Question

class MyOwnClass:

  # list who contains the queries
  queries = []

  # a template dict
  template_query = {}
  template_query['name'] = 'mat'
  template_query['age'] = '12'

obj = MyOwnClass()

query = obj.template_query
query['name'] = 'sam'
query['age'] = '23'
obj.queries.append(query)

query2 = obj.template_query
query2['name'] = 'dj'
query2['age'] = '19'
obj.queries.append(query2)

print obj.queries

It gives me

[{'age': '19', 'name': 'dj'}, {'age': '19', 'name': 'dj'}]

while I expect to have

[{'age': '23'  , 'name': 'sam'}, {'age': '19', 'name': 'dj'}]

I thought to use a template for this list because I’m gonna to use it very often and there are some default variable who does not need to be changed.

Why does doing it the template_query itself changes? I’m new to python and I’m getting pretty confused.

Answer 1

this is because you are pointing to the same dictionary each time … and overwriting the keys …

# query = obj.template_query - dont need this
query = {}
query['name'] = 'sam'
query['age'] = '23'
obj.queries.append(query)

query2 = {} #obj.template_query-dont need this
query2['name'] = 'dj'
query2['age'] = '19'
obj.queries.append(query2)

this should demonstrate your problem

>>> q = {'a':1}
>>> lst = []
>>> lst.append(q)
>>> q['a']=2
>>> lst
[{'a': 2}]
>>> lst.append(q)
>>> lst
[{'a': 2}, {'a': 2}]

you could implement your class differently

class MyOwnClass:
  # a template dict
  @property
  def template_query():
      return {'name':'default','age':-1}

this will make obj.template_query return a new dict each time