Is is possible to have optional keys in string formats using '%' operator? I’m using the logging API with Python 2.7, so I can't use Advanced String Formatting.

My problem is as follow:

>>> import logging>>> FORMAT = '%(asctime)-15s %(message)s %(user)s'
>>> logging.basicConfig(format=FORMAT)>>> logging.warning("It works for:", extra={'user': 'me'})
2016-08-29 11:24:31,262 It works for: me>>> logging.warning("It does't work!")
Traceback (most recent call last):...
KeyError: 'user'
Logged from file <input>, line 1

I want to have an empty string for user if missing. How can I do that?

I tried with a defaultdict, but it fails:

>>> import collections
>>> extra = collections.defaultdict(unicode)
>>> logging.warning("It does't work!", extra=extra)
Traceback (most recent call last):...
KeyError: 'user'
Logged from file <input>, line 1

By contrast, with Jinja2, we can do:

>>> import jinja2
>>> jinja2.Template('name: {{ name }}, email: {{ email }}').render(name="me")
u'name: me, email: '

=> no exception here, just an empty string (for "email").

A) The defaultdict approach works fine, but only if used directly.

>>> import collections
>>> dd=collections.defaultdict(str)
>>> dd['k'] = 22
>>> '%(k)s %(nn)s' % dd
'22 '

B) The extra argument to a log function is used as described in the docs, i.e. not directly as shown above. That's why using a defaultdict instead of a regular dict does not make a difference.

The third keyword argument is extra which can be used to pass adictionary which is used to populate the dict of the LogRecordcreated for the logging event with user-defined attributes.

C) You can use a logging filter to take care of the missing extra data:

import loggingclass UserFilter:def filter(self, record):try:record.userexcept AttributeError:record.user = '<N/A>'return TrueFORMAT = '%(asctime)-15s %(message)s %(user)s'
logging.basicConfig(format=FORMAT)
logging.getLogger().addFilter(UserFilter())logging.warning("It works for:", extra={'user': 'me'})logging.warning("It doesn't work!")
# DATE TIME It doesn't work! <N/A>

Any class with a filter method is fine. It can modify the record in-place and it must return True for accepting the record or False for filtering it out.