CODEWITHSUNDEEP
pythonexceptionfor-loopiterator
jalchy888
jalchy888

Reputation: 155

Raise first exception encountered in for loop after loop completes

I have a loop that may raise an exception on one or more iterations. I want the loop to complete, THEN raise the first exception encountered, in the following example "raise on 4".

Example code:

e = None
for x in range(10):
    try:
        print x
        if x == 4:
            raise Exception('raise on 4')
        if x == 6:
            raise Exception('raise on 6')
    except Exception as e:
        print e
        continue
else:
    if e:
        raise

Output:

0
1
2
3
4
raise on 4
5
6
raise on 6
7
8
9
Traceback (most recent call last):
  File "<stdin>", line 7, in <module>
Exception: raise on 6

I can use the logging module to record them which is fine but I would like to raise on the first Exception if possible.

I am still fairly new to Python so I'm not entirely sure if the way I have constructed the loop with the "else" statement is very Pythonic or correct.

Upvotes: 2

Views: 9326

Answers (2)

Ashwini Chaudhary
Ashwini Chaudhary

Reputation: 250941

you can Append the errors to a list, and raise them later:

In [25]: errors=[]

In [26]: for x in range(10):
        try:
                print x
                if x == 4:
                        raise Exception('raise on 4')
                if x == 6:
                        raise Exception('raise on 6')
        except Exception as e:
                    errors.append(e)
                    continue
   ....:             
0
1
2
3
4
5
6
7
8
9

In [27]: for error in errors:
    raise error
   ....: 
---------------------------------------------------------------------------
Exception                                 Traceback (most recent call last)
<ipython-input-27-1f1d8ab5ba84> in <module>()
      1 for error in errors:
----> 2     raise error

Exception: raise on 4

Upvotes: 1

Martijn Pieters
Martijn Pieters

Reputation: 1121714

You have to store e in a separate variable:

firste = None
for x in range(10):
    try:
        print x
        if x == 4:
            raise Exception('raise on 4')
        if x == 6:
            raise Exception('raise on 6')
    except Exception as e:
        if firste is None:
            firste = e
        continue

if firste is not None:
    raise firste

Now firste is only set when the exception is raised the first time.

You don't need to use else in this case. Only use it when your for loop contains a break statement, which would skip the else suite, otherwise just put the test for firste below the for loop without using the redundant else suite indentation.

Upvotes: 3

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