How can I fix the ArcToolbox error TypeError: cannot concatenate 'str' and 'NoneType' objects?

Question

I am trying to run the ExportHTMLImageMaps tool in ArcGIS 10.3.1 to create HTML image maps to be used as static web geographic navigation maps, but I keep getting the Error TypeError: cannot concatenate 'str' and 'NoneType' objects

I checked each field being used in the tool and each is type String, so I'm not sure where to go from here. Below is a screen cap of my error log. I am a novice at Python and troubleshooting code.

Answer 1

Looking at your screen shot and reading the error message it is saying it cannot join None to a string. This is strongly suggesting that the command

arcpy.env.geographicTransformations

is returning None. So you need to look back at your code to identify where, if at all, you are setting this environment setting?

Answer 2

@Hornbydd is correct and that answer should be accepted. That's the error we get with Python 2 if we try to add two values and one is None.

Python 2.7.11 (v2.7.11:6d1b6a68f775, Dec  5 2015, 12:54:16)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> "message" + None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot concatenate 'str' and 'NoneType' objects

With Python 3 we also get a TypeError but the message has changed.

Python 3.5.2 (v3.5.2:4def2a2901a5, Jun 26 2016, 10:47:25)
[GCC 4.2.1 (Apple Inc. build 5666) (dot 3)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> "message" + None
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Can't convert 'NoneType' object to str implicitly