How to handle exceptions in python using the `with` statement in python?

Question

Assuming this piece of code:

connection = get_some_connection() # maybe from oursql
with connection.cursor() as cursor:
    cursor.execute('some query')

I understand that after finishing cursor.close() will be executed automatically. What about exceptions? Must I put them inside?

connection = get_some_connection() # maybe from oursql
with connection.cursor() as cursor:
    try:
        cursor.execute('some query')
    except IntegrityError, e:
        # handle exceoption

Or is there a nicer way to handle them with the with statement?

cdhowie · Accepted Answer · 2011-08-13 22:43:05Z

4

with x as y: z() is basically syntactic sugar for:

y = x
y.__enter__()
try:
    z()
finally:
    if y.__exit__: y.__exit__()

This is not completely accurate, but that's the gist of it. Note that __exit__() will be passed exception information if an exception was thrown (see the docs) so you can "handle" an exception this way, but this will not keep the exception from being thrown up the call stack.

If you want to handle the exception gracefully and consume it, you need to use a try/catch block. It can be inside the with block or outside of it, as long as the try block is active when the exception is raised.

edited Aug 13, 2011 at 22:43

answered Aug 13, 2011 at 20:11

cdhowie

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1 Comment

Karl Knechtel Over a year ago

Because of how the exception propagates, could you not also put the try/catch immediately outside the with block? ;)

unutbu · Accepted Answer · 2011-08-13 23:46:11Z

2

In the particular case of oursql,

with some_connection.cursor() as cursor:
    do_something_with(cursor)

is equivalent to

cursor = some_connection.cursor()
try:
    do_something_with(cursor)
except:
    some_connection.rollback()
    raise
else:
    some_connection.commit()
finally:
    cursor.close()

As you can see, what a with statement does depends on the context manager (e.g. some_connection.cursor()`).

with connection.cursor() as cursor:
    try:
        cursor.execute('some query')
    except IntegrityError as e:
        # handle exception

may or may not be the correct way to handle an IntegrityError -- you might want to handle the IntegrityError in some outer scope.

For example, if you had some generic function which logs queries such as

def log_query(query):
    logger.info(query)
    with connection.cursor() as cursor:
        cursor.execute(query)

try:
    log_query(query)
except IntegrityError as err:
     # handler error

you may not want to handle IntegrityErrors inside log_query, but rather at a later stage.

edited Aug 13, 2011 at 23:46

answered Aug 13, 2011 at 20:34

unutbu

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1 Comment

Karl Knechtel Over a year ago

Note that if you do handle the error internally, and don't re-raise, then you had better handle it in a way that is compatible with the some_connection.commit() that happens when the cursor's with-block cleans up :)

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