Idempotent PostgreSQL Schema Dump for Existing DB

I'm looking for a way to replicate a PostgreSQL schema to a database which may already have some of it defined. I wonder if there's a simple way to produce DDL that is idempotent (i.e. that can be applied any number of times) from PostgreSQL.
Currently I obtain DDL from my source DB using pgdump -s, and manipulate it using the ad-hoc Python code below. I wonder if there's a cleaner solution.

import re

def MakeDdlIdempotent(sql):
    "Make PGSQL DDL idempotent by changing various 'create' statements to idempotent versions"
    ddlReplacements = {
        "CREATE SCHEMA":   "CREATE SCHEMA IF NOT EXISTS",
        "CREATE TABLE":    "CREATE TABLE IF NOT EXISTS",
        "CREATE INDEX":    "CREATE INDEX IF NOT EXISTS",
        "CREATE VIEW":     "CREATE OR REPLACE VIEW",
        "CREATE FUNCTION": "CREATE OR REPLACE FUNCTION"
        }

    for orig, replacement in ddlReplacements.items():
        sql = sql.replace(orig, replacement)

    addConstraintRe = re.compile(r"ALTER TABLE [^;]+ ADD CONSTRAINT [^;]+;", flags = re.DOTALL | re.MULTILINE)
    sql = addConstraintRe.sub(
        r"""DO $$
BEGIN
  BEGIN
    \g<0>
    EXCEPTION
        WHEN invalid_table_definition OR duplicate_object OR duplicate_table THEN
    END;
END $$;""",
    sql)

    return sql