Using the QGIS Advanced Python Field Calculator, I am able to assign the new field a value from a feature an existing field using value = <Attribute_1>. I run into a problem when I am using more complex code like the following used to calculate the absolute differences in azimuths:

diff = <Az_1> - <Az_2>
absDiff = abs(diff)

if absDiff == 180:
    value = absDiff
elif absDiff < 180:
    value = diff
elif <Az_1> > <Az_2>:
    value = absDiff - 360
    value = 360 - absDiff

When I run the code through a compile and substitute the Az_1 and Az_2 variables with integers, the code works, yet I get the following error when executing the model the code is a part of in QGIS:

TypeError: unsupported operand type(s) for -: 'NoneType' and 'float' 
  • 2
    At least one of <Az_1> values must be NULL. Check <Az_1> values in Attribute Table. Nov 23, 2020 at 19:09
  • What do you mean by "running the code through a compile"? Nov 23, 2020 at 19:12
  • Compiler. Thank you for the help, I will check for null values.
    – tacarn
    Nov 24, 2020 at 13:27
  • I removed Null values by using the Extract by Expression algorithm to extract features where "Az_1 IS NOT NULL AND Az_2 IS NOT NULL" into a new temporary layer, but I am still getting the same error.
    – tacarn
    Nov 24, 2020 at 14:09
  • 1
    Use "Az_1 IS NOT NULL OR Az_2 IS NOT NULL" to extracte features into a temporary layer, then try again. This may work. Nov 24, 2020 at 14:29

1 Answer 1


I came up with the following statement which checks for a 'none' value, and if detected, reassigns it a value of 181. Since 181 is not a valid difference between two azimuths, it can then be deleted using the Extract by Value algorithm. If the value of Az_1 and Az_2 are both numbers, the program calls a function which will run the azimuth calculation code described in the original post.

if Az_1 is None:
    value = 181
elif AZ_2 is None:
    value = 181
    value = abs(AzimuthDifference())

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