Why is the following list comprehension not working in Python?

Question

I have a list called row :

row = ['1234', [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31]]

I want to change row[1] , so that few numbers get replaced with a time and the rest gets replaced with the string 'ABS' . Something like this:

['1234', ['ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', '09:54:59', '09:55:18', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS', 'ABS']]

Here is my code:

for obj in row_objs:
    row[1] = [str(obj.punch.time()) if x==obj.punch.day else 'ABS' for x in row[1]]

But I keep getting ALL the numbers replaced by 'ABS' with the above code.

When I do this:

for obj in row_objs:
    row[1] = [str(obj.punch.time()) if x==obj.punch.day else x for x in row[1]]

The time is placed correctly but the other numbers are still there.

['1234', [1, 2, 3, 4, 5, 6, 7, '09:54:59', '09:55:18', 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31]]

What is going wrong here?

Answer 1

You are replacing the list each iteration. The last obj in your for loop doesn't match any of the integers, so the end result is that you generated a list with only 'ABS' strings. Your second loop works because you take the already-replaced values by re-using x ; that can be the original integer, or the later time() replacement.

Don't use a for loop. Collect all your obj.punch values in a dictionary mapping the day attribute to the str(...time()) result, then use a list comprehension to look up matches:

days = {obj.punch.day: str(obj.punch.time()) for obj in row_objs}
row[1] = [days.get(x, 'ABS') for x in row[1]]

Now you replace row[1] just once.

Answer 2

for obj in row_objs:
    row[1] = [str(obj.punch.time()) if x == obj.punch.day else isinstance(x, int) and 'ABS' or x for x in row[1]]