Python exception handling in list comprehension

Question

I have a Python function called plot_pdf(f) that might throw an error. I use a list comprehension to iterate over a list of files on this function:

Answer 1

You could create a catch object

def catch(error, default, function, *args, **kwargs):
    try: return function(*args, **kwargs)
    except error: return default

Then you can do

# using None as default value
result (catch(Exception, None, plot_pdf, f) for f in file_list)  

And then you can do what you want with the result:

result = list(result)  # turn it into a list
# or
result = [n for n in result if n is not None]  # filter out the Nones

Unfortunately this will not be even remotely C speed, see my question here

Answer 2

try:
    [plot_pdf(f) for f in file_list]  # using list comprehensions
except:
    print ("Exception: ", sys.exc_info()[0])
    continue

If plot_pdf(f) throws an error during execution of comprehension, then, it is caught in the except clause, other items in comprehension won't be evaluated.

It is not possible to handle exceptions in a list comprehension, for a list comprehension is an expression containing other expression, nothing more (i.e. no statements, and only statements can catch/ignore/handle exceptions).

Function calls are expression, and the function bodies can include all the statements you want, so delegating the evaluation of the exception-prone sub-expression to a function, as you've noticed, is one feasible workaround (others, when feasible, are checks on values that might provoke exceptions, as also suggested in other answers).

More here.

Answer 3

You're stuck with your for loop unless you handle the error inside plot_pdf or a wrapper.

def catch_plot_pdf(f):
    try:
        return plot_pdf(f)
    except:
        print("Exception: ", sys.exc_info()[0])

[catch_plot_pdf(f) for f in file_list]