In Python 3.3.0 and 3.2.3:

>>> from inspect import *
>>> def f(a,b):pass
...
>>> formatargspec(getargspec(f))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/inspect.py", line 905, in formatargspec
    spec = formatargandannotation(arg)
  File "/Library/Frameworks/Python.framework/Versions/3.3/lib/python3.3/inspect.py", line 898, in formatargandannotation
    if arg in annotations:
TypeError: unhashable type: 'list'

No such error in 2.7.1:

>>> formatargspec(getargspec(f))
'((a, b), None, None, None)'

It looks like a typo in your code. You should use instead:
formatargspec(*getargspec(f))

Or better:
formatargspec(*getfullargspec(f))

Try with:
def f(a: int, b: float): pass

My bad, but I think documentation can be improved by adding an example.

Here's a patch that adds an example of using `inspect.formatargspec`.

New changeset 9d3b616fc702 by Andrew Svetlov in branch '3.3':
Issue #16243: add example for inspect.formatargspec
http://hg.python.org/cpython/rev/9d3b616fc702

New changeset a0337031a6b7 by Andrew Svetlov in branch 'default':
Merge issue #16243: add example for inspect.formatargspec
http://hg.python.org/cpython/rev/a0337031a6b7

Thanks, Berker.
I feel the patch is good enough to close the issue.
If anybody want to add some value please reopen.