"pymysql.err.InternalError: (1046, u'No database selected')" error message when running Python script from command line - pymysql

Question

I'm trying to run a Python script that connects to a MySQL database through PyMySQL. The script is effectively:
import pymysql
cnx = pymysql.connect(read_default_file = "/directory/my.cnf", cursorclass = pymysql.cursors.DictCursor)
cursor = cnx.cursor()
# Do stuff.
When I run the script in the interpreter, I don't get any errors, but when I try to run it from the command line, I get the following error:
Traceback (most recent call last):
File "s02_prepare_data_RNN.py", line 264, in <module>
(omniture, urls, years, global_regions) = get_omniture_data("omniture_results")
File "s02_prepare_data_RNN.py", line 76, in get_omniture_data
sso_to_accountid = get_sso_accountids()
File "s02_prepare_data_RNN.py", line 31, in get_sso_accountids
cursor.execute(query)
File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/cursors.py", line 134, in execute
result = self._query(query)
File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/cursors.py", line 282, in _query
conn.query(q)
File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/connections.py", line 768, in query
self._affected_rows = self._read_query_result(unbuffered=unbuffered)
File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/connections.py", line 929, in _read_query_result
result.read()
File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/connections.py", line 1125, in read
first_packet = self.connection._read_packet()
File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/connections.py", line 893, in _read_packet
packet.check_error()
File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/connections.py", line 369, in check_error
err.raise_mysql_exception(self._data)
File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/err.py", line 120, in raise_mysql_exception
_check_mysql_exception(errinfo)
File "/home/rdu/malcorn/.local/lib/python2.6/site-packages/pymysql/err.py", line 115, in _check_mysql_exception
raise InternalError(errno, errorvalue)
pymysql.err.InternalError: (1046, u'No database selected')

Answer 1

Modifying my code from:
import pymysql
cnx = pymysql.connect(read_default_file = "/directory/my.cnf", cursorclass = pymysql.cursors.DictCursor)
cursor = cnx.cursor()
# Do stuff.
to:
import pymysql
cnx = pymysql.connect(read_default_file = "/directory/my.cnf", cursorclass = pymysql.cursors.DictCursor)
cursor = cnx.cursor()
if __name__ == "__main__":
# Do stuff.
fixed the error. I had the thought that Python might be trying to execute the queries before the connection was established, so I tried putting the main part of my program under if __name__ == "__main__": and that fixed it. I'm still not 100% what is going on, though. I had assumed the code would wait for the connection to be established before proceeding to the following lines, but this fix suggests that's not the case.
It's also worth noting that I was only getting the error when running the original script from the command line on a server that has Python 2.6. When I ran the original script from the command line on my local machine that has Python 2.7, I did not get the error.
Anyway, if __name__ == "__main__": is good Python style, so I'll make sure to use it in the future.

Answer 2

Sometimes this may simply happen when the database URI is not built correctly. For me, the error occurred when my pymysql connection string (URL) looked like this:
mysql+pymysql://:#localhost/
To fix it, it needed to look like:
mysql+pymysql://<my-database-user-name>:#localhost/<my-database-name> # or replace localhost with your particular host name

Answer 3

If you don't select the database in connection, you must add this line.
connection = pymysql.connect(host=DB_host, user=DB_user, password=DB_password) # NO DB_NAME SPECIFED
cursor = connection.cursor()
cursor.execute(f'CREATE DATABASE IF NOT EXISTS {DB_NAME}') # ADD THIS LINE!
connection.select_db(DB_NAME)