Don't format strings before logging in python

Don’t format strings before logging in python

You should not provide formatted string to loggers in Python, like this:'Logged in: %s' % username)

You should write like this:'Logged in: %s', username)


In mont cases, it is the same as a result. But, internally, the later one contains more information which part of this string represents a username.

You shoud realize the first argument (message) can also be used as a signature.

If you want to aggregate logs, you will group logs by messages, like this:

  • message: ‘Logged in: %s’, args: (‘Ritsu Tainaka’,)
  • message: ‘Logged in: %s’, args: (‘Mio Akiyama’,)

Yes, you will be able to group these logs. They are same log, just username is different.

OK, now consider this grouping with formatted strings, it will be not work:

  • message: ‘Logged in: Ritsu Tainaka’, args: ()
  • message: ‘Logged in: Mio Akiyama’, args: ()

They will be handled as different. Of cause, these messages are totally different.


Sentry, error logging and aggregation platform, it displays logs grouping by these messages.

So, If you use Sentry, you should provide not formatted message to any loggers. Without this, all logs containing some variables will be handled as different. Yes, as thousands of different logs.