arcpy.sa.CostDistance from Python gives error about number of args but executes fine through Toolbox

Question

I am attemping to call arcpy.sa.CostDistance to calculate the cost distance to a source point as well as generate the optional Cost Back link raster:

arcpy.sa.CostDistance("input", cost, "costDist", "", "costBackLink")

Which yields this error when executed within the Python Windows of ArcGIS:

>>> arcpy.sa.CostDistance("input", cost, "costDist", "", "costBackLink")
Runtime error 
Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: CostDistance() takes at most 4 arguments (5 given)

Oddly though, the exact same processing when executed via the Toolbox executes fine. The inline help that appears next to the Python Window when entering a function name also references the 5 arguments.

Why is this happening?

Answer 1

Take a look at the input guidelines here. There are four inputs. Also, the third input is a distance value and should thus be either a hard coded number or a variable representing a number. Your return should be assigned to the operation itself:

returnRaster = arcpy.sa.CostDistance("input", cost, numericValue, "costBackLink")