python 3.x - Why dict is reachable out of function, but list is not? -

three use cases:

1) dict interests reachable

interests = {}   def get_interests():     print('interests:', interests)  # interests: {}     interests['somekey'] = 123     print('interests:', interests)  # interests: {'somekey': 123}  get_interests() 

2) list interests not reachable

interests = []  def get_interests():     print('interests:', interests)  # unboundlocalerror: local variable 'interests' referenced before assignment     interests += [123]     print('interests:', interests)  get_interests() 

3) in case of list, if print, works

interests = []  def get_interests():     print('interests:', interests)  # interests: []     # interests += [123]     # print('interests:', interests)  get_interests() 

what did miss?

try .append() on list:

interests = []  def get_interests():     print('interests:', interests)  # unboundlocalerror: local variable 'interests' referenced before assignment     interests.append(123)     print('interests:', interests)  get_interests() 

the reason you're seeing behavior using += operator you're telling python following: interests = interests + [123] -- declares new variable called "interests" in scope of function instead of modifying global variable "interests".

by using .append() function you're not creating new variable modifying global variable instead.