[Python-Dev] transitioning from % to {} formatting (original) (raw)

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 %-formatting:

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), 'msg':'test'}

will compute current time: same dict used as before

print '%(asctime)s %(msg)s' % {'asctime':DelayedStr(current_time), 'msg':'test'}

Eric.

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