Catch exception in loop head but not in loop body-CodePudding

What's the best way to catch exceptions which occur in the *loop header instead of the whole loop or body.

Take the following example

for value in complex_generator(): # throws exceptions I might want to catch
    ... # do work here - but don't catch any exception

What I don't consider helpful is wrapping the whole loop in a try and except block like so:

try:
    for value in complex_generator(): # throws exceptions I might want to catch
        ... # exceptions raised here will also be caught :(
except Exception:
   ... # handle exception here

Inspired from golang one might encapsulate the try-except-block and always return two elements:

def wrapper(iterable):
    try:
        for value in iterable:
            yield value, None
    except Exception as e:
        yield None, e


for value, err in wrapper(complex_generator()):
    if err != None:
        ... # handle error
    else:
        ... # do work but don't catch any exception here

This however doesn't feel pythonic and a type checker would also require a additional check. Any ideas?

CodePudding user response：

Get the generator first?

try:
  cg = complex_generator()
except Exception as e:
  cg = [] # could alternatively have a success boolean here, and wrap the `for` loop below in an `if`
for value in cg:
  ...

CodePudding user response：

There are two levels of errors: Those that the generator throws, and those that your worker code throws. I would use a nested try:

try:
    for value in complex_generator():
        try:
            # do work here
        except ValueError:
            # catch ValueError and keep going
        except OtherError:
            # catch OtherError and keep going

        # any other error breaks the loop
except ExpectedGeneratorError:
    # handle generator exception here
except:
    # handle more errors

You could extract the inner part into a worker function to keep things tidy.