Using arcpy.da.SearchCursor to obtain values from field that correlates with “year” field?

Question

I am very new to coding.

I am attempting to use SearchCursor to go through the attribute table of a feature class and return a list of unique years from the years field (which I have done). I am now trying to get values from the mean_total field and get an average for these values for each year in my list, having trouble with using SearchCursor to do this.

import arcpy

# data Q:\ADGIS\GIS329\class08_loopingAndGeoprocessing\gis329_class08_dependencies.gdb\Economic_Workforce_MeanIncome


#obtain input feature class from user
in_featureclass = input('Enter the featureclass path: ')

if arcpy.Exists(in_featureclass):
    featureclass_fields = arcpy.ListFields(in_featureclass) # get field objects from the feature class

    available_fields = []
    for field in featureclass_fields:
        #append fields to avaialable_fields list    
        available_fields.append(field.name)

    # Show user what fields are available, ask user to choose fields separated by a space
    selected_fields = input('What fields would you like to examine, separated by a space? ' \
                            'Your options are: {}: '.format(', '.join(available_fields)))
    selected_fields = selected_fields.split() #turn user choices into a list of strings
    #create an empty list to append all unique values in the year field to
    total = 0
    count = 0
    year_list = [] 
    with arcpy.da.SearchCursor(in_featureclass, selected_fields) as rows:
        for row in rows:
            if row[0] and row[0] not in year_list:
                year_list.append(row[0])       
        for year in year_list:
            for rows[1] in year:
                total += total
                count += 1 
    average = total / count
    print(year)
    print(average)

I'm essentially trying to sort the values in row[1] by year and get the average of that value for each year and result in a print statement for each year that looks like "The average of mean_total for 2010 is $34481.67", repeated for each unique year.

Answer 1

The modification below takes advantage of defaultdict, a data structure that allows you to have a default data type of "list" for your dictionary; if the key is not found in the dictionary it doesn't error but instead just creates a list to store your data. Then the average is found by using the built-in functions for lists of sum and length. I'd also recommend adjusting to ensure that "Year" (or the value to average by) is always the first selected field in the cursor, and that the values to average are always the second selected field in the cursor.

import arcpy
from collections import defaultdict

# data Q:\ADGIS\GIS329\class08_loopingAndGeoprocessing\gis329_class08_dependencies.gdb\Economic_Workforce_MeanIncome


#obtain input feature class from user
in_featureclass = input('Enter the featureclass path: ')

if arcpy.Exists(in_featureclass):
    featureclass_fields = arcpy.ListFields(in_featureclass) # get field objects from the feature class

    available_fields = []
    for field in featureclass_fields:
        #append fields to avaialable_fields list    
        available_fields.append(field.name)

    # Show user what fields are available, ask user to choose fields separated by a space
    selected_fields = input('What fields would you like to examine, separated by a space? ' \
                            'Your options are: {}: '.format(', '.join(available_fields)))
    selected_fields = selected_fields.split() #turn user choices into a list of strings
    #create an empty list to append all unique values in the year field to
    # do something to make sure the year is field1, a numeric field is field2...
    year_dict = defaultdict(list)
    with arcpy.da.SearchCursor(in_featureclass, selected_fields) as cursor:
        for row in cursor:
            year_dict[row[0]].append(float(row[1])) # converted to float in case the field contains numbers formatted as string
    for year in year_list.keys():
        total = sum(year_list[year])
        count = len(year_list[year])
        average = total / count
        print("The average for year {} is {}".format(year, average)

Answer 2

You can use pandas module (included in Arcgis since ~v.10.4). It is great when you need to Group by something:

import arcpy
import pandas as pd

#Change to match your data:
fc = r'C:\GIS\data\my_riks.shp'
valuefield = 'area'
datefield = 'datefield'

#Create a dataframe using da.SearchCursor:
df = pd.DataFrame.from_records(data=arcpy.da.SearchCursor(fc,[valuefield,datefield]), columns = [valuefield, datefield])
#Create a new one by grouping:
df2 = df.groupby(df[datefield].dt.year)[valuefield].mean() #https://stackoverflow.com/questions/11391969/how-to-group-pandas-dataframe-entries-by-date-in-a-non-unique-column

Example output:

>>>df2
datefield
1912       127.164617
2008        81.581015
2009      1662.564423
2010     12035.495604
2011     18050.656858
2012      3147.083271
2013     17620.068681
2014        76.132404
2015      4020.356203
2016       497.237620
2017      8471.552898
2018     30982.398062
2019    101183.769650
Name: area, dtype: float64