[Python-ideas] for/else statements considered harmful

Thu Jun 7 15:06:38 CEST 2012

So the subject of the thread seems to hold true. Average developers 
are confused by the current semantic (a problem that needs more than 
abstract p-code to correct) to the point of actively avoiding use of 
the structure.
I agree, however, that breaking all existing code is probably bad. ;)
On 2012年06月07日 00:53:22 +0000, Nick Coghlan said:
> for x in range(20):
> if x > 10:
> break
> except break:
> # Bailed out early
> else:
> # Reached the end of the loop

Seems a not insignifigant number of readers got fixated on the 
alternate keyword for the current behaviour of else (finally in my 
example) and ignored or misinterpreted the -really important part- of 
being able to detect if the loop was skipped (no iterations performed; 
else in my example).
Being able to have a block executed if the loop is never entered is 
vitally important so you can avoid expensive or potentially impossible 
length checks on the iterator before the loop. Take this example:
 sock = lsock.accept()
 for chunk in iter(partial(sock.recv, 4096), ''):
 pass # do something with the chunk
 else:
 pass # no data recieved before client hangup!
Using a temporary varable to simulate this is… unfortunate.
 sock = lsock.accept()
 has_data = False
 for chunk in iter(partial(sock.recv, 4096), ''):
 has_data = True
 pass # do something with the chunk
 if not has_data:
 pass # no data recieved before client hangup!
empty woud be a good keyword to preserve the existing meaning of else, 
but I'm pretty sure that's a fairly common variable name. :/
	— Alice.