How does parse_dates work with pd.read_sql_query

Question

How is Pandas parse_date supposed to work when retrieving data from a MySQL database?

The documentation of Pandas 0.23 gives this information:

parse_dates : list or dict, default: None

List of column names to parse as dates.

Dict of {column_name: format string} where format string is strftime compatible in case of parsing string times, or is one of (D, s, ns, ms, us) in case of parsing integer timestamps.

Dict of {column_name: arg dict}, where the arg dict corresponds to the keyword arguments of pandas.to_datetime() Especially useful with databases without native Datetime support, such as SQLite.

I would like to retrieve for example some data from the MySQL Sakila database.

create table actor
(
    actor_id smallint(5) unsigned auto_increment
        primary key,
    first_name varchar(45) not null,
    last_name varchar(45) not null,
    last_update timestamp not null on update CURRENT_TIMESTAMP,
    constraint idx_unique_id_name
        unique (actor_id, last_name)
)

Here is some sample data:

INSERT INTO sakila.actor (actor_id, first_name, last_name, last_update) VALUES (1, 'PENELOPE', 'None', '2018-05-17 11:08:03');
INSERT INTO sakila.actor (actor_id, first_name, last_name, last_update) VALUES (2, 'NICK', 'WAHLBERG', '2006-02-15 04:34:33');
INSERT INTO sakila.actor (actor_id, first_name, last_name, last_update) VALUES (3, 'ED', 'CHASE', '2006-02-15 04:34:33');
INSERT INTO sakila.actor (actor_id, first_name, last_name, last_update) VALUES (4, 'JENNIFER', 'DAVIS', '2006-02-15 04:34:33');
INSERT INTO sakila.actor (actor_id, first_name, last_name, last_update) VALUES (5, 'JOHNNY', 'LOLLOBRIGIDA', '2018-05-17 11:14:15');
INSERT INTO sakila.actor (actor_id, first_name, last_name, last_update) VALUES (6, 'BETTE', 'Echt', '2018-05-17 11:13:57');
INSERT INTO sakila.actor (actor_id, first_name, last_name, last_update) VALUES (7, 'GRACE', 'MOSTEL', '2006-02-15 04:34:33');
INSERT INTO sakila.actor (actor_id, first_name, last_name, last_update) VALUES (8, 'MATTHEW', 'JOHANSSON', '2006-02-15 04:34:33');
INSERT INTO sakila.actor (actor_id, first_name, last_name, last_update) VALUES (9, 'JOE', 'SWANK', '2006-02-15 04:34:33');
INSERT INTO sakila.actor (actor_id, first_name, last_name, last_update) VALUES (10, 'CHRISTIAN', 'GABLE', '2006-02-15 04:34:33');

I use the default MySQL Python Connector:

    db_connection_url = 'mysql+mysqlconnector://' \
                        + mysql_config_dict['user'] \
                        + ":"  \
                        + mysql_config_dict['password'] \
                        + "@" \
                        + mysql_config_dict['host'] \
                        + ":" \
                        + mysql_config_dict['port'] \
                        + "/"  \
                        + mysql_config_dict['db_name']

    if('ssl_cert' in mysql_config_dict):

        ssl_args = {'ssl_ca':mysql_config_dict['ssl_ca']}

    else:
        ssl_args = ''

With these parameters

mysql_config_dict = {
    'user': 'root',
    'password': '',
    'host':  '127.0.0.1',
    'port': '3306',
    'db_name':  'sakila',
    'ssl_cert': os.getenv('SSL_CERT'),
    'ssl_key': os.getenv('SSL_KEY'),
    'ssl_ca': os.getenv('SSL_CA')
}

for obtaining an engine.

The Python snippet to retrieve the result set:

df = pd.read_sql_query('SELECT a.actor_id, a.last_name, a.last_update FROM sakila.actor a',parse_dates={'last_update':'%Y%m%d %H:%M:%S'},con=mysql_conn)

I obtain a KeyError:

Traceback (most recent call last):
  File "~/Development/python-virtual-env/lib/python3.5/site-packages/pandas/core/indexes/base.py", line 2442, in get_loc
    return self._engine.get_loc(key)
  File "pandas/_libs/index.pyx", line 132, in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5280)
  File "pandas/_libs/index.pyx", line 154, in pandas._libs.index.IndexEngine.get_loc (pandas/_libs/index.c:5126)
  File "pandas/_libs/hashtable_class_helper.pxi", line 1210, in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas/_libs/hashtable.c:20523)
  File "pandas/_libs/hashtable_class_helper.pxi", line 1218, in pandas._libs.hashtable.PyObjectHashTable.get_item (pandas/_libs/hashtable.c:20477)
KeyError: 'last_update'

When I use

df = pd.read_sql_query('SELECT a.actor_id, a.last_name, a.last_update FROM sakila.actor a',parse_dates=True,con=mysql_conn)

it works but I can see in the DataFrame view of IntelliJ that the column name of the column 'last_update' is prefixed with the Byte literal: b'last_update', which is strange.

What is the correct usage here when I want to treat multiple columns as date columns. Thanks!

score 4 · Answer 1 · answered May 24 '18 at 14:41

I pass the field names in a list to parse_dates when I invoke pd.read_sql with:

df= pd.read_sql(query, 
                connection, 
                parse_dates=['Date_of_creation', 
                             'Date_of_termination']
                )

You mentioned doing it with a dictionary for custom formatting:

fmt='%Y%m%d %H:%M:%S'

df= pd.read_sql(query, 
                connection, 
                parse_dates={'Date_of_creation':fmt,
                             'Date_of_termination':fmt}
                )

How does parse_dates work with pd.read_sql_query

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