ENH: Join - Add a parameter to check for duplicates

[jiri-kulik]

Is your feature request related to a problem?

It is arguably more common than not to join dataframes on a unique index. Optional check for uniqueness would help to prevent hidden errors and time spent on debugging.

Describe the solution you'd like

DataFrame.join should get a new parameter on_unique that would, if set to True, check if the index/columns on which joining is performed have duplicates and raise error if yes. Default should be False to keep backward compatibility.

API breaking implications

Default set to False ensures no issues with compatibility.

[samukweku]

A workaround at the moment would be to use pandas merge and use the validate argument to check for uniques. Pass left_index and right_index as True. It would be nice though to have the validate argument in the join function

[jreback]

.join calls merge under the hood

so would accept a PR to add validate kwarg

i believe there is another issue about this - pls check

[gaotian98]

Hi @jreback , I submitted a PR. Would you please take a look?