I have this piece of code that I am not understanding very well, especially the part with the lambda function without arguments.

class uniform:
    def __init__(self,a,b)
        self.a=a
        self.L=b-a

    def __call__(self,randfun):
        f=lambda : self.a+self.L*randfun()
        return f

@uniform(-1,1)
def gen():
    from random import random
    return random()
#print 5 random numbers
for _ in range(5):
    print(gen()) 

If I remove that lambda resulting in f=self.a+self.L*randfun() I get this type of error: Exception has occurred: TypeError 'float' object is not callable. Why is this happenning? Same thing if I do return self.a+self.L*randfun() but nothing happens if I do print(self.a+self.L*randfun()).

Why is that lambda function without arguments essential to this piece of code? What does it do?