Popping key causes dict derived from object to revert to object

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]))

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?

Cheers,

Loris

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