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copied!<p>I've always found having two completely separate types for the same basic data structure (arrays) to be an awkward design, but not a real problem in practice. (Every language has its warts, Python included, but this isn't an important one.)</p> <blockquote> <p>Why does anyone care if a variable lives at a different place in memory than when it was originally allocated? This whole business of immutability in Python seems to be over emphasized.</p> </blockquote> <p>These are different things. Mutability isn't related to the place it's stored in memory; it means the <em>stuff it points to</em> can't change.</p> <p>Python objects can't change location after they're created, mutable or not. (More accurately, the value of id() can't change--same thing, in practice.) The internal storage of mutable objects can change, but that's a hidden implementation detail.</p> <pre><code>>>> x='hello' >>> id(x) 1234567 >>> x='good bye' >>> id(x) 5432167 </code></pre> <p>This isn't modifying ("mutating") the variable; it's creating a new variable with the same name, and discarding the old one. Compare to a mutating operation:</p> <pre><code>>>> a = [1,2,3] >>> id(a) 3084599212L >>> a[1] = 5 >>> a [1, 5, 3] >>> id(a) 3084599212L </code></pre> <p>As others have pointed out, this allows using arrays as keys to dictionaries, and other data structures that need immutability.</p> <p>Note that keys for dictionaries do not have to be completely immutable. Only the part of it used as a key needs to be immutable; for some uses, this is an important distinction. For example, you could have a class representing a user, which compares equality and a hash by the unique username. You could then hang other mutable data on the class--"user is logged in", etc. Since this doesn't affect equality or the hash, it's possible and perfectly valid to use this as a key in a dictionary. This isn't too commonly needed in Python; I just point it out since several people have claimed that keys need to be "immutable", which is only partially correct. I've used this many times with C++ maps and sets, though.</p>

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