CODEWITHSUNDEEP
pythonpython-3.x
Szymon
Szymon

Reputation: 633

Handle and checking whether an exception occurred?

I have a function that handles an exception, and later I want to know if exception occurs. Here is my code:

def cx_runquery(query, conn_obj, fetch='all'):
    """
    Run Query on the database

    :param query: Query to execute (str)
    :param conn_obj: Connection Object (obj)
    :param fetch: Parameter with default='all' (str)
    """
    cur = conn_obj.cursor()

    try:
        cur.execute(query)
        if fetch == 'all':
            return cur.fetchall()
        elif fetch == 'one':
            return cur.fetchone()
        else:
            raise ValueError('not a vaild option')
    except cx_Oracle.DatabaseError as e:
        error, = e.args
        return error

Above function called and return data or error to the variable like following:

output = cx_runquery('SELECT somerow FROM sometable', ora_conn, fetch='one')

If an error occurs (for example ORA-00942: table or view does not exist) then output variable is cx_Oracle._Error object.

I wrote the following test, but I don't know it's good approach?

if isinstance(output, cx_Oracle._Error):
    output = 'Database errror occured'

In the pseudo code I want to do following:

if cx_Oracle._Error occured:
    do something ...
else
    do something ...

How to checking whether an exception (cx_Oracle._Error in this case) occurred?

Upvotes: 1

Views: 706

Answers (3)

emnoor
emnoor

Reputation: 2708

Do not include the try block in the function. Use try block where you want to catch it!

def cx_runquery(query, conn_obj, fetch='all'):
    """
    Run Query on the database

    :param query: Query to execute (str)
    :param conn_obj: Connection Object (obj)
    :param fetch: Parameter with default='all' (str)
    """
    cur = conn_obj.cursor()

    cur.execute(query)
    if fetch == 'all':
        return cur.fetchall()
    elif fetch == 'one':
        return cur.fetchone()
    else:
        raise ValueError('not a vaild option')

Then use it like:

try:
    output = cx_runquery('SELECT somerow FROM sometable', ora_conn, fetch='one')
except cx_Oracle.DatabaseError as e:
    output = "Database error occurred"
    # do other error handling

Upvotes: 1

Anurag
Anurag

Reputation: 3114

IMO the best way to achieve is to raise the exception from child function and catch it in parent function. like -

def parent_function(query, conn_obj, fetch):
    try:
        cx_runquery(query, conn_obj, fetch='all')

        #no error do something

    except:
        #error occured do somthing

def cx_runquery(query, conn_obj, fetch='all'):
    """
    Run Query on the database

    :param query: Query to execute (str)
    :param conn_obj: Connection Object (obj)
    :param fetch: Parameter with default='all' (str)
    """
    cur = conn_obj.cursor()

    try:
        cur.execute(query)
        if fetch == 'all':
            return cur.fetchall()
        elif fetch == 'one':
            return cur.fetchone()
        else:
            raise ValueError('not a vaild option')
    except cx_Oracle.DatabaseError as e:
        error, = e.args
        raise

So in parent_function you can perform different actions on different behavior.

Even you can raise your custom exception or New exception from child function like

raise Excetion('some exception')

Also you can raise the same type cx_Oracle.DatabaseError exception from child function and in parent function catch that specific type of exception and do required actions

Upvotes: 1

jeffknupp
jeffknupp

Reputation: 6264

In Python, you can return multiple results from a function. You might think about returning results, error where one of those values is None. That way, the client code can check explicitly if an error condition exists. This is especially useful if you start to encounter exceptions that are not of the type cx_Oracle._Error.

The other approach would be to simply let the exception go uncaught and let your calling code deal with the error. This is a more idiomatic approach, as you really can't do anything useful with the error within the function itself. Leave it up to the client what happens when an exception is raised.

Upvotes: 1

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