Accessing object attributes in a for loop results in same attribute [Python]

Question

Using a for loop I am trying to append dictionaries to an object attribute:

do = direct_objects[0].point_logs
for direct_object in direct_objects: #Contains two objects
            print ( do is direct_object.point_logs) #Gives true both times
            do = direct_object.point_logs
            print(direct_object.id) #objects are different instances
            dates = generate_dates_interval('2017-01-01', '2017-04-01', 14) #Generates dates with an interval of 14 days for the given period

            for i in range(0, len(dates)-1):

                point_logs, interval_logs, cum_logs = get_point_logs(direct_object, dates[i],
                                                                     dates[i+1]) # gets logs for given period

                if point_logs is not None:
                    direct_object.point_logs.append(point_logs)
                if interval_logs is not None:
                    direct_object.interval_logs.append(interval_logs)
                if cum_logs is not None:
                    direct_object.cum_logs.append(cum_logs)

I want to loop through the dates and append all the logs to an instance of DirectObject's point_logs. However what I get is that, instead of being two seperate lists, the direct_object.point_logs becomes a a big list of all the point logs for both direct objects.

class DirectObject(object):
    def __init__(self, id, created_date, deleted_date, name, type, point_logs = [], interval_logs = [], cum_logs= [],
                 services = None):
        self.id = id
        self.created_date = created_date
        self.deleted_date = deleted_date
        self.name = name
        self.type = type
        self.point_logs = point_logs
        self.interval_logs = interval_logs
        self.cum_logs = cum_logs
        self.services = services

    def __str__(self):
        return str(self.name) + " id: " + str(self.id) + " type: " + str(self.type)
    def __repr__(self):
        return str(self.name) + " id: " + str(self.id) + " type: " + str(self.type)

Why is this happening?

`do` isn't a python keyword BTW. it's a `c` keyword. The mystery is why it is highlighted like a keyword. — Jean-François Fabre, Apr 05 '17 at 11:29
the highlighting seems to have a language-merged keyword database, so C keywords appear highlighted as well. — Jean-François Fabre, Apr 05 '17 at 11:34
direct_object.point_logs is a list and so is point_logs(probably). When you append a list to a list, it becomes a bigger list. What is wrong here? — alDiablo, Apr 05 '17 at 11:38
@alDiablo That's correct, but I expect two direct_objects.point_logs lists (for the two different instances). Instead I get one direct_object.point_logs list. — k88, Apr 05 '17 at 11:42
Note: `point_logs = [], interval_logs = [], cum_logs= []`. This is a common [Python gotcha](https://pythonconquerstheuniverse.wordpress.com/category/python-gotchas)(not sure if the link is the best). — CristiFati, Apr 05 '17 at 11:48

score 1 · Accepted Answer · edited May 23 '17 at 12:09

1

Since everything is an object in Python, the empty list you use as the default argument is too an object. More importantly, it is the same object! Like writing a = b = [] You have to try other recipes, such as the one here.

What is the pythonic way to avoid default parameters that are empty lists?

edited May 23 '17 at 12:09

Community

1
1

answered Apr 05 '17 at 11:47

Elan

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Thank you that answers my question – k88 Apr 05 '17 at 11:49

Accessing object attributes in a for loop results in same attribute [Python]

1 Answers1