CODEWITHSUNDEEP
pythonpython-3.xpython-2.7exception
Evan
Evan

Reputation: 557

How to create a python exception with multiple causes?

How do I raise a python exception with multiple causes, similar to Java's addSuppressed() feature? For example I have list of multiple methods to try and if none of them work I want to raise an exception that includes the exceptions from all the methods that were tried. I.e.:

exceptions = []
for method in methods_to_try:
  try:
    method()
  except Exception as e:
    exceptions.append(e)
if exceptions:
  raise Exception("All methods failed") from exceptions

But this code fails because the raise ... from ... statement expects a single exception and not a list. Python 2 or 3 solutions are acceptable. All back traces and exception messages must be preserved.

Upvotes: 6

Views: 1550

Answers (3)

Shriram V
Shriram V

Reputation: 1568

Once could raise each exception from the other to chain it. The only issue is with the cause order which may be misleading.

def combine_exc(exceptions):
   c_exc = None
   for ex in reversed(exceptions):
      try:
         if c_exc is None:
            raise ex
         else:
            raise ex from c_exc
      except Exception as ex1:
         c_exc = ex1

   return c_exc

Upvotes: 1

Stop harming Monica
Stop harming Monica

Reputation: 12620

Just pass the exceptions as arguments when creating the last exception.

for method in methods_to_try:
    try:
        method()
    except Exception as e:
        exceptions.append(e)
if exceptions:
    raise Exception(*exceptions)

They will be available en the args attribute.

Upvotes: 3

Bernhard
Bernhard

Reputation: 2251

Use logging instead

If using the logging module is acceptable for you, I guess it would be the cleanest to log the individual exceptions you caught and only raise a single overall exception.

If you only want to output the log-messages in case all tried functions error, this is an ideal case for the buffered logger from the logging cookbook You could modify it to always log at debug level, but raise the level to error, if all subfunctions raise an error.

Upvotes: 0

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