Featureclass to Dictionary

When working with Featureclasses and Tables, it is necessary to sometimes compare one dataset to another and make updates. You can do this with nested cursors, but it can get confusing, circular, and costly regarding speed and performance. It is better to read one dataset into an organized structure and operate against it than trying to nest cursors. I'll provide some methods of creating dictionaries and then demonstrate how you could use it.

A simple way to avoid nested cursors is to load all the data into a dictionary using a search cursor. Then use dictionary lookups to retrieve new data. For example, this code below creates a dictionary of attribute values from a search cursor.

fc = r'C:\NGA 489\USA.gdb\Cities' 

# Create a dictionary of fields and values, using the objectid as key
# Get a list of field names from the Featureclass.
fields = [fld.name for fld in arcpy.ListFields(fc)] 

# Create empty dictionary
fc_dict = {} 

# Start the search cursor
with arcpy.da.SearchCursor(fc, fields) as sCur: 
    for row in sCur: 
        # Add the attribute values from the cursor to the avalache_dict using the OBJECTID as the key
        fc_dict[row[fields.index('OBJECTID')]] = {k: v for k, v in zip(sCur.fields, row)} 

This code converts all features into a dictionary using a field (TYPE) as the key, and adds the feature's UIDENTvalue to a list to create a group of UIDENT values for each TYPE.

fc = r'C:\NGA 489\USA.gdb\Hydro' 
fields = ['OBJECTID', 'TYPE', 'HYDRO_', 'UIDENT'] 

fc_dct = {} 

with arcpy.da.SearchCursor(fc, fields) as sCur: 
    for row in sCur: 
        if dct.get(row[fields.index('TYPE')]): # gets the list index of ‘TYPE’ from the fields list as index of row 
            # Append the UIDENT value to the list
            fc_dct[row[fields.index('TYPE')]].append(row[3]) 
        else: 
            # Create the list for the accountno and add the space value.
            fc_dct[row[fields.index('TYPE')]] = [row[3]]

This example creates a dictionary of dictionaries using the OBJECTID as the key and {field:value, …} for all features in dictionary comprehension. We will discuss the list and dictionary comprehension construct in more detail in lesson 3.

fc = r'C:\NGA 489\USA.gdb\Hydro'
fields = [f.name for f in arcpy.ListFields(fc)] 

# Create a dictionary of fields and values, using the objectid as key
sCur = arcpy.da.SearchCursor(fc, fields)

fc_dct = {row[fields.index('OBJECTID')]: {k: v for k, v in zip(sCur.fields, row)} for row in sCur} # zip is a built in method that creates a field:value, one to one representation of the field to the value in the row, like a zipper. 

Once the data is in the dictionary structure, you can iterate over the dictionary within an Update or Insert cursor to save the new data. For example:

# Create InsertCursor to add new features to a Featureclass

fc = r'C:\NGA 489\USA.gdb\Hydro'
# filter out the objectid field since it is autogenerated
fields = [f.name for f in arcpy.ListFields(fc) if f.name not in ['OBJECTID']] 
with arcpy.da.InsertCursor(fc, fields) as iCur: 
    # Iterate over the dictionary items.  Note that the dictionary may be in the { objectid : {field: value, ...}, ...} 
    # format so there is a need to get the inner dictionary of fields and values 
    for k, v in fc_dct.items(): 
        # k = objectid
        # v = {field: value, ...}
        # get list of values in the order of the cursor fields.
        iRow = [v.get(f) for f in iCur.fields]
        iCur.insertRow(iRow) 

or as an Update and iterating over the features from the cursor and matching to the key in the dictionary by OBJECTID:

with arcpy.da.UpdateCursor(fc, [‘OBJECTID’, 'TYPE', 'UIDENT']) as uCur: 
    for row in uCur: 
        vals = fc_dct.get(row[0]) 
        if vals: 
            row[1] = vals['TYPE'] 
            row[2] = vals['UIDENT'] 
            uCur.updatetRow(row)