Is An Element of a Sequence an Object?
I'm learning about Python. A book I'm reading about it says "... a string in Python is a sequence. A sequence is an ordered collection of objects". This implies that each character in a string is itself an object. This doesn't seem right to me, but since I'm just learning Python I questioned the author about this. He gave an example the displays the ids of string slices. These ids are all different, but I think that's because the slicing operation creates objects. I'd like to suggest an explanation of what a sequence is that doesn't use the word 'object' because an object has a specific meaning in Python. Am I on the right track here? Cordially, Jon Forrest -- https://mail.python.org/mailman/listinfo/python-list
Re: Is An Element of a Sequence an Object?
On 6/3/2017 12:38 PM, Thomas Jollans wrote: >> I'd like to suggest an explanation of what a sequence is >> that doesn't use the word 'object' because an object has >> a specific meaning in Python. >> >> Am I on the right track here? > > No, strings don't internally store the characters as objects, and yes, > the slicing operation creates objects. However, strings *are* sequences, > sequences *are* ordered collections of objects. I don't see how both can be true. "Object" has a clear meaning in Python, and the contents of a sequence don't meet the requirements, as I understand them. If there were some way of measuring the number of objects in a program, then it would be easy to prove (or disprove) my hypothesis. In other words this program a = "abc" would have the same number of objects as this program a = "abcdefghijklmnopqrstuvwzyz" > The sentence "A sequence is an ordered collection of objects" means that > a sequence has (ordered) elements, you can access these elements, and > when you do that, you will get an object (seeing as everything is an > object). The distinction between an "object" and "element" is key here. (This might be seen as pedantic, but I think it's important to be clear, especially in a book intended for beginners, as I am. > Hope this helps. Thanks for taking the time to reply. I'm somebody who reads technical books *very* carefully so the distinction between objects and elements is important to me. Cordially, Jon Forrest -- https://mail.python.org/mailman/listinfo/python-list
Re: Is An Element of a Sequence an Object?
On 6/3/2017 5:03 PM, Ben Finney wrote: Jon Forrest writes: I'm learning about Python. A book I'm reading about it Can you say which book, and where in the book it says this? With all due respect, I'd rather not. The author has been very responsive when I raised this issue, and I don't want to make him look bad. To phrase it that way might be slightly misleading, though. It is *not* true, for example, to say that the characters in the string already exist as objects. That's exactly what I was thinking, and why I raised the issue. I think, rather, that you may have an incorrect understanding of a sequence. The objects produced by an operation on a sequence – a slice, or iterating the sequence – do not necessarily exist as distinct objects before that operation. I wasn't thinking so much about the objects produced by an operation on a sequence as I was about when the sequence is created. A trivial sequence is "abc" As I understand it, this is one object, not three. The original excerpt implies the later. Jon -- https://mail.python.org/mailman/listinfo/python-list
Re: Is An Element of a Sequence an Object?
On 6/3/2017 4:58 PM, Chris Angelico wrote: A sequence doesn't necessarily "contain" anything. Maybe not always, but doesn't "abc" contain three characters? Is 'contain' the right word? As has been mentioned, a range object is a sequence, but it creates integer objects lazily. So there must be a class of sequence, like a literal string, that contains multiple characters stored as one object, and another class, like range(50), that doesn't store anything, and only produces values when it's iterated over. Plus, a list is a sequence, but in this case it's clear that each element of a list is an object. Jon -- https://mail.python.org/mailman/listinfo/python-list
Re: Is An Element of a Sequence an Object?
On 6/3/2017 5:23 PM, Steve D'Aprano wrote: On Sun, 4 Jun 2017 05:10 am, Jon Forrest wrote: We can fix the book's statement by changing it to: A sequence is an ordered collection of *elements* ... That's exactly what I was thinking, but then there'd have to be a clear definition of "element". Jon -- https://mail.python.org/mailman/listinfo/python-list
Question About When Objects Are Destroyed
Consider the following Python shell session (Python 3.6.2, Win64): >>> def givemetwo(): ... x = 'two' ... print(id(x)) ... >>> givemetwo() 1578505988392 So far fine. My understanding of object existence made me think that the object referred to by x would be deleted when the givemetwo() function returned, like a local variable in C. However, this isn't true, as shown by the following in the same session: >>> import ctypes >>> print (ctypes.cast(1578505988392, ctypes.py_object).value) two This shows that the object still exists, which was a surprise. Will this object ever be deleted? I'm learning about function decorators which, as my early studies tell me, depend on calling a function defined inside another function. This suggests that objects defined inside functions are never deleted, otherwise function decorators would never work. (I'm not 100% sure my understanding of function decorators is correct since I'm still learning about them). What's the right way to think about this? Cordially, Jon Forrest -- https://mail.python.org/mailman/listinfo/python-list
Re: Question About When Objects Are Destroyed
On 8/4/2017 4:34 PM, gst wrote: 'two' is a so called constant or literal value .. (of that function). Why not attach it, as a const value/object, to the function itself ? So that a new string object has not to be created each time the function is called. Because anyway strings are immutable. So what would be the point to recreate such object every time the function is called ? This was just an example program, not meant to do anything meaningful. I would think that the same object behavior would occur if I dynamically created an object in that function. Jon -- https://mail.python.org/mailman/listinfo/python-list
Question About When Objects Are Destroyed (continued)
Perhaps the reason the variable isn't destroyed is shown by the following (again, in the same session): >>> import sys >>> sys.getrefcount(1578505988392) 3 So, maybe it's not destroyed because there are still references to it. But, what are these references? Will the reference count ever go to zero? Jon -- https://mail.python.org/mailman/listinfo/python-list
