[Python-Dev] Re: lists v. tuples

Glyph Lefkowitz glyph@twistedmatrix.com
Sat, 15 Mar 2003 22:19:30 -0600

Hash: SHA1

On Saturday, March 15, 2003, at 12:35 AM, python-dev-request@python.org 

> Then I realize that C++ has exactly this feature (it's called "const"),
> and that I find it to be an annoyance far more often than I find it
> handy. And I begin to question.

I have thought about this as well; I think that the problem is that in 
C++, you have to declare "const" *everywhere* -- you can't just pass a 
mutable data structure and have the "right thing" happen the way it 
most obviously should.  Arguably when one is mucking about at such a 
low level as is common in C++, this is something that you have to be 
really careful about, but I still think that it's handled badly in the 

Imagine for a moment that dictionaries and lists in Python had a 
"const" method which would immutabilify them (if that's even a word).  
The following example:

>> const char* strrev(const char* torev) {
>> 	// reverse the string
>> }
>> ...
>> 	char* x = "1234";
>> 	char* y = (char*) strrev((const char*)x); // tell me I've been bad, 
>> g++!
>> ...

As opposed to:

>> def strrev(s):
>>     "Please pass me immutable data!"
>>     # reverse the string
>> ...
>> 	x = '1234'
>> 	y = strrev(x.const()).copy()
>> ...

I think that the latter is far less likely to annoy.  Of course, in 
this hypothetical example, I can design all kinds of convenient 
behavior for these Python mutability operations to have, like 'copy' 
always returning a mutable shallow copy of the data structure in 
question, and '.const()' making an object immutable and then returning 

This smells like another unformed PEP I don't have the time to think 
about or implement :-(, but I would definitely like to see mutability 
guarantees worm their way into the language at some point, too.

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