Re: Popping key causes dict derived from object to revert to object

Loris Bennett wrote:

Hi,
 I am using SQLAlchemy to extract some rows from a table of 'events'.
 From the call to the DB I get a list of objects of the type
    sqlalchemy.orm.state.InstanceState
 I would like to print these rows to the terminal using the 'tabulate'
package, the documentation for which says
    The module provides just one function, tabulate, which takes a list of
   lists or another tabular data type as the first argument, and outputs
   a nicely formatted plain-text table
 So as I understand it, I need to convert the InstanceState-objects to,
say, dicts, in order to print them.  However I also want to remove one
of the keys from the output and assumed I could just pop it off each
event dict, thus:
               event_dicts = [vars(e) for e in events]
     print(type(event_dicts[0]))
     event_dicts = [e.pop('_sa_instance_state', None) for e in event_dicts]
     print(type(event_dicts[0]))

vars() returns the __dict__ attribute of the object.  It may not be a good idea to modify that dictionary directly (it will also affect the object), although it might be OK if you're not going to do anything else with the original objects.  To be safer, you could copy the event objects:
     event_dicts = [dict(vars(e)) for e in events]
or:
     event_dicts = [vars(e).copy()]

However, this prints
    <class 'dict'>
   <class 'sqlalchemy.orm.state.InstanceState'>
 If I comment out the third line, which pops the unwanted key, I get
    <class 'dict'>
   <class 'dict'>
 Why does popping one of the keys cause the elements of the list to
revert back to their original class?

As Dieter pointed out, the main problem here is that pop() returns the value removed, not the dictionary with the rest of the values.  You probably want something more like:
     for e in event_dicts:
         del e['_sa_instance_state']
(There's not really any point popping the value if you're not going to do anything with it - just delete the key from the dictionary)
--
Mark.