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In the Wrapping Up section, there are some very interesting points that weren't covered in the article. - Why is "list += seq" not the same as "list = list +

• RodericDay 13y ago
  I think two different objects that evaluate to the same thing pass the `==` test, but is checks for identity? >>> a = list() >>> b = list() >>> a is b False >>> a==b True >>>
  • nslocum 13y ago
    You're correct. Further, there are some odd quirks because of the way Python caches small Integers for efficiency. > 2 + 2 is 4 True > 1000 + 1 is 1001 False
    • Thrymr 13y ago
      > 2 + 4 is 4 True I sure hope not.
      • nslocum 13y ago
        lol, thanks, i fixed it
    • alecrn 13y ago
      (I think you meant 2 + 2 is 4)
    • audeyisaacs 13y ago
      For more details: http://stackoverflow.com/questions/306313/python-is-operator-behaves-unexpectedly-with-integers http://stackoverflow.com/questions/306313/python-is-operator...
      • nslocum 13y ago
        After a little trial and error, it appears that Python caches the values in range(-5,257). > -6 + 0 is -6 False > -5 + 0 is -5 True > 256 + 0 is 256 True > 257 + 0 is 257 False
        • wahnfrieden 13y ago
          (CPython)
• rflrob 13y ago
  > list += seq uses the __iadd__ method, which almost certainly uses the exact same mechanism as the list.extend() method. When this method isn't defined (c.f. the Counter class from the collections module), a += b works like a = a + b. > from collections import Counter > c = Counter() > c_old = c > d = Counter() > c += d > '__iadd__' in dir(c) False > c_old is c False > a = [] > a_old = a > b = [] > a += b > '__iadd__' in dir(a) True > a_old is a True
• njharman 13y ago
  I'm not sure about list += one. Would have to dissemble to determine "why". The '+' operator is not defined for all possible sequence types (could never be given user definable types). So, in many instances (e.g. adding tuple to list) l=l+s will TypeError. += for list must be defined to "cast" the sequence into list. Something like "l = l + list(s)". List constructor must iterate over the sequence to create list. By definition a sequence must be iterable. So l += s will work for all (s). "is" operator determines if two things are the same thing, contrast with two things evaluate to the same value, "==". It's probably implementation dependent but "same thing" is same memory location. CPython "interns" (keeps a table/cache) of small integers rather than allocating them anew (for speed and space efficiency). All names referring to the value of integer '4' are referring to the same value. Possible cause integers are immutable. Were as names referring to larger ints may or may not be referring to the same value, even when those values are equal. Default mutable arguments have been talked about a lot. Google it. You should already grok it after learning what OA taught you about names / reference and knowning that function/method definitions are executed once (at import/"parse"). So, the list that the name "mutable" refers to is created once, when defined (which is usually once But inner functions and programatically generated functions could be more). All calls of that function will be using the same name "mutable" all of which refer to the same list. def func(mutable=[]): mutable.append(len(mutable)) print mutable I'm not sure what is meant by class attribute and esp, given that he just wrote whole article on how names (attributes are names) don't have types, values do, what is meant by list or int attribute.
• true_religion 13y ago
  > 1000 + 1 is not 100 'is' actually checks memory locations and not equity. Because of the way Python caches small integers, anything from 0 through X will map to the same memory location. But a large integer (say 1001) will map to a new memory location each time its created.
  • wahnfrieden 13y ago
    To get more specific, this object aliasing is an implementation detail. You can't expect different Python implementations to do it the same.