Q:

In a PostgreSQL CHECK constraint using a boolean stored procedure written in PL/Python, can the detail of the error message be updated?

The qustion's in the (albeit long) title. I have code in this fashion:
-- create the tables for models
CREATE TABLE invoice(
    id SERIAL PRIMARY KEY,
    value VARCHAR(8)
      CONSTRAINT valid_money CHECK(validate_invoice_money(value))
);

with this procedure:
CREATE OR REPLACE FUNCTION validate_invoice_money(test VARCHAR) RETURNS BOOLEAN AS $$
    import plpy
    import re

    if (re.match("^[0-9]+\\.[0-9]{2}$", test) == None):
        return False

    return True
$$ LANGUAGE plpython3u IMMUTABLE;

(the specifics of this example are not important, I know there's better ways to validate a value than this, such as using the MONEY type.)
When an insert is attempted that fails this check, I get the following error:
ERROR  : new row for relation "invoice" violates check constraint "valid_model"
DETAIL : Failing row contains (1, "notvalid").

(the error/detail descriptors are my own code, but a PostgreSQL error has an error and a detail field provided.)
Is there any way for me to change the "detail" portion of this error from my Python procedure?

A:

In Postgres 9.6+ you can raise an error with detailed messages from a plpython function using utility functions, e.g.:
CREATE OR REPLACE FUNCTION validate_invoice_money(test VARCHAR) 
RETURNS BOOLEAN AS $$
    import re

    if (re.match("^[0-9]+\\.[0-9]{2}$", test) == None):
        plpy.error("custom exception message",
            detail="some info about exception",
            hint="hint for users")
        return False

    return True
$$ LANGUAGE plpython3u IMMUTABLE;

insert into invoice
values (default, 'notvalid');

ERROR:  plpy.Error: custom exception message
DETAIL:  some info about exception
HINT:  hint for users
CONTEXT:  Traceback (most recent call last):
  PL/Python function "validate_invoice_money", line 8, in <module>
    hint="hint for users")
PL/Python function "validate_invoice_money"

