Question

In a class Week, I have created a dictionary using the .fromkey() method that contains a string key for Monday-Saturday and initializes them all to an empty list:

self.weekDict = dict.fromkeys(dayNameList, [])

I have a function that then iterates over a list of employees, and, within each iteration, iterates over their work days and appends an employee to the corresponding weekDict list of each day:

def setWeekDict(employees):
            for employee in employees:
                for day in employees[employee].workDays:
                    self.weekDict[day].append(employee)
                    print "employee: " + employee
                    print "weekday/list: " + str(day) + str(self.weekDict[day])

I expect the weekDict lists to reflect everyone who is scheduled to work on a given day. Instead, it appears that the list for each day accumulates the total number of times I’ve iterated over each day for the employee. My print statements output this:

employee: Addison
weekday/list: Saturday['Addison']
employee: Addison
weekday/list: Monday['Addison', 'Addison']
employee: Addison
weekday/list: Tuesday['Addison', 'Addison', 'Addison']
employee: Addison
weekday/list: Wednesday['Addison', 'Addison', 'Addison', 'Addison']
employee: Addison
weekday/list: Thursday['Addison', 'Addison', 'Addison', 'Addison', 'Addison']

Obviously I only want ‘Addison’ or any other employee to appear once per day, and I can’t understand what in my for loop would be contributing to each list adding the employee name + the amount of times the employee name has been added to other lists before. I can only think that dict.fromkeys() creates keys that actually all point to the same list, but I didn’t think that was how the fromkeys() function worked.

Answer 1

fromkeys() if used with mutable objects can lead to such behavior because it assigns the same object(same id()) to every key of the dictionary.

In [16]: dic=dict.fromkeys("spam",[])

In [17]: [id(x) for x in dic.values()]    #all of them are same objects
Out[17]: [141885196, 141885196, 141885196, 141885196]

In [18]: dic['s'].append(1)   #changing one changes others as well

In [19]: dic
Out[19]: {'a': [1], 'm': [1], 'p': [1], 's': [1]}

So, dict-comprehensions must be used with mutable objects:

In [20]: dic={x:[] for x in "spam"}

In [21]: [id(x) for x in dic.values()]               #unique objects
Out[21]: [141884972, 141848300, 142262988, 141895980]

In [22]: dic['s'].append(1)

In [23]: dic
Out[23]: {'a': [], 'm': [], 'p': [], 's': [1]}