String concatenation vs. string formatting

[Benjamin Kaplan]

On Fri, Jul 8, 2011 at 1:18 PM, Andrew Berg <bahamutzero8825 at gmail.com>wrote:

> Is it bad practice to use this
> > logger.error(self.preset_file + ' could not be stored - ' +
> > sys.exc_info()[1])
> Instead of this?
> > logger.error('{file} could not be stored -
> > {error}'.format(file=self.preset_file, error=sys.exc_info()[1]))
>
>
> Other than the case where a variable isn't a string (format() converts
> variables to strings, automatically, right?) and when a variable is used
> a bunch of times, concatenation is fine, but somehow, it seems wrong.
> Sorry if this seems a bit silly, but I'm a novice when it comes to
> design. Plus, there's not really supposed to be "more than one way to do
> it" in Python.
>

String formatting is the One Right Way here. It's fine to use string
concatenation for a few things, but the operation is O(n^2) because each
concat occurs one at a time: Python allocates space for a string the size of
the first 2 things, copies the contents over. Then allocate a string the
size of that string plus the third string and copy the contents over. It can
get pretty slow if you're building a really big string With string
formatting, Python creates a single string large enough to copy all the
formatting arguements in and then copies the contents over once.

Also, string formatting (especially using the new syntax like you are) is
much clearer because there's less noise (the quotes all over the place and
the plusses) and it's better for dealing with internationalization if you
need to do that.
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