block scope?

[Neal Becker]

One thing I sometimes miss, which is common in some other languages (c++),
is idea of block scope. It would be useful to have variables that did not
outlive their block, primarily to avoid name clashes. This also leads to
more readable code. I wonder if this has been discussed?

[James Stroud]

Neal Becker wrote:

One thing I sometimes miss, which is common in some other languages (c++),
is idea of block scope. It would be useful to have variables that did not
outlive their block, primarily to avoid name clashes. This also leads to
more readable code. I wonder if this has been discussed?

Probably, with good code, block scope would be overkill, except that I
would welcome list comprehensions to have a new scope:
pyi
------------------------------------------------------------
Traceback (most recent call last):
File "<ipython console>", line 1, in <module>
<type 'exceptions.Nam eError'>: name 'i' is not defined

py[i for i in xrange(4)]
[0, 1, 2, 3]
pyi # hoping for NameError
3

[Paul Rubin]

James Stroud <js*****@mbi.uc la.eduwrites:

Probably, with good code, block scope would be overkill, except that I
would welcome list comprehensions to have a new scope:

Block scope is a win because it gets rid of the uncertainty of whether
the variable is used outside the block or not. The "good code" theory
(just manually don't use the variable elsewhere) doesn't always hold
up under release deadline pressure and so on and doesn't make sense
anyway. What's the point of NOT having block scope if you don't want
to allow for creating variables in inner blocks and using them in
other blocks? I think it's best to require creating the variable
in a mutually enclosing scope if you want to use it that way.

[John Nagle]

Paul Rubin wrote:

James Stroud <js*****@mbi.uc la.eduwrites:

>>Probably, with good code, block scope would be overkill, except that I
would welcome list comprehensions to have a new scope:

Block scope is a win because it gets rid of the uncertainty of whether
the variable is used outside the block or not.

In a language with few declarations, it's probably best not to
have too many different nested scopes. Python has a reasonable
compromise in this area. Functions and classes have a scope, but
"if" and "for" do not. That works adequately.

Javascript got it wrong. They have declarations, but the default,
in the absence of a declaration, is global, not local or an error.
Bad design. It's a result of retrofitting declarations to a language,
which usually has painful aftereffects.

John Nagle

[Paul Rubin]

John Nagle <na***@animats. comwrites:

In a language with few declarations, it's probably best not to
have too many different nested scopes. Python has a reasonable
compromise in this area. Functions and classes have a scope, but
"if" and "for" do not. That works adequately.

I think Perl did this pretty good. If you say "my $i" that declares
$i to have block scope, and it's considered good practice to do this,
but it's not required. You can say "for (my $i=0; $i < 5; $i++) { ... }"
and that gives $i the same scope as the for loop. Come to think of it
you can do something similar in C++.

[James Stroud]

Paul Rubin wrote:

John Nagle <na***@animats. comwrites:

> In a language with few declarations, it's probably best not to
have too many different nested scopes. Python has a reasonable
compromise in this area. Functions and classes have a scope, but
"if" and "for" do not. That works adequately.

I think Perl did this pretty good. If you say "my $i" that declares
$i to have block scope, and it's considered good practice to do this,
but it's not required. You can say "for (my $i=0; $i < 5; $i++) { ... }"
and that gives $i the same scope as the for loop. Come to think of it
you can do something similar in C++.

How then might one define a block? All lines at the same indent level
and the lines nested within those lines?

i = 5
for my i in xrange(4):
if i: # skips first when i is 0
my i = 100
if i:
print i # of course 100
break
print i # i is between 0 & 3 here
print i # i is 5 here
Doesn't leave a particularly bad taste in one's mouth, I guess (except
for the intended abuse).

James

[Neal Becker]

James Stroud wrote:

Paul Rubin wrote:

>John Nagle <na***@animats. comwrites:

>> In a language with few declarations, it's probably best not to
have too many different nested scopes. Python has a reasonable
compromise in this area. Functions and classes have a scope, but
"if" and "for" do not. That works adequately.

I think Perl did this pretty good. If you say "my $i" that declares
$i to have block scope, and it's considered good practice to do this,
but it's not required. You can say "for (my $i=0; $i < 5; $i++) { ... }"
and that gives $i the same scope as the for loop. Come to think of it
you can do something similar in C++.

