update app.py

app.py

@@ -90,8 +90,9 @@ if main_selection:
 
 # Earth Engine Index Calculator Section
 st.header("Earth Engine Index Calculator")
+
 # Load band information based on selected dataset
-if main_selection and sub_selection:
+if main_selection and sub_selection:  # Now safe because sub_selection is initialized
     dataset_bands = data[main_selection]["bands"].get(sub_selection, [])
     st.write(f"Available Bands for {sub_options[sub_selection]}: {', '.join(dataset_bands)}")
 
@@ -113,20 +114,21 @@ if main_selection and sub_selection:
 
     # Show custom formula input if bands are selected
     if selected_bands:
+        # Provide a default formula based on the number of selected bands
         if len(selected_bands) == 1:
             default_formula = f"{selected_bands[0]}"
             example = f"'{selected_bands[0]} * 2' or '{selected_bands[0]} + 1'"
-        else:
+        else:  # len(selected_bands) == 2
             default_formula = f"({selected_bands[0]} - {selected_bands[1]}) / ({selected_bands[0]} + {selected_bands[1]})"
             example = f"'{selected_bands[0]} * {selected_bands[1]} / 2' or '({selected_bands[0]} - {selected_bands[1]}) / ({selected_bands[0]} + {selected_bands[1]})'"
 
         custom_formula = st.text_input(
             "Enter Custom Formula",
             value=default_formula,
-            help=f"Use only these bands: {', '.join(selected_bands)}. Examples: {example}. Note: Actual band availability depends on the data."
+            help=f"Use only these bands: {', '.join(selected_bands)}. Examples: {example}"
         )
 
-        # Validate the formula syntax
+        # Validate the formula
         def validate_formula(formula, selected_bands):
             allowed_chars = set(" +-*/()0123456789.")
             terms = re.findall(r'[a-zA-Z][a-zA-Z0-9_]*', formula)
@@ -366,15 +368,9 @@ def calculate_custom_formula(image, geometry, selected_bands, custom_formula, re
         band_values = {}
         band_names = image.bandNames().getInfo()
         
-        # Check if all selected bands are available in the image
-        missing_bands = [band for band in selected_bands if band not in band_names]
-        if missing_bands:
-            error_msg = f"The following bands are not available in this image: {', '.join(missing_bands)}. Available bands: {', '.join(band_names)}. Please adjust your formula or check the data for your region and date range."
-            st.error(error_msg)
-            return ee.Image(0).rename('custom_result').set('error', error_msg)
-
-        # Only proceed if all bands are available
         for band in selected_bands:
+            if band not in band_names:
+                raise ValueError(f"Band '{band}' not found in the dataset.")
             band_values[band] = image.select(band)
 
         reducer = get_reducer(reducer_choice)
@@ -409,7 +405,7 @@ def calculate_custom_formula(image, geometry, selected_bands, custom_formula, re
     except Exception as e:
         st.error(f"Unexpected error: {e}")
         return ee.Image(0).rename('custom_result').set('error', str(e))
-        
+
 # Function to calculate index for a period
 def calculate_index_for_period(image, roi, selected_bands, custom_formula, reducer_choice):
     return calculate_custom_formula(image, roi, selected_bands, custom_formula, reducer_choice)