The Python Apprentice

Mutability of dictionaries

The keys in a dictionary should be immutable, although the values can be modified. Here's a dictionary which maps the element symbol to a list of mass numbers for different isotopes of that element:

>>> m = {'H': [1, 2, 3],...      'He': [3, 4],...      'Li': [6, 7],...      'Be': [7, 9, 10],...      'B': [10, 11],...      'C': [11, 12, 13, 14]}

See how we split the dictionary literal over multiple lines. That's allowed because the curly braces for the dictionary literal are open.

Our string keys are immutable, which is a good thing for correct functioning of the dictionary. But there's no problem with modifying the dictionary values in the event that we discover some new isotopes:

>>> m['H'] += [4, 5, 6, 7]>>> m{'H': [1, 2, 3, 4, 5, 6, ...