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    <p><a href="http://mysql-python.sourceforge.net/MySQLdb-1.2.2/private/_mysql_exceptions-module.html" rel="noreferrer">This link</a> shows the various types of Errors that you can catch. <code>MySQLdb.Error</code> is the standard base class from which all other MySQL Errors are derived.</p> <p>I usually use <code>MySQLdb.Error</code> because it lets you focus on errors relating to MySQLdb itself. By contrast <code>StandardError</code> will catch almost all the exceptions (not something you want if you want better debugging capability). Plus the use of <code>MySQLdb.Error</code> allows you to display the exact error message (<strong>MySQL</strong> error number and all) so that you can debug it faster.</p> <p>Coming to the first part of the question, in case of database statements it is (usually) necessary to rollback transactions (if they are supported) in case of error.</p> <p>The methodology that I follow is to wrap each <code>execute</code> statement in a try except clause (catching <code>MySQLdb.Error</code>) and using rollback if there is an an error before printing the error message and exiting. </p> <p>However, there is a catch. In MySQLdb the changes that you make to DB are <strong>not</strong> actually written to the database until you explicilty call commit. So, logically, rollback is not necessary. </p> <p>As an example,</p> <pre><code>conn = MySQLdb.connection(db=, host=, passwd=, user=) cur = conn.cursor() #Say you have a table X with one entry id = 1 and total = 50 cur.execute("update X set total = 70 where id = 1") #Actual DB has not yet changed cur.execute("update X set total = 80 where id = 1") #Actual DB has still not changed </code></pre> <p>If you exit the program <em>without</em> commiting, the value in DB will still be 50 because you never called commit().</p> <p>This is how you would ideally do it:</p> <pre><code>conn = MySQLdb.connection(db=, host=, passwd=, user=) cur = conn.cursor() #Say you have a table X with one entry id = 1 and total = 50 try: cur.execute("update X set total = 70 where id = 1") except MySQLdb.Error,e: print e[0], e[1] conn.rollback() cur.close() conn.close() #print lengthy error description!! sys.exit(2) #Note: Value in table is still 50 #If you do conn.commit() here, value becomes 70 in table too!! try: cur.execute("update X set total = 80 where id = 1") except MySQLdb.Error,e: print e[0], e[1] conn.rollback() cur.close() conn.close() #print lengthy error description!! sys.exit(2) #Value in DB will be #a) 50 if you didn't commit anywhere #b) 70 if you committed after first execute statement conn.commit() #Now value in DB is 80!! cur.close() conn.close() </code></pre>
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