>>> from estudiantat import Expedient
>>> from estudiantat import Assignatura

--ini-enunciat-
>>> e = Expedient()
>>> e.total_credits()
0
>>> a = Assignatura('20888', 4.5)
>>> a in e
False
>>> len(e)
0
>>> e[a] = 5.0
>>> len(e)
1
>>> e[a]
5.0
>>> e.mitjana()
5.0
>>> a in e
True

--fi-enunciat-
>>> b = Assignatura('240111', 9)
>>> c = Assignatura('240222', 3)
>>> d = Assignatura('240333', 2.5)
>>> e[a] = 10
>>> e[b] = 6
>>> e[c] = 0
>>> e[d] = 8
>>> e.total_credits()
19.0
>>> print(round(e.mitjana(), 2))
6.26
>>> f = Assignatura('240444', 7.5)
>>> e[f] = 8
>>> e.total_credits()
26.5
>>> print(round(e.mitjana(), 2))
6.75