Python syntax returning only part of the if/elif statement in Field Calculator?

Question

Using ArcGIS 10.7 desktop.

I want to reclassify a land use field using Field Calculator. Grasslands are conditionally reclassified using a 'veg_type' field (Avena, or Bromus) and a CWHR land use field (grassland values = Annual Grassland, AGS, Perennial Grassland). All other values in the CWHR field should stay the same.

When I run the Python Parser syntax the Grassland_Reclass field returns "Avena Grassland" (gridcode = 1) for the assigned land use values, but also for grasslands categorized as "Bromus' (gridcode = 0). The default CWHR values are added, but the elif condition (gridcode = 0) is not returned. I've also tried with the veg_type field but I get the same result.

def Reclass(gridcode, CWHR_TYPE):
  if gridcode == 1 and CWHR_TYPE == "Annual Grassland" or CWHR_TYPE == "AGS" or CWHR_TYPE == "Perennial Grassland":
    return "Avena Grassland"
  elif gridcode == 0 and CWHR_TYPE == "Annual Grassland" or CWHR_TYPE == "AGS" or CWHR_TYPE == "Perennial Grassland":
    return "Bromus Grassland"  
  else:
    return CWHR_TYPE

Expression: Grassland_Reclass =

Reclass(!gridcode!, !CWHR_TYPE!)

Answer 1

Logical operator precedence and order of operators is causing your issue. Try grouping your ors in parentheses:

e.g.

Wrong:

def Reclass(gridcode, CWHR_TYPE):
  if gridcode == 1 and CWHR_TYPE == "Annual Grassland" or CWHR_TYPE == "AGS" or CWHR_TYPE == "Perennial Grassland":
    return "Avena Grassland"
  elif gridcode == 0 and CWHR_TYPE == "Annual Grassland" or CWHR_TYPE == "AGS" or CWHR_TYPE == "Perennial Grassland":
    return "Bromus Grassland"
  else:
    return CWHR_TYPE

print(Reclass(0,"Annual Grassland"))
print(Reclass(1,"Annual Grassland"))
print(Reclass(0,"Perennial Grassland"))
print(Reclass(1,"Perennial Grassland"))

Output:

Bromus Grassland
Avena Grassland
Avena Grassland
Avena Grassland

Right:

def Reclass(gridcode, CWHR_TYPE):
    if gridcode == 1 and (CWHR_TYPE == "Annual Grassland" or CWHR_TYPE == "AGS" or CWHR_TYPE == "Perennial Grassland"):
        return "Avena Grassland"
    elif gridcode == 0 and (
            CWHR_TYPE == "Annual Grassland" or CWHR_TYPE == "AGS" or CWHR_TYPE == "Perennial Grassland"):
        return "Bromus Grassland"
    else:
        return CWHR_TYPE


print(Reclass(0, "Annual Grassland"))
print(Reclass(1, "Annual Grassland"))
print(Reclass(0, "Perennial Grassland"))
print(Reclass(1, "Perennial Grassland"))

Output:

Bromus Grassland
Avena Grassland
Bromus Grassland
Avena Grassland

Even better, use the in operator (thanks @fatih_dur):

Right:

def Reclass(gridcode, CWHR_TYPE):
    if gridcode == 1 and CWHR_TYPE in ("Annual Grassland", "AGS", "Perennial Grassland"):
        return "Avena Grassland"
    elif gridcode == 0 and CWHR_TYPE in ("Annual Grassland", "AGS", "Perennial Grassland"):
        return "Bromus Grassland"
    else:
        return CWHR_TYPE

Answer 2

What you are seeing is due to the fact that and takes precedence over or. a and b or c is the same as (a and b) or c and not a and (b or c), as you assumed.

As mentioned in the other answer, you can use parentheses to change the precedence (because parentheses have a higher precedence than either of and and or), or even better, use in. You can also turn your checks around, like this:

def Reclass(gridcode, CWHR_TYPE):
    if CWHR_TYPE not in ("Annual Grassland", "AGS", "Perennial Grassland") or gridcode not in (0, 1):
        return CWHR_TYPE
    return "Avena Grassland" if gridcode == 0 else "Bromus Grassland"