Question

I found some unexpected behavior when debugging my application. Does anyone know why I get the results described below?

from google.appengine.ext import ndb

class Person(ndb.Model):
  name = ndb.StringProperty()
  shared = ndb.BooleanProperty(default=False)

class Department(ndb.Model):
  name = ndb.StringProperty()
  persons = ndb.KeyProperty(kind=Person, repeated=True)

  @property
  def all_department_resources(self):    
    emp_list = self.persons
    for p in Person.query().filter(Person.shared == True):
      emp_list.append(p.key)
    return emp_list 

p1 = Person(name='Jane').put()
p2 = Person(name='Siri').put()
p3 = Person(name='Joe', shared=True).put()
Department(name='Finance',persons=[p1,p2]).put()

print '\n*First run*'
for d in Department.query():
  print '>>' + d.name
  for p in d.all_department_resources:   
    print p.get().name

print '\n*Second run:*'
for d in Department.query():
  print '>>' + d.name
  for p in d.all_department_resources:   
    print p.get().name

Output:

*First run*
>>Finance
Jane
Siri
Joe

*Second run:*
>>Finance
Jane
Siri
Joe
Joe

My question is simple: Where did the second Joe come from?

Answer 1

In all_department_resources you add Joe to the self.persons list. And on the second call you add it again. You can confirm this behavior by executing your text block a third time and find three Joes….

Why?

emp_list = self.persons

does not make a copy of the list, but references it. Everything you do to emp_list happens in self.persons too.

What can I do about it?
Either make a copy with

emp_list = list(self.persons)

Or you can just add the Query to the return statement:

  @property
  def all_department_resources(self):    
    return self.persons + Person.query().filter(Person.shared == True)