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To: python-list@python.org
From: Terry Reedy <tjreedy@udel.edu>
Subject: Re: Time we switched to unicode? (was Explanation of this Python language feature?)
Date: Tue, 25 Mar 2014 19:55:39 -0400
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On 3/25/2014 11:18 AM, Steven D'Aprano wrote:

> The thing is, we can't just create a =E2=88=91 function, because it doe=
sn't work
> the way the summation operator works. The problem is that we would want=

> syntactic support, so we could write something like this:
>
>      p =3D 2
>      =E2=88=91(n, 1, 10, n**p)

Of course we can. If we do not insist on separating the dummy name from=20
the expression that contains it. this works.

def sigma(low, high, func):
     sum =3D 0
     for i in range(low, high+1):
         sum +=3D func(i)
     return sum

p =3D 2
print(sigma(1, 10, lambda n: n**p))
 >>>
385
which looks correct. This corresponds to a notation like so

10
=E2=88=91 n: n**p
1

which, for ranges, is more sensible that the standard. Lambda is an=20
explicit rather than implicit quotation.

If you do insist on separating two things that belong together, you can=20
quote with quote marks instead and join within the function to get the=20
same answer.

def sig2(var, low, high, exp):
     func =3D eval("lambda {}: {}".format(var, exp))
     sum =3D 0
     for i in range(low, high+1):
         sum +=3D func(i)
     return sum

p =3D 2
print(sig2('n', 1, 10, 'n**p'))
 >>>
385

To me, both these apis are 'something' like the api with implicit=20
quoting, which is impossible for function calls, but common in=20
statements. (This is one reason to make a construct a python statement=20
rather than function. Jumping is another) Using the same api, one could=20
instead expand the template to include the for loop and use exec instead =

of eval.

def sig3(var, low, high, exp):
     loca =3D {}
     exec(
'''\
sum =3D 0
for {} in range({}, {}):
     sum +=3D {}\
'''.format(var, low, high+1, exp), globals(), loca)

     return loca['sum']

print(sig3('n', 1, 10, 'n**p'))


> This cannot be an ordinary function, because if it were, the n**p
> expression would be evaluated before passing the result to the function=
=2E

So would the dummy parameter name n.

> We want it to delay evaluation, like a list comp:
>
>      [n**p for n in range(1, 11)]
>
> the expression n**p gets evaluated inside the list comp.

Which is to say, it is implicitly quoted

> That cannot be written as a function either:
>
>      list_comp(n**p, n, range(1, 11)

It can be with explicit quoting, similar to how done for sigma.  Some=20
lisps once and maybe still have 'special' functions that implicitly=20
quote certain arguments. One just has to learn which functions and which =

parameters. I prefer having implicit quoting relegated to a limited set=20
of statements, where is it pretty clear what must be quoted for the=20
statement to make any sense. Assignment is one example, while, for, and=20
comprehensions are others.

--=20
Terry Jan Reedy