How to bind a name with multiple objects or values in python

Question

I saw in a book about language description that says

On the other hand, a name can be bound to no object (a dangling pointer), 
one object (the usual case), or several objects (a parameter name in a 
 recursive function).

How can we bind a name to several objects? Isnt that what we call an array for example where all elements have the same name but with index? For a recursive function like the example here:

x = 0
def f(y):
    global x
    x += 1
    if x < 4 :
        y +=100
        f(y)

    else: return
f(100)

Is the name y binded with multiple values that are created recursively since the nametable has already the y name binded to an initial value which is being reproduced with recursion?

EDITED Just press here Visualizer and see what it generates. :)

I have serious doubt if said "book" is going to help you understand Python in any meaningful way. — jsbueno
– jsbueno, Commented Oct 1, 2012 at 15:54
@jsbueno Yes its kinda old and it has the fundamentals in language description The Anatomy of Programming Languages by Alice E. Fischer & Frances S. Grodzinsky. So i guess in python we cant have a single name refering to multiple objects right? — BugShotGG
– BugShotGG, Commented Oct 1, 2012 at 16:10
@GeoPapas I think part of the reason that jsbueno made that comment is because there is likely a significant difference between what the book you're reading refers to as a "name" and what you think of when you talk about a "name" in python. Python doesn't use traditional 'pass-by-reference' or 'pass-by-value' calling methods. Instead it uses something that might be described as 'call-by-object'. See effbot.org/zone/call-by-object.htm — Wilduck
– Wilduck, Commented Oct 1, 2012 at 16:35
@Wilduck So even in recursive its passed as a copy and a new "instance" that happens to have the same name (here for eg is y) is a different name in the nametable? — BugShotGG
– BugShotGG, Commented Oct 1, 2012 at 18:41
@GeoPapas You're getting closer! Instead of a "nametable" though, python stores a dictionary for each scope. See this answer for a little more detail (stackoverflow.com/a/370380/173292). The implication of this is that you can never have two of the same name in a single scope (for the same reason you can't have two of the same key in a python dictionary). So, while y may exist in a dictionary for a different scope, you have no way of accessing it, since you can only access the variables in the current scope's dictionary. — Wilduck
– Wilduck, Commented Oct 1, 2012 at 18:54

jsbueno · Accepted Answer · 2012-10-01 15:58:40Z

No. A name is bound to one single object . When we are talking about Python - it is either bound to a single object in a given context, or do not exist at all.

What happens, is that the inner workings may have the name defined in several "layers" - but your code will only see one of those.

If a name is a variable in a recursive function, you will only see whatver is bound to it in the current running context - each time there is a function call in Python, the execution frame, which is an object which holds several attributes of the running code, including a reference to the local variables, is frozen. On the called function, a new execuciton frame is created, and there, the variable names are bound again to whatever new values they have in the called context. Your code just "see" this instance.

Then, there is the issue of global variables and builtin objects in Python: if a name is not a local variable in the function execution context, it is searched in the globals variables for the module (again, just one of those will be visible).ANd if the name is not defiend in the globals, them, Python looks for it in globals().__builtins__ that is your last call.

Community · Accepted Answer · 2017-05-23 10:29:30Z

3

If I understand you correctly, you're asking about what rules Python has for creating variables in different scopes. Python uses lexical scoping on the function level.

It's hard to tell exactly what you're getting at with the code you've written, but, while there may be a different value associated with y in different scopes (with a value of y defined at each level of recursion), your code will only ever be able to see one at a time (the value defined at the scope in which you're operating).

To really understand scoping rules in Python, I would have a look at PEP 227. Also, have a look at this Stack Overflow question.

Finally, to be able to speak intelligently about what a "name" is in Python, I suggest you read about how Python is a "Call-By-Object" language.

At this point, we are capable of understanding that, instead of a "nametable", python uses a dictionary to hold what is accessible in a given scope. See this answer for a little more detail. The implication of this is that you can never have two of the same name in a single scope (for the same reason you can't have two of the same key in a python dictionary). So, while y may exist in a dictionary for a different scope, you have no way of accessing it, since you can only access the variables in the current scope's dictionary.

edited May 23, 2017 at 10:29

CommunityBot

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answered Oct 1, 2012 at 16:01

Wilduck

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2 Comments

BugShotGG Over a year ago

@Wildduck i ve added a link to visualizer check it if u like :)

Wilduck Over a year ago

I understand what your code does. It's just that there's a mismatch between how you've phrased your question, and how Python actually works, which makes it difficult to give you a satisfying, straightforward answer. I've tried to provide some resources to make it easier for you to understand how Python works under the hood. I hope the links are helpful.

Marcin · Accepted Answer · 2012-10-01 16:18:03Z

0

The key is:

several objects (a parameter name in a recursive function).

The passage is almost certainly not referring to arrays, but simply to the fact that in a recursive function (or any function, but a recursive function is likely to have multiple activations at one time), a parameter may be bound to a different value in each recursive call.

This does not mean that you can access each such object in every stack frame; indeed the point of the technique is to ensure that only one such value is accessible in each stack frame.

answered Oct 1, 2012 at 16:18

Marcin

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Comments

pepr · Accepted Answer · 2012-10-01 20:07:15Z

Firstly, you should mention in the question that the sentence from the book is not related explicitly to Python (as jsbueno wrote, one name is bound to exactly one object in Python).

Anyway, name bound to no object is a bit inaccurate. Generally, names are related to variables, and name related to a dangling pointer is the name of that pointer variable.

When speaking about the variable scope (i.e. the part of code where the variable is used), one variable name can be used only for a single value at a time. However, there may be other parts of code, independent on the one where we think about that variable. In the other part of code, the same name can be used; however, the two variables with the same name are totally isolated. This is the case of local variables also in the case of function bodies. If the language allows recursion, it must be capable to create another isolated space of local variable even for another call of the same function.

In Python, each function can also access outer variables, but it is more usual to use the inner, local variables. Whenever you assign a name some value, it is created in the local space.

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