why cannot assign to function call

[Joe Strout]

rurpy at yahoo.com wrote:

>> I never claimed that you *couldn't* have copy semantics in C; you can do
>> almost anything you want in C (or C++).  But the *normal* usage of an
>> array is via a pointer, in which case the semantics are exactly the same
>> as in Python, Java, REALbasic, .NET, etc.
> 
> Arrays are the only datatype in C that don't use
> copy-like assignment.  Everything else does.

No, arrays are just one reference type; pointers are another (and in 
most ways, these are the same thing).  When dealing with objects in C++, 
one routinely handles them with pointers, so that's the use case which 
is analogous to Python -- all the value types can be ignored for the 
sake of comparison.  (C is not an OOP language, of course, but even 
there, all but the most trivial of structs are usually allocated on the 
heap and passed around via pointers, just like in C++, Java, .NET, RB, 
and Python.)

>> Ah.  OK then, I guess I missed you're point.  You're absolutely right;
>> many languages have both reference types and value types.  Python is a
>> bit unusual in that it has only reference types.  I would have picked a
>> different example to illustrate that, but it's true nonetheless.
> 
> If one accepts that there are a "lot" of people
> who post in here that clearly are surprised by
> Python's assignment semantics, and further appear
> to expect assignment to have copy-like semantics,
> then where is that expectation coming from?

I think it comes from people stumbling across posts in this forum 
claiming that Python has unusual assignment semantics.  I wish people 
would stop saying that, as it causes a lot of confusion.

> How would anyone develop that expectation if (from
> a different post in this thread), "[Python's] idea
> of assignment is the same as anyone else's."

I can think of two ways:

1. They're new to programming in general, and would have had the same 
expectation for any other language.  OR,

2. They already understand some other language, and then they come here 
and read wild claims that Python's assignment and parameter-passing 
semantics are different from other languages.  Duped by this claim, they 
  conclude that, if it's unlike other languages, then Python must have 
copy semantics.

> If you maintain that reference-like assignment
> is very common and something every programmer is
> accustomed to, then where are they getting the
> copy-like assignment expectations from?

Reference-like assignment IS very common (see 
<http://www.strout.net/info/coding/valref/>).  So, see above.

> I agree that most of the time, when one is using
> large (memory) composite "objects", and one needs
> to pass, or access them by different names, one will
> often use references to do so in order to avoid
> expensive copies or to get desired "shared" behavior.

Right.

> But (with the exception of C arrays [*1]), doing so
> requires some special syntax in all the languages I
> mentioned (AFAIK).

Whether you consider it "special" or not, pointers are extremely common 
in C.  Even more so in C++, which is the closest thing to an OOP 
language in the list of moldy languages you mentioned.

You also mentioned VBA -- if that's anything like VB, it does NOT 
require any special syntax; a variable is a reference type if its 
declared type is a class or string, and a simple type if it's anything 
else (just like in .NET, Java, and REALbasic).

> So it still seems to me that this is a likely
> explanation to why there is frequent misunderstanding
> of Python's assignments, and why responding to such
> misunderstandings with, "Python's assignments are
> the same as other languages'", is at best not helpful.

I don't think so.  More likely, people are being confused by the claim 
that Python's assignments are NOT like other languages, when in fact 
they are.

Best,
- Joe