Role of comma when adding a dictionary entry in Python

Question

I have a simple question.

I have a dictionary: table = collections.defaultdict(set), and a previously defined grammar consisting of rules like the following:

Rule(('Noun', ('money',)))
Rule(('Noun', ('book',)))
Rule(('S', ('book',)))

Now, when I type this, nothing happens.

for rule in grammar:
    if rule.symbols == ("book"):
        table[col - 1, col].add(rule.head)

When I type this, the entry is added.

for rule in grammar:
    if rule.symbols == ("book",):
        table[col - 1, col].add(rule.head)

The only difference between the two is the comma behind "book". What does this comma do and why is it necessary?

Answer 1

In the first case, ("book") the parens are just a way of grouping the expression. The value of that expression is just the string "book".

In the second case, it's creating a tuple, with one element in it.

Answer 2

You need add comma to make it a tuple, otherwise it's just a string.