When the merge executes I get an error 001156 on OID 1 that says i can't write value 'A' to output field ALPHA.

If I try to append one to the other I get the same error. They were originally the same feature class why won't they go back together?

  • To which line does the error refer? The CalculateField or the Merge?
    – Tom
    Commented Jan 24, 2019 at 17:42
  • To the merge line
    – jtgis
    Commented Jan 24, 2019 at 17:53
  • 2
    If you used an expression in CalculateField you could avoid needing to split and re-merge. Something like arcpy.CalculateField_management("in_memory/originalfc","ALPHA",'"A" if !attr! <= 0 else "B"',"PYTHON")
    – mikewatt
    Commented Jan 24, 2019 at 18:16
  • 2
    Does it work when performed in a regular workspace like a file geodatabase?
    – PolyGeo
    Commented Jan 24, 2019 at 19:16
  • @Polygeo I have the same type of issue. When I am working with a regular file geodatabase merging works fine, when I am working with in memory workspace I get the same error as above..
    – GeoF
    Commented Jul 9, 2021 at 9:02

2 Answers 2


IN_MEMORY is a sort of simplified "geodatabase" but obviously in memory not on a disk. Now the important thing to appreciate is that it is like a geodatabase and thus honours the many specification, but not all, of a geodatabase.

I believe you have been unlucky enough to have chosen a geodatabase RESERVED field name, ALPHA. Suggest you call it something else like "myALPHA".


I got this error again and the reason it is coming up for me has to do with domains and in_memory feature classes and combining datasets in arcpy.


FeatureClass1 has attribute [NAME] with a coded value of "MR" and the domain shows "Mister" (that fc was a selection from a larger dataset and the domain assigned in the gdb carried over to the in_memory)

FeatureClass2 has attribute [NAME] with a value of "MR" no domain attached

If I merge the two I get error 001156. If I append FeatureClass1 to FeatureClass2 I get error 001156.

But if I append FeatureClass2 to FeatureClass1 it works!

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