def trisum(n, csum):
    if n == 0:
        return csum
    else:
        return trisum(n - 1, csum + n)

print(trisum(1000, 0))

>>> def trisum(n, csum):
...     while True:                     # change recursion to a while loop
...         if n == 0:
...             return csum
...         n, csum = n - 1, csum + n   # update parameters instead of tail recursion

>>> trisum(1000,0)
500500

lambda f: (lambda x: x(x))(lambda y: f(lambda *args: y(y)(*args)))

lambda f: (lambda x: x(x))(lambda y: f(lambda *args: lambda: y(y)(*args)))

def bet(func):
    b = (lambda f: (lambda x: x(x))(lambda y:
          f(lambda *args: lambda: y(y)(*args))))(func)
    def wrapper(*args):
        out = b(*args)
        while callable(out):
            out = out()
        return out
    return wrapper

>>> from recursion import *
>>> fac = bet( lambda f: lambda n, a: a if not n else f(n-1,a*n) )
>>> fac(5,1)
120
>>> fibo = bet( lambda f: lambda n,p,q: p if not n else f(n-1,q,p+q) )
>>> fibo(10,0,1)
55

>>> bet( lambda f: lambda n: 42 if not n else f(n-1) )(50000)
42

def bet1(func):
    def wrapper(*args):
        out = func(lambda *x: lambda: x)(*args)
        while callable(out):
            out = func(lambda *x: lambda: x)(*out())
        return out
    return wrapper

class _RecursiveCall(Exception):
  def __init__(self, *args):
    self.args = args
def _recursiveCallback(*args):
  raise _RecursiveCall(*args)
def bet0(func):
    def wrapper(*args):
        while True:
          try:
            return func(_recursiveCallback)(*args)
          except _RecursiveCall as e:
            args = e.args
    return wrapper

>>> f = bet0( lambda f: lambda n: (lambda x: x) if not n else f(n-1) )
>>> f(5)(42)
42