Assignment versus binding [was Re: unintuitive for-loop behavior]

[Rustom Mody]

On Tuesday, October 4, 2016 at 8:11:41 AM UTC+5:30, Steve D'Aprano wrote:
> On Mon, 3 Oct 2016 04:15 pm, Jussi Piitulainen wrote:
> 
> > Steve D'Aprano writes:
> >> Why shouldn't people say that binding and assignment are the same
> >> thing in Python? What's the difference?
> > 
> > Outside Python, (lambda x : f(x)) is said to "bind" x. It's different
> > from assigning a new value to x. It's similar to how quantifiers in
> > logic are said to bind their variables, so that variables bound to
> > different quantifiers are different variables.
> 
> o_O
> 
> Me: "How is binding different from assignment?"
> 
> You: "This is an example of binding: lambda x: f(x). Its different from
> assigning to x. Clear now?"
> 
> 
> What's "outside python"? Can you give some examples? Presumably you don't
> actually mean "every single programming language apart from Python", even
> if that's what it sounds like.
> 
> Anyway, I've done my own research, and what I've found is:
> 
> - Some people consider assignment and binding to be synonyms. This is
> especially common in OOP languages (like Python), and procedural languages.

Here’s C++
http://www.informit.com/articles/article.aspx?p=376876
roughly using initialization where Jussi/Greg/myself/etc are using binding



> 
> - Some people consider them to be different. This is especially common in
> functional languages with an academic emphasis.
> 
> For those who consider them different, the distinction is usually along
> these lines:
> 
> - Assignment associates a mutable label to a value. So if I say "x = 1",
> that sets up a labelled box and puts 1 inside that box. Then if I say "x =
> 2", the box is updated to contain 2 instead of 1.
> 
> - Binding is more like setting up a label or identifier for a constant. For
> example, in a legal contract, you might say words to the effect of:
> 
>    John and Jane Smith, of 123 Evergreen Terrace, Metropolis ("the Client")
> 
> and then everywhere the contract refers to "the Client", you are supposed to
> mentally substitute "John and Jane Smith, of ...". Its a fixed label for a
> fixed value. Even if the value is mutable (John and Jane have a baby, and
> the baby is now automatically covered by the contract, the value cannot be
> replaced with a new value (you can't swap out John and Jane and replace
> them with their neighbours Fred and Ethel, you have to cancel the contract
> and write a new one).
> 
> So for these languages, bindings relate to a *fixed* association between a
> label and value. Whether the *value* itself is mutable or immutable, the
> association is fixed.

Are you suggesting that python can be understood without this distinction?
Consider the ‘mutable default gotcha’ in which the mutable is assigned but not mutated

tl;dr 
Many concepts are needed to understand programming languages
Some of them may be reified into the language; all cannot be.
“Not reified into the language” ≠ “Not a necessary concept for the language”
If you have variables+scopes you need (the concept of) binding
As understood by language-theorists, eg:
http://homepage.cs.uiowa.edu/~slonnegr/plf/Book/Chapter9.pdf