running scripts of codeblocks in arcpy with syntax error

Question

I am having a trouble in running the following scripts:

import arcpy
arcpy.env.workspace = r'C:\MyData\my_geodatabase.gdb'
max = 123
min = 0
mean_s = 12.3
expression1 = "test(!mydensity!)"
codeblock1 = """def test(value):
   if value <(%mean_s% + %min%)/2:     # if value <(12.3 + 0)/2:
      return "cat1"
   elif value < (%max% + %mean_s%)/2 and value >=(%min% + %mean_s%)/2:    #elif value < (123 + 12.3)/2 and value >=(12.3+ 0)/2:
      return "cat2"
   """
a ="a"
b = "b"
arcpy.CalculateField_management(a, b,expression1,"PYTHON", codeblock1)

this python script returns with error : arcgisscripting.ExecuteError: Failed to execute. Parameters are not valid. ERROR 000989: Python syntax error: Parsing error SyntaxError: invalid syntax (line 2) Failed to execute (CalculateField).

but the python works if I did the change in the code indicated after the (# ...)in the code above(in other words,if I change those %mean%,%max%,%min% to the real numbers, it works)

I guess it is because the codeblock in python wont recognize the max min mean_s defined at the beginning or something else goes wrong?

Answer 1

"my gdb" is nothing. it's a variable that you haven't declared. Instead, you want to put the path of your geodatabase in there; e.g.:

arcpy.env.workspace = r'C:\MyData\my_geodatabase.gdb'

You will run into other errors after that, but this is the current one.

You need to end the triple-quotes in your codeblock1. On the empty line above

a='a'

insert

"""

EDIT: Okay, with the new corrections to your code, we can get to the meat of it: your codeblock1 doesn't actually have the values; you're only putting in the variable names. Try this:

codeblock1 = """def test(value):
   if value <({0} + {1})/2:
      return "cat1"
   elif value < ({2} + {0})/2 and value >=({1} + {0})/2:
      return "cat2"
   """.format(mean_s, min, max)

Also, please do not assign min and max as variable names, since they're built-in Python variables. You won't be able to use the min and max functions anymore.

Answer 2

I assume the paths, layer (a) and fields (b) are just representative. The code block of a field calculation written as a string has no access to variables defined in a python script. For your code to work you have to do value substitution in the string using the python format method. Your calculation also needs to output a value even when both of the conditions you wrote in your codeblock are false, so you need to include a final else clause. For example, if value = 70 then both original conditions fail (both 70 < 6.15 is False and (70 < 67.65 and 70 >= 6.15) is False).

import arcpy
arcpy.env.workspace = r'C:\MyData\my_geodatabase.gdb'
myMax = 123
myMin = 0
myMean_s = 12.3
expression1 = "test(!mydensity!)"
codeblock1 = """def test(value):
  if value < ({0} + {1})/2:     # if value <(12.3 + 0)/2:
    return 'cat1'
  elif value < ({2} + {0})/2 and value >=({1} + {0})/2:    #elif value < (123 + 12.3)/2 and value >=(12.3+ 0)/2:
    return 'cat2'
  else
    return None""".format(myMean_s, myMin, myMax)

a = "a"
b = "b"
arcpy.CalculateField_management(a, b, expression1,"PYTHON_9.3", codeblock1)