Why is the migration to py3k a concern?
For example I have libraries which use string%dictionary
substitution where the dictionary is actually an object
which emulates a dictionary. The __getitem__ for
the object can be very expensive and is only called when
needed by the string substitution.
In py3k string%dictionary is going away. Why?
I have no idea.
The replacement is a string.format(...) method
which supports dictionary calling.
string.format(**dictionary)
But dictionary
calling doesn't support dictionary emulation.
So in the example below the substitution works
but the call fails.
=== code
class fdict(dict):
def __getitem__(self, item):
return "got("+item+")"
def fn(**d):
print d["boogie"]
if __name__=="__main__":
fd = fdict()
print "attempting string substitution with fake dictionary"
print
print "hello there %(boogie)s" % fd # <-- works
print
print "now attempting function call with fake dictionary"
print
fn(**fd) # <-- fails
=== output
% python2.6 dtest.py
attempting string substitution with fake dictionary
hello there got(boogie)
now attempting function call with fake dictionary
Traceback (most recent call last):
File "dtest.py", line 17, in <module>
fn(**fd)
File "dtest.py", line 7, in fn
print d["boogie"]
KeyError: 'boogie'
==== end of output
Consequently there is no simple way to translate
my code, I think. I suspect you will find this kind of subtle
issue in many places. Or worse, you won't find it
until after your program has been installed
in production.
It's a damn shame because
if string%dict was just left in it wouldn't be an issue.
Also, if making f(**d) support dict emulation
has any negative performance implications
then I don't want it please.
sigh. -- Aaron Watters
===
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