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I'm a -0.9 on this one. I really like that Python is powerful, but
also a great pedagogical language. <br>
<br>
I don't like that whereas before you could teach someone {} creates a
dict, but now you have to say {} creates a dict, if there are colons
inside, or it's empty, but otherwise creates a frozenset. I also don't
like that it will be easy (especially coming from Certain Other
Languages) to make the following error:<br>
<br>
my_dict = {<br>
'a', 'b',<br>
'c', 'd',<br>
'e', 'f'<br>
}<br>
<br>
Moreover, I don't like that once you've done that, you'll be able to do<br>
<br>
if 'c' in my_dict:<br>
...<br>
<br>
and have it evaluate as true even though you aren't dealing with a
dict. Those are ugly gotchas for something so basic to the language.
Newcomers will run into it a lot.<br>
<br>
As a half-baked alternative thought, what about using {{ 'a', 'b', 'c'
}} for the syntax. It's visually clearer, and still syntactically
unambiguous, because a dict can't have another dict as a key, and a
(frozen)set can't have a dict inside it. Thinking about possible
issues--what if you come across a construct with three opening braces
at the start? ({{{)<br>
<br>
* It can't be a dict within a dict within a dict, because a dict is not
hashable, so can't be a key.<br>
* It can't be a dict within a frozenset, because a dict is not hashable.<br>
* It can (and must) be a frozenset within a dict, because a frozenset
is hashable, and hence a valid dict key.<br>
<br>
Four braces? ({{{{)<br>
* I think this has to be a frozen set within a frozen set.<br>
<br>
More?<br>
* The outer one is a dict if an odd number of braces, everything else
is a frozen set.<br>
<br>
Obviously, the more complex cases get less visually clear, but (1) they
are not common cases, and (2) they are still unambigous, and will often
be made clearer by (a) the placement of closing brackets, and (b)
decent syntax highlighting.<br>
<br>
Any major holes in this idea? <br>
<br>
Cheers,<br>
Cliff<br>
<br>
<br>
<br>
<br>
<br>
Nick Coghlan wrote:
<blockquote cite="mid479A08BB.6010101@gmail.com" type="cite">
<pre wrap="">Mark Summerfield wrote:
</pre>
<blockquote type="cite">
<pre wrap="">On 2008-01-25, Guido van Rossum wrote:
</pre>
<blockquote type="cite">
<pre wrap="">For the record, I'm thinking Raymond has won this argument fair and
square, and I'm withdrawing my opposition.
I hope it isn't too confusing that {1: 1} creates a *mutable* dict
while {1} creates an *immutable* frozenset. I still find this slightly
inelegant. But the practicality of being able to treat set literals as
compile-time constants wins me over.
</pre>
</blockquote>
<pre wrap="">So this will produce:
frozenset() # empty frozen set
{1} # 1 item frozen set
{1, 2} # 2 item frozen set
{} # empty dict
{1:1} # 1 item dict
{1:1, 2:2} # 2 item dict
</pre>
</blockquote>
<pre wrap=""><!---->
More completely:
() # empty tuple
(1,) # 1 item tuple
(1, 2) # 2 item tuple
[] # empty list
[1] # 1 item list
[1, 2] # 2 item list
{} # empty dict
{1:1} # 1 item dict
{1:1, 2:2} # 2 item dict
frozenset() # empty frozen set
{1} # 1 item frozen set
{1, 2} # 2 item frozen set
set() # empty mutable set
set({1}) # 1 item mutable set
set({1, 2}) # 2 item mutable set
So with Raymond's proposal we will have syntax for two immutable
literals (tuples, frozensets) and two mutable container displays (lists,
dicts).
Yes, there will be a few anomalies to learn in this list:
- 1-tuples require a trailing comma
- {} is a dict rather than a frozen set
- frozen sets are immutable while dicts are mutable
Do these anomalies make this area of the language syntax harder to
learn? Almost certainly - the 1-tuple anomaly has been tripping people
up for years. Despite any reservation, are there valid reasons for
having these anomalies in Py3k? As far as I am concerned, yes there
are*, and I believe that is Guido's view as well.
Cheers,
Nick.
*Taking them from the top:
- 1-tuples require a trailing comma to differentiate them from the use
of parentheses for mere expression grouping. Expression grouping is kind
of important, and this anomaly in the syntax is a small price to pay for
making that work intuitively.
- {} is used extensively in existing code (both operational code and
code in documentation and other examples). Py3k may lower the bar for
'acceptable breakage' in the realm of backwards compatibility, but it
doesn't get rid of it altogether - and changing the meaning of {} fails
to clear even that lowered hurdle. Also, as Marcin pointed out, an empty
frozenset() is pretty useless, while an empty dict() is common.
- making set literals immutable provides excellent optimisation
opportunities, which is important because it is a concern for speed
which is likely to lead to the use of a set in the first place. It is
also convenient in that set() is a lot easier to type than frozenset(),
so going from an immutable literal to a mutable container is easier than
going the other way would have been.
</pre>
</blockquote>
<br>
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