For discussing the best approach for handling type errors in Python 3

type-error-handling-conditional-vs-try-except.py

def increment_list_by(list, num):
    '''Increment all items in a list by `num`
    provided all items are `int` types.
    Return modified list if successful, or original list otherwise.'''
    incremented_list = []

    # =============
    # Note: below are 2 separate approaches to implement this function
    # =============

    # Approach 1: handle type error with conditional
    for idx, item in enumerate(list):
        if not (type(item) is int or type(item) is float):
            print('Cannot increment list with non-integer values, returning as-is')
            return list
        incremented_list.append(list[idx] + 1)
    return incremented_list

    # Approach 2: handle type error with try-except
    for idx, item in enumerate(list):
        try:
            incremented_list.append(list[idx] + 1)
        except TypeError:
            print('Cannot increment list with non-integer values, returning as-is')
            return list
    return incremented_list


# TEST (same result for both implementations)

print(increment_list_by([1, 3, 99, 3, 2, 9.9], 1))
#=> [2, 4, 100, 5, 5, 5, 4, 3, 10.9]

print(increment_list_by([1, 3, 99, 3, '2', 9.9], 1)) # note '2' instead of 2
#=> Cannot increment list with non-integer values, returning as-is
# [1, 3, 99, 4, 4, 4, 3, '2', 9.9]

After discussing with some friends, I'd pick approach #2 in this case.

See this relevant discussion on Stackoverflow for how to think about choosing one or the other (it's not a hard rule): https://stackoverflow.com/questions/12265451/ask-forgiveness-not-permission-explain