Comment by DangitBobby
2 months ago
I guess I have pandas brain because I definitely want to drop duplicates, 100% of the time I'm worried about duplicates and 99% of the time the only thing I want to do with duplicates is drop them. When you've got 19 columns it's _really fucking annoying_ if the tool you're using doesn't have an obvious way to say `select distinct on () from my_shit`. Close second at say, 98% of the time, I want to a get a count of duplicates as a sanity check because I know to expect a certain amount of them. Pandas makes that easy too in a way SQL makes really fucking annoying. There are a lot of parts on pandas that made me stop using it long ago but first class duplicates handling is not among them.
And the API is vastly superior to SQL is some respects from a user perspective despite being all over the place in others. Dataframe select/filtering e.g. df = df[df.duplicated(keep='last')] is simple, expressive, obvious, and doesn't result in bleeding fingers. The main problem is the rest of the language around it with all the indentations, newlines, loops, functions and so on can be too terse or too dense and much hard to read than SQL.
Duplicates in source data are almost always a sign of bad data modeling, or of analysts and engineers disregarding a good data model. But I agree that this ubiquitous antipattern that nobody should be doing can still be usefully made concise. There should be a select distinct * operation.
And FWIW I personally hate writing raw SQL. But the problem with the API is not the data operations available, it's the syntax and lack of composability. It's English rather than ALGOL/C-style. Variables and functions, to the extent they exist at all, are second-class, making abstraction high-friction.
Oooh buddy how's the view from that ivory tower??
But seriously I'm not in always in control of upstream data, I get stuff thrown over to my side of the fence by an organization who just needs data jiggled around for one-off ops purposes. They are communicating to me via CSV file scraped from Excel files in their Shared Drive, kind of thing.
Do what you gotta do, but most of my job for the past decade has been replacing data pipelines that randomly duplicate data with pipelines that solve duplication at the source, and my users strongly prefer it.
Of course, a lot of one-off data analysis has no rules but get a quick answer that no one will complain about!
2 replies →
Duplicates are a sign of reality. Only where you have the resources to have dedicated people clean and organize data do you have well modeled data. Pandas is a power tool for making sense of real data.
> Duplicates in source data are almost always a sign of bad data modeling
Nope. Duplicates in source data(INPUT) is natural, correct and MUST be supported or almost all data become impossible.
What is the actual problem is the OUTPUT. Duplicates on the OUTPUT need to be controlled and explicit. In general, we need in the OUTPUT a unique rowby a N-key, but probably not need it to be unique for the rest, so, in the relational model, you need unique for a combination of columns (rarely, by ALL of them).
You articulate your case well, thank you!
I always warn people (particularly junior people) though that blindly dropping duplicates is a dangerous habit because it helps you and others in your organization ignore the causes of bad data quickly without getting them fixed at the source. Over time, that breeds a lot of complexity and inefficiency. And it can easily mask flaws in one's own logic or understanding of the data and its properties.
When I'm in pandas (or was, I don't use it anymore) I'm always downstream of some weird data process that ultimately exported to a CSV from a team that I know has very lax standards for data wrangling, or it is just not their core competency. I agree that duplicates are a smell but they happen often in the use-cases that I'm specifically reaching to pandas for.
Exactly. It’s not that getting rid of duplicates is bad, is that they may be a symptom of something worse. E.g. incorrect aggregation logic