CODEWITHSUNDEEP
pythonsqlalchemytry-except
I159
I159

Reputation: 31189

Try two expressions with `try except`

I have two expressions. I need to try one expression, if it is raise an exception try another, but if the second raises an exception too - to raise the exception.

I tried this, but it is looks ugly and I am not sure it is the best way to solve this issue:

try:                                                           
    image = self.images.order_by(func.random()).limit(1)       
except:                                                        
    try:                                                       
        image = self.images.order_by(func.rand()).limit(1)     
    except ProgrammingError:                                   
        raise ProgrammingError(                                
            'The database engine must be PostgtreSQL or MySQL')

How do you do it?

Upvotes: 8

Views: 15215

Answers (5)

kindall
kindall

Reputation: 184270

Use a loop:

for methname in ("random", "rand"):
    try:
        image = self.images.order_by(getattr(func, methname)()).limit(1)
        break
    except ProgrammingError:
        continue
else:
    raise ProgrammingError("The database engine must be PostgtreSQL or MySQL")

The loop's else clause is executed only if the loop terminates normally (i.e., without a break) which is why we break after doing the image assignment. If you consider this too tricksy, because the else clause is so infrequently used with for, then this would also work:

image = None
for methname in ("random", "rand"):
    try:
        image = self.images.order_by(getattr(func, methname)()).limit(1)
    except ProgrammingError:
        continue
if not image:
    raise ProgrammingError("The database engine must be PostgtreSQL or MySQL")

Upvotes: 7

Oleh Prypin
Oleh Prypin

Reputation: 34116

Making a separate function is very helpful.

def get_random_image(self):
    for rand in func.random, func.rand:
        try:                                                           
            return self.images.order_by(rand()).limit(1)
        except ProgrammingError:                                                        
            pass
    raise ProgrammingError('This database engine is not supported')

Upvotes: 5

Martijn Pieters
Martijn Pieters

Reputation: 1123420

In this particular case, I'd actually try and detect the database before selecting the function. Can you reach the database connection from your code? If so, just switch on the drivername:

random = None
if drivername == 'postgres':
    random = func.random
elif drivername == 'mysql':
    random = func.rand
else:
    raise ValueError('This module requires that you use PostgreSQL or MySQL')

Then, when selecting images, use the random value:

image = self.images.order_by(random()).limit(1)

Upvotes: 3

Michel Keijzers
Michel Keijzers

Reputation: 15367

If you want to check if rand or random is a function of a class, you also could use

if 'rand' in dir(some object of a class)

Upvotes: 0

Michel Keijzers
Michel Keijzers

Reputation: 15367

Actually it MIGHT be a design flaw. Raising exceptions is to act on an event that should normally not occur. If you want to do something functionally into an except (except for handling the exception), than it looks like the first statement that you want to try is not a statement that should get an exception at all.

So instead of:

try:
    do statement 1
except ...
    try:
        do statement 2
    except:

think about :

if (statement_1 result == ...)
    try:
         do statement 2
    except:

Upvotes: 1

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