Try/Except not working when fetching from SQLite database - sql

Question

I am bulding an app that extracts different attributes from an XML file in my iTunes library export. Since not every song has a genre, finding one per song will not always work. When the program inserts a genre into the 'Genre', it creates an automatic id
CREATE TABLE Genre (
id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT UNIQUE,
name TEXT UNIQUE
);
The thing is, when there is no 'Genre' found in the XML file, nothing is inserted, meaning I cannot later fetch the id created by the addition of something to put it in a tabke in order to do relational queries. To stop this, I put the insert and fetch inside a try/except that would catch the
TypeError: 'NoneType' object is not subscriptable
error when it is unable to
cur.fetchone()[0]
. In the except I established the genre as "No Genre" to generate the unique id for the case that there were no genre found. This, even though it catches an exception in the try block, isnt running. Error:
TypeError Traceback (most recent call last)
~\Documents\python\tracks.py in <module>
83 cur.execute('SELECT id FROM Artist WHERE name = ? ', (genre, ))
---> 84 genre_id = cur.fetchone()[0]
85 except:
TypeError: 'NoneType' object is not subscriptable
During handling of the above exception, another exception occurred:
TypeError Traceback (most recent call last)
~\Documents\python\tracks.py in <module>
87 cur.execute('''INSERT OR IGNORE INTO Genre (name) VALUES ( ? )''', ( genre, ) )
88 cur.execute('SELECT id FROM Artist WHERE name = ? ', (genre, ))
---> 89 genre_id = cur.fetchone()[0]
90
91
TypeError: 'NoneType' object is not subscriptable
Help! Why isn't the try/except not working?
try:
cur.execute('''INSERT OR IGNORE INTO Genre (name) VALUES ( ? )''', ( genre, ) )
cur.execute('SELECT id FROM Artist WHERE name = ? ', (genre, ))
genre_id = cur.fetchone()[0]
except:
genre = "No Genre"
cur.execute('''INSERT OR IGNORE INTO Genre (name) VALUES ( ? )''', ( genre, ) )
cur.execute('SELECT id FROM Artist WHERE name = ? ', (genre, ))
genre_id = cur.fetchone()[0]

Answer 1

Okay, solution first
import sqlite3
conn = sqlite3.connect('systeminfo.db')
cur = conn.cursor()
genre = 'test'
try:
cur.execute('''INSERT OR IGNORE INTO Genre (name) VALUES ( ? )''', ( genre, ) )
cur.execute('SELECT id FROM Artist WHERE name = ? ', (genre, ))
genre_id = cur.fetchone()
if genre_id is None:
print('Select did not find any artist for {} genre'.format(genre))
else:
print('Select resulted in Artist ID {}'.format(genre_id[0]))
except Exception as e:
print('Exception: {}'.format(e))
raise Exception(e)
conn.commit()
conn.close()
See, there's a possibility that cur.fetchone() can result in a row or None if there is no row. So, let's do an if..then to check for None.
Here's what seems to be happening. Scroll way at the bottom of the post to find your answer.
Table
CREATE TABLE Artist (
id integer not null primary key autoincrement,
name text,
genre text
);
CREATE TABLE Genre (
id integer not null primary key autoincrement,
name text unique
);
Data
sqlite> select * from Artist;
Run Time: real 0.000 user 0.000086 sys 0.000054
sqlite> select * from Genre;
Run Time: real 0.000 user 0.000092 sys 0.000064
sqlite>
Basically, there's no data.
Your code
import sqlite3
conn = sqlite3.connect('systeminfo.db')
cur = conn.cursor()
genre = 'test'
try:
cur.execute('''INSERT OR IGNORE INTO Genre (name) VALUES ( ? )''', ( genre, ) )
cur.execute('SELECT id FROM Artist WHERE name = ? ', (genre, ))
genre_id = cur.fetchone()[0]
except:
cur.execute('''INSERT OR IGNORE INTO Genre (name) VALUES ( ? )''', ( genre, ) )
cur.execute('SELECT id FROM Artist WHERE name = ? ', (genre, ))
genre_id = cur.fetchone()[0]
conn.commit()
conn.close()
Issue
Your errors are happening in 2 places. In your try block, there's an error at genre_id = cur.fetchone()[0]. Once the error is hit, the control goes to except block. In that block, the code is repeated. That means, the error is repeated. In that block, there is no error handling because it IS error handling. So, python throws another error in except block for the same thing genre_id = cur.fetchone()[0].
Clean up the issue
import sqlite3
conn = sqlite3.connect('systeminfo.db')
cur = conn.cursor()
genre = 'test'
try:
cur.execute('''INSERT OR IGNORE INTO Genre (name) VALUES ( ? )''', ( genre, ) )
cur.execute('SELECT id FROM Artist WHERE name = ? ', (genre, ))
genre_id = cur.fetchone()[0]
except Exception as e:
print('Exception: {}'.format(e))
conn.commit()
conn.close()
Alright. In except we want to handle the exception and perhaps print out the exception. So, we will use except Exception as e. Now, error information is in e. We print that out and that's it.
$ python myfile.py
Exception: 'NoneType' object is not subscriptable
But what if I want to show where the error is?
Add raise Exception(e) right under print('Exception: {}'.format(e)). So, the result becomes this:
$ python myfile.py
Exception: 'NoneType' object is not subscriptable
Traceback (most recent call last):
File "testy.py", line 9, in <module>
genre_id = cur.fetchone()[0]
TypeError: 'NoneType' object is not subscriptable
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "testy.py", line 12, in <module>
raise Exception(e)
Exception: 'NoneType' object is not subscriptable
That raise statement lists out on the screen where issue happened.