defaultdict_test.py

from collections import defaultdict
def run():
   print("RUNNING")
   return 'val'

test = {'store':1}
print(test.get('store', run))
print(test.get('not_stored', run))

test2 = defaultdict(run)

test2['store'] = 1
print(test2['store'])
print(test2['not_stored']) # calls function

# output:
# 1
# <function run at 0x102877ae8>
# 1
# RUNNING
# val