dict.fromkeys() shared mutable value

Question

Accepted Answer

dict.fromkeys(keys, value) maps every key to the same single value object. When that value is a mutable list, all keys share the same list - so appending to d['x'] is visible through d['y'] and d['z']. To get independent lists per key, use a comprehension instead: {k: [] for k in ['x', 'y', 'z']}. This is the same aliasing trap as list multiplication, just expressed through a different API.