Update page 'Day 3 reading notes'

Day 3 reading notes.md

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+## 3.1
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+Note that the `type()` and `id()` builtin methods work both on themselves, and each other.
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+Experiment: what is the class of the `type()` method?  If you use `dir()` on the type class, there's an interesting method at the end of the list of the class contents, what does it do?
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+The reason why the language reference is specific about saying that containers contain *references* to objects is that it's trying to tell programmers that Python does not *copy* objects to make data structures.  This means that modifying objects inside containers will modify the one, original version of the object.  Keep in mind that third-party libraries may not always make this guarantee, especially when it comes to big data, for performance reasons.
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+Keep in mind that Python itself may sometimes change the identity of an object, such as when promoting an integer to a bignum because it got too large, but that doesn't affect the other guarantees expressed here.
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