Your min_key and max_key variables are calculated using min and max function respectively.
You may remove None's, but there’s also node.key (that’s probably always present) which becomes the new minimum/maximum.
You don’t always return None, you do it only when the node is None.
but sir, we are not returning anything other than None either.
You return is_bst_node, min_key and max_key.
The last two values are result of min() and max() functions. And they accept a list of up to three values. One of these values is node.key which probably is always different than None.
That’s where the values (different than None) are from.
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