# In [1]: f = lambda : 1 In [2]: x = f()
# In [3]: x = (lambda : 1)()
# In [4]: from collections import defaultdict In [5]: tree = lambda : defaultdict(tree) In [6]: root = tree() # how to root = (lambda : ...)()
root = (lambda f: f(f))(lambda f: defaultdict(lambda: f(f)))