concurrent.futures does not throw exception when using try except

Question

When using concurrent.futures.ThreadPoolExecutor() it doesn't throw an exception for try except.

For eg:

num = [1,2,3,4,5]
def div():
    for i in num:
        return i/0
    print('Done')

try:
    div()
except:
    print('Not Divisible by 0')

The above code prints the except part but the below code doesn't

num = [1,2,3,4,5]
def div(i):
    return i/0

try:
    with concurrent.futures.ThreadPoolExecutor() as executor:
        executor.map(div, num)
    print('Done')
except:
    print('Not Divisible by 0')

Answer 1

executor.map does not raise the exception right away. Instead that exception will be raised when the value is retrieved from the iterator. For example try this,

>>> num = [1,2,3,4,5]
>>>
>>> def div(i):
...     return i/0
...
>>> import concurrent.futures
>>>
>>> with concurrent.futures.ThreadPoolExecutor() as executor:
...     result = executor.map(div, num)
...
>>> result # Not evaluated yet
<generator object Executor.map.<locals>.result_iterator at 0x0000019E60A36570>
>>> list(result) # This will raise the `ZeroDivisionError` exception

So to get the expected behavior, evaluate the return value of executor.map within the try block.