Print statement under __len__ definition leads to constantly repeating console output

I've used Python for years without dabbling in the art of classes very much, and decided it's time to get some insight on the topic. I thus decided to play around with them a bit, and was in particular playing around with the dunder functionality. I made the following class:

class TestClass:
    
    def __init__(self,y):
        self.x = [1,2,3]
        self.y = y
    
    def __len__(self):
        print('gottem')
        return len(self.x)
    
    def __str__(self):
        print('gottem again')
        return str(self.y)

Once I instantiate the class within the console, all subsequent console statements are concatenated with 'gottem', see console snippet below:

>>> 1+1
Out[2]: 2

>>> a = TestClass(3)
gottem

>>> 2+2
Out[4]: 4gottem

>>> len(a)
gottem
Out[5]: 3gottem

I've tried removing the __len__ definition, but I observe no similar behaviour with the __str__ definition, which indeed seems to function as I would expect. The behaviour goes away once I redefine a, so that the class instantiation is dropped. I am completely stumped as to what causes this behaviour, and why it only happens for __len__. Is there an obvious explanation behind this?

Info: I am running Anaconda's Python 3.7.6 distribution through Spyder.

Disclaimer: I absolutely tried to search for some information on this, but I either have no clue what to search for, or alternatively am doing something so fundamentally wrong that the questions don't even exist.