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I'm trying to create this result (see the first row below):

using this data (see the selected row):

As you can see, the two vector tables have a common field, that relate themselves, the input is "gridcode" and the output must be "id_group".

I'm not expert and I've expent too much time trying to get a solution, nevertheless I can't find anyone.

The pictures are just an example, because the real shapefiles are from around 1M of features.

I was trying to implement this solution from Comparison between two fields of two tables using ArcPy cursors? without success, perhaps because I it takes so much time to create the dictionary.

import arcpy
from time import strftime

print "Start script: " + strftime("%Y-%m-%d %H:%M:%S")

def_agrupada = r'neighbours'
deff = r'entry_data'

fields_deff = ["gridcode", "data", "Shape_Area"]


list1 = [r[0] for r in arcpy.da.SearchCursor(deff, fields_deff)]
list2 = list(set(list1))
list2.sort()

dicc = 
for f in list2:
 dicc[f] = [r[:] for r in arcpy.da.SearchCursor(deff, fields_deff) if r[0] == f]
dicc


print "Finished script: " + strftime("%Y-%m-%d %H:%M:%S")

You are wasting time creating a list set and sorting it and then running a cursor over and over to match one value at a time from the ordered data. The dictionary eliminates all of that. The dictionary key is inherently stored as a set and is always stored without any predefined order. The power of a dictionary is that it can access all keys in any order with virtually no lag time. After loading a dictionary randomly you can output the dictionary records in key sort order using sorted(dict),

See my Turbo Charging Data Manipulation with Python Cursors and Dictionaries blog for the best ways to load a dictionary. Load the dictionary using this code. It will load the data into the dictionary in about 2 to 5 minutes if your data has around 1 million records.

You could run into a memory issue if you are using Python 2.7. If you run the code in Python 3.4 it should be fine.

import arcpy
from time import strftime

print "Start script: " + strftime("%Y-%m-%d %H:%M:%S")

def_agrupada = r'neighbours'
deff = r'entry_data'

fields_deff = ["gridcode", "data", "Shape_Area"]

dicc = r[0]:(r[1:]) for r in arcpy.da.SearchCursor(deff, fields_deff)

print "Finished script: " + strftime("%Y-%m-%d %H:%M:%S")

According with the @Richard Fairhurst help, I was able to adapt and implement their code to solve my problem, below is the code. I do not know if it is optimised, but it's working for me without problems.

import arcpy
from time import strftime

updateFC = r'neighbours'
deff = r'data'

fields_deff = ["gridcode", "data", "Shape_Area"]
updateFieldsList = [u'id_group', u'n_events', u'ini_date', u'end_date', u'ini_ha', u'end_ha', u'n_fids', u'fids', u'dates_ev', u'ha_ev']

def getIniDate(relateDict, key_value):
 list1 = [f[0] for f in relateDict[key_value]]
 return min(list1)

def getEndDate(relateDict, key_value):
 list1 = [f[0] for f in relateDict[key_value]]
 return max(list1)

def getArea(relateDict, key_value):
 listaa = relateDict[key_value]
 datos = list(set([f[0] for f in listaa]))
 datos.sort()
 area = 
 for d in datos:
 cuenta = 0
 for e in listaa:
 if d == e[0]:
 cuenta += e[1]
 area.append(cuenta)
 return area,datos

def getFids(relateDict, key_value):
 listaa = relateDict[key_value]
 fids = list(set([f[0] for f in listaa]))
 return fids

print "Start script: " + strftime("%Y-%m-%d %H:%M:%S")
relateDict = 
with arcpy.da.SearchCursor(deff, fields_deff) as relateRows:
 for relateRow in relateRows:
 relateKey = relateRow[0]
 if not relateKey in relateDict:
 relateDict[relateKey] = [relateRow[1:]]
 else:
 relateDict[relateKey].append(relateRow[1:])
del relateRows, relateRow


with arcpy.da.UpdateCursor(updateFC, updateFieldsList) as updateRows:
 for updateRow in updateRows:
 # store the Join value of the row being updated in a keyValue variable
 key_value = updateRow[0]
 # verify that the keyValue is in the Dictionary
 if keyValue in relateDict:
 # transfer the value stored under the keyValue from the dictionary to the updated field.
 n_events = len(getFids(relateDict, key_value))
 ini_date = getIniDate(relateDict, key_value)
 end_date = getEndDate(relateDict, key_value)
 ini_area = getArea(relateDict, key_value)[0][0]
 end_area = sum(getArea(relateDict, key_value)[0])
 n_fids = len(relateDict[key_value])
 dates_ev = ";".join(getArea(relateDict, key_value)[1])
 ha_ev = ';'.join(str(round(x,0)) for x in getArea(relateDict, key_value)[0])
 updateRow[1] = n_events
 updateRow[2] = ini_date
 updateRow[3] = end_date
 updateRow[4] = ini_area
 updateRow[5] = end_area
 updateRow[6] = n_fids
 updateRow[8] = dates_ev
 updateRow[9] = ha_ev
 updateRows.updateRow(updateRow)
print "Finished script: " + strftime("%Y-%m-%d %H:%M:%S")