[Python] Why do two Python dictionaries initiated by default parameter point to the same...

I am using Python 3.12 and wanted to initiate a dictionary with a passed value. If nothing was passed, an empty dictionary should be created in the outer dictionary.

The following example:

def result_dict[T](init: dict[str, T] = {}) -> dict[str, dict[str, T]]:
return dict.fromkeys(["x", "y"], init)


dict_1 = result_dict()
dict_2 = result_dict()

dict_1["x"]["one"] = "foo"
dict_2["y"]["two"] = "bar"

print("dict_1: ", dict_1)
print("dict_2: ", dict_2)
print("Is same?: ", dict_1 == dict_2)


Output:

dict_1: {'x': {'one': 'foo', 'two': 'bar'}, 'y': {'one': 'foo', 'two': 'bar'}}
dict_2: {'x': {'one': 'foo', 'two': 'bar'}, 'y': {'one': 'foo', 'two': 'bar'}}
Is same?: True


I assume this is because the parameter to a dict is always a reference and therefore if you set it as the default parameter, the interpreter always takes the same default dict. As a developer, I would expect the default parameter to point to a new object.

Is this a bug in python or expected behaviour?

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