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.