[Python] In the dict class, why does the update method not call __setitem__? [duplicate]

Very simple example:

class MyDict(dict):
def __setitem__(self, key, value):
print(f"setting new dictionary item {key}:{value}.")
return super().__setitem__(key, value)

d1 = MyDict(a=100, b=200)
d1.update(c=300)
d1['d'] = 400


I would have expected the message for setting c:300 to be printed, but it isn't, it's printed only for d:400. I would have expected the update method to call setitem. Why is this expectation mistaken?

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