Why this result?

Question

here's a code,
which result should be and why?

def func(par=[]):
par.append("a")
return par
print(len(func()+func()+func()))

Answer 1

Is the secret in len function's code?

Festo, an automation company, puts this clue in its Linkedin feed.

Answer 2

ty wet water ))) but try executing the code and the result is 7.