if
each row exists before trying the insert is too much overhead.

I tried in plpython:

create or replace function testme() returns bool as
$$
try:
plpy.execute("insert into
reports.survey_types(typeid,name)values(1,'test')")

return true

except:

plpy.notice('the constraint was violated')
return false
$$language 'plpythonu';

where the insert violates a unique constraint and it never gets to the
except. The function dies and doesn't get to my except with:

WARNING: plpython: in function testme:
DETAIL: <class 'plpy.SPIError'>: Unknown error in PLy_spi_execute_query

ERROR: duplicate key violates unique constraint "survey_types_pkey"
CONTEXT: SQL statement "insert into
reports.survey_types(typeid,name)values(1,'test')"