CODEWITHSUNDEEP
pythontypescontainers
jimmyc3po
jimmyc3po

Reputation: 471

I'd like to know what's going on this Python program. I've included the code

Here's the code: http://paste.pocoo.org/show/238093/

My main questions right now are:

  1. Is Line 37 mainly the gist of this program? And does it simply calculate this once and then print the result? Ex: self.start + key*self.step with start=1, key=4, step=2 [prints 9]

  2. where does the variable 'value' actually come into play here? Line 39.

  3. Not worried about the "Exceptions" part of the program. I pretty much understand what it's doing.

  4. Lastly, and you really don't have to answer this one as it's probably better as another question "down the road" but I really do not see how __getitem__, __setitem__...etc...you still have to write in your own code to "make it do stuff". :) I'm just not getting what's so "special" about these special methods.

Upvotes: 1

Views: 111

Answers (1)

Walter Mundt
Walter Mundt

Reputation: 25271

  1. Yes, more or less.
  2. This is the exception. If someone assigns a value to a particular index, the sequence remembers that and will return that value instead of calculating it. Note that the code here does not actually use this function.
  3. Random comment instead: the last 3 lines of the getitem function could be much more concisely implemented as return self.changed.get(key, self.start + key*self.step) -- dict.get lets you provide a default to return if a key is missing.
  4. They're "special" only in that they let you override what happens when someone does yourthing[foo] or yourthing[foo] = bar. You see the first going on here; the second is what happens if someone does s[5] = 100 -- the 100 ends up as the value of a __setitem__ call.

Upvotes: 3

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