[Python] 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?

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