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copied!<p>multiplying a single reference data type would simply create multiple references of the same type. What it means is that [True]*N is actually N times the same instance of the element [True]</p> <p>Thus changing one would inadvertently change the others </p> <p>As you can see in the following example, </p> <pre><code>>>> grid = [[True]]*10 >>> grid = [True]*10 >>> [id(e) for e in grid] [505379788, 505379788, 505379788, 505379788, 505379788, 505379788, 505379788, 505379788, 505379788, 505379788] </code></pre> <p>It shows all the elements are actually the same instance.</p> <p>But because here as the element is not a mutable Type, changing won't be an issue here as, changing one of the element would simply assign a new instance.</p> <p>Problem happens with a mutable type</p> <pre><code>>>> [id(e) for e in grid] [66523744, 66523744, 66523744, 66523744, 66523744, 66523744, 66523744, 66523744, 66523744, 66523744] >>> grid[0][0]=False >>> [id(e) for e in grid] [66523744, 66523744, 66523744, 66523744, 66523744, 66523744, 66523744, 66523744, 66523744, 66523744] >>> grid [[False], [False], [False], [False], [False], [False], [False], [False], [False], [False]] </code></pre> <p>To get over it, you need to understand which are mutable types and refrain from duplicating it but instead create new multiple instances of the same mutable types</p> <p>So here as a list is a mutable type, you need to create multiple instances, possibly through list comprehension</p> <pre><code>[[False]*N for _ in range(N)] </code></pre>

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