It’s a feature.
When you type q = f you are making q point to the same array as f. So whatever changes you make to q are also visible in f. This is extremely useful, but you must be aware of it.
Whenever f is a mutable data structure, q = f followed by mutation of q will be visible in f.
If you want to avoid this, write q = copy(f) instead. (In some cases you even need q = deepcopy(f).)
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