Add method to drop scheme from database

[frankenjoe]

To avoid that a user removes a scheme that is used in a table, it could make sense to introduce Database.drop_scheme() to raise an error in that case.

[frankenjoe]

@hagenw what is your take here? Since we also don't have methods to drop media, raters, attachments etc. I wonder if we should simply close the issue as I don't see us implementing all those methods at the moment.

[hagenw]

I'm not completely sure if we can argue that we also don't have drop methods for media, raters, attachments as we have audformat.Database.drop_tables() which addresses also one of the important parts of the database.

But I think it is not very urgent as you can delete a scheme with del and most of the time you do it only if really needed. And if you delete a needed scheme everything else seems to work as expected, you can even save and load the database, e.g.

import audformat


db = audformat.Database('db')

db.schemes['spk'] = audformat.Scheme('str', labels=['a', 'b'])
index = audformat.filewise_index(['f1', 'f2'])
db['files'] = audformat.Table(index)
db['files']['spk'] = audformat.Column(scheme_id='spk')
db['files']['spk'].set(['a', 'a'])

db.schemes['label'] = audformat.Scheme('int')
index = audformat.segmented_index(['f1', 'f1'], [0, 1], [1, 2])
db['segments'] = audformat.Table(index)
db['segments']['spk'] = audformat.Column(scheme_id='spk')
db['segments']['spk'].set(['a', 'a'])

and then

>>> del db.schemes['spk']
>>> db
name: db                   
source: ''                                                                          
usage: unrestricted         
languages: []                                                                       
schemes:                            
  label: {dtype: int}
tables:                                                                                  
  files:                                              
    type: filewise                                                                       
    columns:                                                                   
      spk: {scheme_id: spk}                 
  segments:                                                                    
    type: segmented
    columns:                                                                                    
      spk: {scheme_id: spk}        
>>> db.save('test-db')
>>> db = audformat.Database.load('test-db')
>>> db
name: db
source: ''
usage: unrestricted
languages: []
schemes:
  label: {dtype: int}
tables:
  files:
    type: filewise
    columns:
      spk: {scheme_id: spk}
  segments:
    type: segmented
    columns:
      spk: {scheme_id: spk}

The only not so nice think is the error message you are getting at the moment when the deleted scheme is needed:

>>> df = db['files'].get()
...
BadKeyError: "Bad key 'spk', expected one of ['label']"
>>> df = db['segments'].get()
...
BadKeyError: "Bad key 'spk', expected one of ['label']"