Create dict key only when needed

[Helge Stenstrom]

I'd like to do the following, which doesn't work:

a = {}
for key in keys:
    a[key] += stringFunction()

where keys contains some duplicates. Put in another way:

a = {}
a[17] += "foo"
a[4711] += "foo"
a[4711] += "bar"
would give a = {17: "foo", 4711: "foobar"}

but that doesn't work, because the keys of the dict must exist before
the += operator can be used.

How can this problem be solved in a readible way?

An ugly solution would be:


# -------- Start of example  ---------
def addStuff(dict, key, data):
    if dict.has_key(key):
        dict[key] += data
    else:
        dict[key] = data
# =========== UNIT TEST ==========
import unittest
class Ok(unittest.TestCase):
    def test1(self):
        a = {}
        addStuff(a, 17, "foo")
        addStuff(a, 4711, "foo")
        addStuff(a, 4711, "bar")
        self.assertEqual(a, {17: "foo", 4711: "foobar"})
if __name__ == '__main__':
    unittest.main()
# -------- End of example  ---------

How can the += syntax be used instead of calling a function?

Cheers,
Helge