[Python-ideas] immutable classes [was: pre-PEP: Default Argument Expressions]

[Jan Kanis]

On the 'using old semantics when you really want to' part, that's very  
well possible with a decorator under the proposed semantics:

def caching(**cachevars):
	def inner(func):
		def wrapper(**argdict):
			for var in cachevars:
				if not var in argdict:
					argdict[var] = cachevars[var]
			return func(**argdict)
		return wrapper
	return inner

@caching(cache={})
def foo(in, cache):
	result = bar(in)
	cache[in] = result
	return result

This implementation of caching doesn't handle positional args, but it can  
be made to. One such decorator would still be a net win of several hundred  
lines of code in the standard lib.


Of course, IMHO, the real fix to this is to 1) have default expressions be  
evaluated at calltime, and 2) have _all_ lexical variables be bound at  
definition time and 3) make them immutable. Then something like

lst = []
for i in range(10):
	lst.append(lambda i: i*i)

would work. That would be a real win for functional programming. (good  
thing)
Unfortunately Guido's decided not to support (1), and (2) has been  
proposed some time ago and didn't make it. In both cases because it would  
be to big a departure from how Python currently works. (3) is quite  
impossible in a language like python. <mode=dreaming> I just hope if  
python were designed today it would have done these. </mode>

- Jan