You can do that using count:
my_dict = {i:MyList.count(i) for i in MyList}
>>> print my_dict #or print(my_dict) in python-3.x
{'a': 3, 'c': 3, 'b': 1}
Or using collections.Counter:
from collections import Counter
a = dict(Counter(MyList))
>>> print a #or print(a) in python-3.x
{'a': 3, 'c': 3, 'b': 1}