How to use string formatting to dynamically assign variables

Question

In Python, I am populating an object to model the configuration, environment and other aspects related to rsyslog on the local machine (RHEL 6.4 Python 2.6.4) I loop through many instances of the rsyslog daemon (this_instance_number), and many attributes (attribute) defined in a configuration file.

With those value I populate the object.

Given the following variables:

this_instance_number=5
attribute="startupscript"

The following line of code,

print ('rsyslog.instance%(instance_number)s.%(attribute)s' %\
      {"instance_number": this_instance_number, "attribute": attribute})

will print this text:

rsyslog.instance5.startupscript

How can I then assign the attribute that text would refer to to a value based on that format string?

For example, if hard coded, I would assign:

rsyslog.instance5.startupscript="/etc/init.d/sample"

But, I want to assign it something like this:

('rsyslog.instance%(instance_number)s.%(attribute)s' %\
      {"instance_number": this_instance_number, "attribute": attribute}) = variable

Martijn Pieters · Accepted Answer · 2014-03-12 12:12:58Z

2

You'd use getattr() to dynamically retrieve attributes:

instance = getattr(rsyslog, 'instance{}'.format(this_instance_number))
print getattr(instance, attribute)

and setattr() to assign:

instance = getattr(rsyslog, 'instance{}'.format(this_instance_number))
setattr(instance, attribute, variable)

or, for a more generic approach with arbitrary depth:

def get_deep_attr(obj, *path):
    return reduce(getattr, path, obj)

def set_deep_attr(obj, value, *path)
    setattr(get_deep_attr(obj, path[:-1]), path[-1], value)

print get_deep_attr(rsyslog, 'instance{}'.format(this_instance_number), attribute)
set_deep_attr(rsyslog, variable, 'instance{}'.format(this_instance_number), attribute)

edited Mar 12, 2014 at 12:12

answered Mar 12, 2014 at 10:59

Martijn Pieters

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3 Comments

Justin Haynes Over a year ago

Martijn, how then do we populate deeper objects? If I need to edit my original question I will. In a nutshell I'm looping to create rsyslog.<instance#>, and within that loop looping to create the rsyslog.<instance#>.<attribute>s. Your setattr appears to be setting the object's attribute back to whatever you just read, but I will be setting unique values. Is it possible to use settattr like this?: setattr(rsyslog, instance5.startupscript, "/etc/init.d/sample")? By extension then how do we replace "instance5.startupscript" dynamically?

Martijn Pieters Over a year ago

@JustinHaynes: updated with a generic approach to allow for arbitrary-depth attribute chains.

Martijn Pieters Over a year ago

@JustinHaynes: the original approach will still work just fine though, you must've futzed some variable somewhere in your loop.

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