How to generate a list of unique years from date field?

Question

I have data that was collected over many years and I'd like to be able to run a python script tool in ArcToolbox (ArcGIS 10.1) to get a list of unique years from the date field.

I found an example of how to format the date field, but this example lists the year from every record, and I just want a list of unique years.

# Imports
import arcpy
from datetime import datetime

# Input Table
fc = arcpy.GetParameterAsText(0)

# Date Field
field = arcpy.GetParameterAsText(1)

rows = arcpy.SearchCursor(fc)
for row in rows:
    datetimeVal = row.getValue(field)
    formattedTime = datetime.strftime(datetimeVal, "%Y")
    arcpy.AddMessage(formattedTime)

I can get a list of unique dates using a search cursor, but I just want the year and not the entire date field:

# imports
import arcpy
from datetime import datetime

# Input Table
inputTable = arcpy.GetParameterAsText(0)

# Date Field
field = arcpy.GetParameterAsText(1)

# Use SearchCursor with list comprehension to return a
#  unique set of values in the specified field
values = [row[0] for row in arcpy.da.SearchCursor(inputTable, (field))]
uniqueValues = set(values)
arcpy.AddMessage(uniqueValues)

How can I get the date in the example above, format it to just get the year, and then run the search cursor to get a list of unique years?

Answer 1

Here is one approach that uses the datetime module to parse the year from a datetime object. The SearchCursor is wrapped in a generator expression to extract unique years from the "date" field in the table.

import arcpy
from datetime import date

table = r'C:\path\to\your.gdb\table'

print set(date.strftime(row[0], "%Y") for row in arcpy.da.SearchCursor(table, "date"))

Result:

>>> 
set(['2008', '2009', '2011', '2022'])

Answer 2

By combining parts from the two different python scripts you've shown, you can accomplish what you're after.

# Imports
import arcpy
from datetime import datetime

# Input Table
fc = arcpy.GetParameterAsText(0)

# Date Field
field = arcpy.GetParameterAsText(1)

# Search Cursor
rows = arcpy.da.SearchCursor(fc,field)

# Create an empty list to fill with years
allYears = []

for row in rows:
    datetimeVal = row[0]
    formattedTime = datetime.strftime(datetimeVal, "%Y")
    # add the current row's year to the list
    allYears.append(formattedTime)

#create a unique set of the years
uniqueYears = [ year for year in set(allYears) ]
# sort years
uniqueYears.sort()

uniqueYears is a list that will contain a unique set of years from your input table. If your input table has Null values in the date field you'll need to test for those or you'll raise a TypeError.

Answer 3

I agree with Adam. Another option though if you have access to your data in a standard database, would be use a combination of datepart and group by. something similar to:

select datepart('yyyy',[your_date_field])
from [your_data_set]
group by datepart('yyyy',[your_date_field])