[Python-Dev] transitioning from % to {} formatting

Eric Smith eric at trueblade.com
Fri Oct 2 00:58:31 CEST 2009

Vinay Sajip wrote:
> Good point as far as the general case is concerned, though it's perhaps not that
> critical for logging. By which I mean, it's not unreasonable for
> Formatter.__init__ to grow a "style" keyword parameter which determines whether
> it uses %-, {}- or $-formatting. Then the formatter can look for '%(asctime)s',
> '{asctime}' or '$asctime' according to the style. 

It's tangential, but in the str.format case you don't want to check for 
just '{asctime}', because you might want '{asctime:%Y-%m-%d}', for example.

But there are ways to delay computing the time until you're sure it's 
actually being used in the format string, without parsing the format 
string. Now that I think of it, the same technique could be used with 

import datetime

class DelayedStr:
     def __init__(self, fn):
         self.fn = fn
         self.obj = None
     def __str__(self):
         if self.obj is None:
             self.obj = self.fn()
         return self.obj.__str__()

def current_time():
     print "calculating time"
     return datetime.datetime.now()

# will not compute current time
print '%(msg)s' % {'asctime':DelayedStr(current_time),

# will compute current time: same dict used as before
print '%(asctime)s %(msg)s' % {'asctime':DelayedStr(current_time),


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