Python defaultdict with string as type of value

Question

Need to create a defaultdict, value type is normal Python string, it seems my below method does not work? Post compile error message. Using Python 2.7 and wondering any good ideas how to fix? Thanks.

Code

import collections

a = collections.defaultdict("")

a[1]="Hello"
a[2]="World"

print a

Error Message

  a = collections.defaultdict("")
TypeError: first argument must be callable

Answer 1

As the error says, the first argument has to be a callable that produces the value you want. Use str:

a = collections.defaultdict(str)

If necessary, you can create a wrapper with a lambda function:

a = collections.defaultdict(lambda: 'initial')

Answer 2

It's right. You have an instance of a string instead of something that returns a string. What you want is collections.defaultdict(str). That is because str() returns "". You could have found out what the type was like this:

>>> type("")
<class 'str'>

Therefore, if you didn't know the name of the class, you could have done this:

a = collections.defaultdict(type(""))

or

a = collections.defaultdict("".__class__)