How then might one define a block? All lines at the same indent level
and the lines nested within those lines?

i = 5
for my i in xrange(4):
if i: # skips first when i is 0
my i = 100
if i:
print i # of course 100
break
print i # i is between 0 & 3 here
print i # i is 5 here
Doesn't leave a particularly bad taste in one's mouth, I guess (except
for the intended abuse).

James

Yes, the above is pretty much what I had in mind. +1.

[irstas]

On Apr 7, 6:48 am, James Stroud <[e-mail] la.eduwrote:

Neal Becker wrote:

One thing I sometimes miss, which is common in some other languages (c++),
is idea of block scope. It would be useful to have variables that did not
outlive their block, primarily to avoid name clashes. This also leads to
more readable code. I wonder if this has been discussed?

Probably, with good code, block scope would be overkill, except that I
would welcome list comprehensions to have a new scope:

Generator expressions have a new scope, and in Python 3.0 list
comprehensions will have one as well (according to http://www.python.org/dev/peps/pep-0289/
). It's a fix that might break existing code so it couldn't be
introduced in "minor" versions like 2.4 and 2.5.

[Alex Martelli]

Neal Becker <nd*******@gmai l.comwrote:
...

i = 5
for my i in xrange(4):
if i: # skips first when i is 0
my i = 100
if i:
print i # of course 100
break
print i # i is between 0 & 3 here
print i # i is 5 here
Doesn't leave a particularly bad taste in one's mouth, I guess (except
for the intended abuse).

James

Yes, the above is pretty much what I had in mind. +1.

I prefer Java's approach (14.4.2 in the JLS 2nd edition): forbid "inner"
blocks from shadowing variables in "outer" ones. I quote:
"""
If a declaration of an identifier as a local variable of the same
method, constructor, or initializer block appears within the scope of a
parameter or local variable of the same name, a compile-time error
occurs.
Thus the following example does not compile:

class Test {
public static void main(String[] args) {
int i;
for (int i = 0; i < 10; i++)
System.out.prin tln(i);
}
}
This restriction helps to detect some otherwise very obscure bugs.
"""
I entirely agree with the JLS here, having fought just such bugs in C++
and other languages that lack the restriction in question. I just wish
Python had adopted the same restriction regarding nested functions, when
proper lexical scoping was introduced -- I argued for it at the time,
but backwards compatibility blocked its introduction. There are
definitely NOT many Java-specific language characteristics that I like,
but this one is a winner!-) [[but, I disagree with the lack in Java of
a similar restriction against shadowing between instance variables and
local variables, and the weak rationale for that in the JLS:-)]].
Alex

[Steve Holden]

Alex Martelli wrote:

Neal Becker <nd*******@gmai l.comwrote:
...

>>i = 5
for my i in xrange(4):
if i: # skips first when i is 0
my i = 100
if i:
print i # of course 100
break
print i # i is between 0 & 3 here
print i # i is 5 here
Doesn't leave a particularly bad taste in one's mouth, I guess (except
for the intended abuse).

James

Yes, the above is pretty much what I had in mind. +1.

I prefer Java's approach (14.4.2 in the JLS 2nd edition): forbid "inner"
blocks from shadowing variables in "outer" ones. I quote:
"""
If a declaration of an identifier as a local variable of the same
method, constructor, or initializer block appears within the scope of a
parameter or local variable of the same name, a compile-time error
occurs.
Thus the following example does not compile:

class Test {
public static void main(String[] args) {
int i;
for (int i = 0; i < 10; i++)
System.out.prin tln(i);
}
}
This restriction helps to detect some otherwise very obscure bugs.
"""
I entirely agree with the JLS here, having fought just such bugs in C++
and other languages that lack the restriction in question. I just wish
Python had adopted the same restriction regarding nested functions, when
proper lexical scoping was introduced -- I argued for it at the time,
but backwards compatibility blocked its introduction. There are
definitely NOT many Java-specific language characteristics that I like,
but this one is a winner!-) [[but, I disagree with the lack in Java of
a similar restriction against shadowing between instance variables and
local variables, and the weak rationale for that in the JLS:-)]].

What do you think the chances are of this being accepted for Python 3.0?
It is indeed about the most rational approach, though of course it does
cause problems with dynamic namespaces.

regards
Steve
--
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