No Dangling Else Trap

One additional benefit of using indentation is that the “dangling else ambiguity” is impossible in Python. For example, here is some C++ code:

• • if (x > 0)

    • o

      • if (y > 0)

        • + z = 1;
    else
    • o z = 5;

The code sets z to 1 if both x and y are greater than 0, and it looks like it will set z to 5 if x is less than or equal to 0. But in fact, it sets z to 5 only if x is greater than 0 and if y is less than or equal to 0. Here is what it means in Python:

• • if x > 0:

    • o

      • if y > 0:

        • + z = 1
        else:
        • + z = 5

And if we really want z set to 5 if x is less than or equal to 0, we would write this:

• • if x > 0:

    • o

      • if y > 0:

        • + z = 1
    else:
    • o z = 5

Thanks to Python’s indentation-based block structure, we avoid the “dangling else” trap.