It is not syntactic sugar, "x := 10" is an assignment expression in contrast with "x = 10", which is a statement.
Hence the former can be used in contexts like "if x := 10: pass", which is the whole point of the PEP.
replies(2):
Hence the former can be used in contexts like "if x := 10: pass", which is the whole point of the PEP.
For example, import mod is NOT defined as
mod = eval(open("mod.py").read())
That's why := is just syntactic sugar; there are no new semantics.
I don't think that's right; what expression/statement is `x := y` equivalent to? I'm thinking in particular about using mutable collections to emulate assignment in a lambda, e.g.
>>> counter = (lambda c: lambda: (c.append(c.pop() + 1), c[0])[1])([0])
>>> counter()
1
>>> counter()
2
>>> counter()
3
>>> counter = (lambda c: lambda: (c := c + 1))(0)
Maybe it's equivalent to using a `=` statement, but statements are forbidden inside lambdas. Maybe the lambdas are equivalent to `def ...` functions, but what would their names be? Even if we made the outer one `def counter(c)...` the resulting value would have a different `func_name` (`counter` versus `<lambda>`).
Even the `if` examples that are scattered around this page don't seem to have an equivalent. For example:
if (x := foo() is not None):
do_something()
x = foo()
if x is not None:
do_something
if (sys.stdout.write(x), x := foo() is not None)[1]:
do_something
x = foo()
if (sys.stdout.write(x), x is not None)[1]:
do_something
if foo(x) and (x := bar()):
do_something
PS: I think this `:=` is a good thing, and writing the above examples just reminded me how infuriating it is when high-level languages distinguish between statements and expressions, rather than having everything be an expression!
Take a more familiar example:
x, y = (y, x)
z = (y, x)
x = z[0]
y = z[1]
del(z)
z = "hello world"
x, y = (y, x)
print(z)
(lambda z: (x := z[0], y := z[1]))((y, x))
class A(object):
def __init__(self, x):
super(A, self).__setattr__('x', x)
def __setattr__(self, name, value):
if name == "x":
raise Exception("Don't override 'x'")
return super(A, self).__setattr__(name, value)
>>> a = A('foo')
>>> a.y = 'bar'
>>> print(repr({'x': a.x, 'y': a.y}))
{'y': 'bar', 'x': 'foo'}
>>> a.x, a.y = (a.y, a.x)
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 6, in __setattr__
Exception: Don't override 'x'
On the one hand, that's a pretty crappy program. But on the other it demonstrates that "use a temporary variable" is not "easy" in the general case (which is what language implementations must handle).
> You need to use a temporary variable but then your example is easy.
Yes this example, of `if foo(x) and (x := bar()):`, would be easy with a temporary variable. But there are infinite variations we can make:
if foo(x) and (x := bar()):
if foo(x) or (x := bar()):
if (x := baz()) and foo(x) and (x := bar()):
if foo(x, y) and (x := bar()) and baz(x) and (y := quux()):
...
I would suggest that if you can express the exact same semantics with a "few" more lines then it's just sugar.
In the case of x := y, it's always possible to rewrite the program with a "few" extra lines where it means the same thing. It's just combining the assignment and expose operations.
Unless you can provide an example where that isn't true, it's just sugar, i.e. unneeded, but maybe desired, syntax.
I agree. The important question is what we mean by "the exact same semantics". I would say that observational equivalence is the most appropriate; i.e. that no other code can tell that there's a difference (without performing unpredictable side-effects like parsing the contents of the source file). Python is a really difficult language for this, since it provides so many hooks for redefining behaviour. For example in many languages we could say that 'x + x' and 'x * 2' and 'x << 1' are semantically the same (they double 'x'), but in Python those are very different expressions, which can each invoke distinct, arbitrary code (a `__mul__` method, an `__add__` method, etc.). The fact they often do the same thing is purely a coincidence (engineered by developers who wish to remain sane).
It's fine if we only care about the 'black box' input/output behaviour, but at that point it no longer matters which language we're using; we could have something more akin to a compiler rather than desugaring into expressions from the same language.
> it's always possible to rewrite the program
There's an important distinction here too. Are we saying that "a semantically equivalent program exists"? That's a trivial consequence of Turing completeness (e.g. there's always an equivalent turing machine; and an equivalent lambda calculus expression; and an equivalent Java program; etc.)
Are we saying that an algorithm exists to perform this rewriting? That would be more useful, since it tells us that Rice's theorem doesn't apply for this case (otherwise it might be impossible to tell if two programs are equivalent or not, due to the halting problem).
Are we saying that we know an algorithm which will perform this rewriting? This is the only answer which lets us actually run something (whether we call that an "elaborator", a "compiler", etc.). Yet in this case I don't know of any algorithm which is capable of rewriting Python involving `:=` into Python which avoids it. I think such an algorithm might exist, but I wouldn't be surprised if Python's dynamic 'hooks' actually make such rewriting impossible in general.
I certainly don't think that a local rewrite is possible, i.e. where we can swap out any expression of the form `x := y` without changing any other code, and keep the same semantics. If it is possible, I would say that such a local, observational equivalence preserving rewrite rule would qualify for the name "syntactic sugar".
> It's just combining the assignment and expose operations.
I'm not sure what you mean by "expose", and a search for "python expose" didn't come up with anything. It would be nice to know if I've missed out on some Python functionality!
What makes you say that? I would say it's crucial. Syntactic sugar is anything where we can say "Code of the form 'foo x y z...' is defined as 'bar x y z...'" where both forms are valid in the same language. Such a definition, by its very nature, gives us an automatic translation (look for anything of the first form, replace it with the second).
> It just means that in all cases a human can rewrite it without the new syntax and get the same semantics.
Yet that's so general as to be worthless. I'm a human and I've rewritten Java programs in PHP, but that doesn't make Java "syntactic sugar" for PHP.
How about integer arithmetic? That's the programming language Goedel used for his incompleteness theorems (specifically, he showed that the semantics of any formal logical system can be implemented in Peano arithmetic, using Goedel numbering as an example).
I wouldn't call that a useful definition though. There are reasons why we don't treat RAM as one giant binary integer.