>>> class Registrar(object):
...   def __init__(self):
...     self.called = 0
...   def __call__(self, *args, **kwargs):
...     self.called += 1
>>> BUILD = '''
... target(
...   dependencies=[target2()]
... )
... '''
>>> t, t2 = Registrar(), Registrar()
>>> exec(BUILD, {'target': t, 'target2': t2}, {})
>>> t.called, t2.called
(1, 1)