Why does JSONDecoder deserialize JSON String to Python Objects intuitively?

I have a root Class, College which holds many Course classes

 class Course:

    def __init__(self, name, url, hidden, semester="SS16"):
        self.name = name
        self.url = url
        self.hidden = hidden
        self.semester = semester

    def __str__(self):
        return ('Course: (Name: %s, Semester: %s, url: %s, hidden: %s)' % (self.name, self.semester, self.url, str(self.hidden)))


class College:

    def __init__(self, url='http://dummy', courses=set()):
        self.url = url
        self.courses = courses

    def __str__(self):
        s = ('College: (url: %s, Courses:[' % (self.url))
        s += ', '.join(str(v) for v in self.courses)
        return s + "])"

Now I want to save/load my College Class to/from a JSON file. For Encoding I created a custom JSONEncoder class, which seems to work fine:

from json import JSONEncoder
from json import JSONDecoder
class CollegeEncoder(JSONEncoder):

    def default(self, o):
        courses = list(map(lambda v: {'name': v.name, 'url': v.url,
                                      'hidden': v.hidden, 'semester': v.semester}, o.courses))
        return {'url': o.url, 'courses': courses}

For decoding I wrote a simple function:

def from_json(dct):
    if 'url' in dct:
        return College(dct['url'])

Now if I test JSON encoding/decoding like this:

myCollege = College()
myCollege.courses.add(Course("Course1", "url1", False))
myCollege.courses.add(Course("Course2", "url2", False))
myCollege.courses.add(Course("Course3", "url3", False))

dump = CollegeEncoder().encode(myCollege)
college = JSONDecoder(object_hook=from_json).decode(dump)
print(college)

At this point was expecting to get only a plain College object with just the URL, but I get my whole College with all its courses and their attributes

 College: (url: http://dummy, Courses:[Course: (Name: Course1, Semester: SS16,
 url: url1, hidden: False), Course: (Name: Course2, Semester: SS16, url: url2,
 hidden: False), Course: (Name: Course3, Semester: SS16, url: url3, hidden: False)])

But I don't understand how my 'object_hook' function knows how to deserialize the sub-classes. Should I keep it that way or expand custom deserialization? And how could I prevent my from_json function from behaving that way by default?

Solution

This is not the JSON decoder; you used a mutable default argument in your College.__init__ method:

def __init__(self, url='http://dummy', courses=set()):

The courses default value is created once, when the function is created. You then add values to this one, essentially global set:

myCollege.courses.add(Course("Course1", "url1", False))

That set is never emptied, so when you create a new College instance, the set is still there:

>>> College.__init__.__defaults__
('http://dummy', {<__main__.Course object at 0x10f55cda0>, <__main__.Course object at 0x10f55cc50>, <__main__.Course object at 0x10f55ccf8>})
>>> foo = College()
>>> foo.courses
{<__main__.Course object at 0x10f55cda0>, <__main__.Course object at 0x10f55cc50>, <__main__.Course object at 0x10f55ccf8>}
>>> print(foo)
College: (url: http://dummy, Courses:[Course: (Name: Course3, Semester: SS16, url: url3, hidden: False), Course: (Name: Course1, Semester: SS16, url: url1, hidden: False), Course: (Name: Course2, Semester: SS16, url: url2, hidden: False)])

See "Least Astonishment" and the Mutable Default Argument. Set courses to a sentinel instead, and create a new set in the __init__ method if the default is still set to the sentinel. None is a good choice here:

class College:
    def __init__(self, url='http://dummy', courses=None):
        self.url = url
        self.courses = courses or set()