Debugging (displaying) SQL command sent to the db by SQLAlchemy

Question

I have an ORM class called Person, which wraps around a person table:

After setting up the connection to the db etc, I run the statement:

people = session.query(Person).all()

The person table does not contain any data (as yet), so when I print the variable people, I get an empty list.

I renamed the table referred to in my ORM class People, to people_foo (which does not exist).

I then run the script again. I was surprised that no exception was thrown when attempting to access a table that does not exist.

I therefore have the following 2 questions:

1. How may I setup SQLAlchemy so that it propagates db errors back to the script?
2. How may I view (i.e. print) the SQL that is being sent to the db engine?

If it helps, I am using PostgreSQL.

[Edit]

I am writing a package. In my __main__.py script, I have the following code (shortened here):

### __main__.py
import common # imports logging and defines logging setup funcs etc

logger = logging.getLogger(__name__)


def main():    
    parser = OptionParser(usage="%prog [options] <commands>",
                          version="%prog 1.0")

    commands = OptionGroup(parser, "commands")

    parser.add_option(
        "-l",
        "--logfile",
        dest="logfile",
        metavar="FILE",
        help="log to FILE. if not set, no logging will be done"
    )

    parser.add_option(
        "--level",
        dest="loglevel",
        metavar="LOG LEVEL",
        help="Debug level. if not set, level will default to low"
    )

    # Set defaults if not specified
    if not options.loglevel:
        loglevel = 1
    else:
        loglevel = options.loglevel

    if not options.logfile:
        logfilename = 'datafeed.log'
    else:
        logfilename = options.logfile

    common.setup_logger(False, logfilename, loglevel) 

       # and so on ...



        #### dbfuncs.py


import logging

    # not sure how to 'bind' to the logger in __main__.py
    logging.getLogger('sqlalchemy.engine').setLevel(logging.INFO)

    engine = create_engine('postgres://postgres:pwd@localhost:port/dbname', echo=True)

[Edit2]

Common module sets the logger up correctly, and I can use the logger in my other modules that import common.

However in dbfuncs module, I am getting the following error/warning:

No handlers could be found for logger "sqlalchemy.engine.base.Engine

Answer 1

In addition to echo parameter of create_engine() there is a more flexible way: configuring logging to echo engine statements:

import logging
logging.basicConfig()
logging.getLogger('sqlalchemy.engine').setLevel(logging.INFO)

See Configuring Logging section of documentation for more information.

Answer 2

You can see the SQL statements being sent to the DB by passing echo=True when the engine instance is created (usually using the create_engine() or engine_from_config() call in your code).

For example:

engine = sqlalchemy.create_engine('postgres://foo/bar', echo=True)

By default, logged statements go to stdout.