Question

I’m a python newbie so I apologize in advance if this question has been asked before.

I am building out an application in GAE and need to generate a report that contains the values for a user-defined subset of fields.

For example, in my db model, CrashReport, I have the following fields:

• entry_type
• entry_date
• instance_id
• build_id
• crash_text
• machine_info

I present a user with the above list as a checkbox group from which they select. Whichever fields the user selects, I then create a report showing all the values in the datastore, but only for the fields that they selected. For example, if from the above list, the user selects the build_id and crash_text fields, the output might look like this:

build_id  crash_text
0.8.2      blown gasket
0.8.2      flat tire
0.8.1      crack
…

So the question is, how exactly do I only access the values for the fields which the user has defined?

Answer 1

Given a model instance mi and an attribute name an, getattr(mi, an) will fetch the value of attribute an for object mi. It will raise AttributeError if object mi has no attribute by that name; if you want to avoid this, try e.g. getattr(mi, an, None).

So if you have a list of attribute names la,

[getattr(mi, x, None) for x in la]

gives you a list of the values of the various attributes whose names are in la for a given instance mi.

If you also have a list of instances li, and want a list of lists (one outer list per instance, each with all the attribute values),

[[getattr(mi, x, None) for x in la] for mi in li]

And so forth. These are all general Python concepts and operations and they apply to instances of models that you have fetched from the datastore, no less (and no more) than to any other Python objects.