Safe in/decreasing value without select-for-update

t.py

from sqlalchemy import Column, Integer, String
from sqlalchemy import create_engine
from sqlalchemy.orm import sessionmaker
from sqlalchemy.ext.declarative import declarative_base

import logging

logging.basicConfig()

Base = declarative_base()

# Define a unsigned type for avoiding negative value
# drop domain if exists uint4;
# create domain uint4 as integer check(value > 0);

# sql for creating demo table
# drop table if exists t;
# create table t(id integer primary key, name text, counter uint4);

class T(Base):
    __tablename__ = 't'
    id = Column(Integer, primary_key=True)
    name = Column(String(20))
    counter = Column(Integer)

db_url = 'postgresql+psycopg2://localhost/zzb_shard_1'
engine = create_engine(db_url, echo=True)

Session = sessionmaker(bind=engine)

session = Session()
# create a obj if it does not exist
data = { 'id': 1, 'name': 'dawn', 'counter': 1}
t = session.query(T).get(data['id'])
if not t:
    t = T(**data)
    session.add(t)
    session.commit()

# increase numberic column
t.counter = T.counter + 1
print("*NOTE*:\n - type: %s\n - value: %s" % (type(t.counter), t.counter))
session.commit()
session.close()