[Python-Dev] On Syntax Extensibility (again)

2003年6月09日 21:30:07 +0200

still thinking about this stuff.
At 01:19 07.06.2003 +0200, Samuele Pedroni wrote:
>I thought python-dev archives would be a better place for this than my hd. 
>Warning: terse.
>>I have come to the conclusion that I should agree with Guido:
>>- generators do not let really abstract over things like exception 
>handling etc.

what I had in mind were generators like this one:
def all_lines(file_list_filename):
 for fn in open(file_list_filename,'r'):
 fn = fn.rstrip()
 for line in open(fn,'r'):
 yield line
for line in all_lines("all_files"):
 <do-stuff>
what if we want to release resources (here in this toy example namely close 
files) in a timely fashion in face of exception also in <do-stuff>.
all of these touch on the issue:
- non-support of 'yield' inside try-finally
- PEP 288: Generators Attributes and Exceptions
- PEP 310: Reliable Acquisition/Release Pairs
- proposals for syntax extensibility through Ruby/Smalltalk-like 
first-class blocks
so all these proposals should be considered together and in an organic way.
With current Python a possible rewrite of the above would be:
class all_lines(object):
 def __init__(self,file_list_fname):
 self.listf = open(file_list_fname,'r')
 self.f = None
 def __iter__(self):
 for fn in self.listf:
 fn = fn.rstrip()
 self.f = open(fn,'r')
 for line in self.f:
 yield line
 self.f.close()
 self.f = None
 def close(self):
 self.listf.close()
 if self.f: self.f.close()
x = all_lines("all_files")
try:
 for line in x:
 <do-stuff>
finally:
 x.close()
kind of obfuscated.
maybe-generators-should-grow-a-close-method-ly y'rs - Samuele