get error using ThreadPoolExecutor with map

Question

I have noticed that my code does not output an error or break when there is an error inside function(x) when it is called by ThreadPoolExecutor():

import concurrent.futures

def function(x):
   # do sth invalid such as
   x = y + 1 #there is no y variable

input_list = [1,2,3]
with concurrent.futures.ThreadPoolExecutor() as executor: 
    executor.map(function,input_list)  

How can I get the error output using map() instead of submit() for the executor?

Answer 1

executor.map returns an iterable that must be iterated to get individual results, including any exception that might have been thrown:

import concurrent.futures

def function(x):
    # do sth invalid such as
    if x == 2:
        raise ValueError("I don't like 2")
    return x, x ** 2

input_list = [1,2,3]
with concurrent.futures.ThreadPoolExecutor(3) as executor:
    results = executor.map(function,input_list)
    try:
        for x, return_value in results:
            print(f'{x} ** 2 = {return_value}')
    except Exception as e:
        print(e)

Prints:

1 ** 2 = 1
I don't like 2

Or you can iterate slightly differently and get the same results with:

import concurrent.futures

def function(x):
    # do sth invalid such as
    if x == 2:
        raise ValueError("I don't like 2")
    return x, x ** 2

input_list = [1,2,3]
with concurrent.futures.ThreadPoolExecutor(3) as executor:
    results = executor.map(function,input_list)
    while True:
        try:
            x, return_value = next(results)
        except StopIteration:
            break
        except Exception as e:
            print(e)
        else:
            print(f'{x} ** 2 = {return_value}')

However, if you use class ThreadPool from package multiprocessing.pool, then you can get all the results, that is get results beyond any exception that may have been thrown (using imap):

from multiprocessing.pool import ThreadPool

def function(x):
    # do sth invalid such as
    if x == 2:
        raise ValueError("I don't like 2")
    return x, x ** 2

input_list = [1,2,3]
with ThreadPool(3) as executor:
    results = executor.imap(function,input_list)
    while True:
        try:
            x, return_value = next(results)
        except StopIteration:
            break
        except Exception as e:
            print(e)
        else:
            print(f'{x} ** 2 = {return_value}')

Prints:

1 ** 2 = 1
I don't like 2
3 ** 2 = 9