Whenever you use pyexec you’re invoking the Python parser, which makes for an unfair comparison. It’s better to use it to create a Python function then call that:
@pyexec """
def fib(n):
a, b = 0, 1
while a < n:
print(a, end=' ')
a, b = b, a+b
print()
""" => fib
fib(10)
This example uses @pyexec’s handy => notation for getting variables into and out of Python.