Confusion about change from `pl.count()` to `pl.len()` about polars HOT 3 CLOSED

What is counter-intuitive? We don't have a notion of null rows, so we cannot make an ignore_nulls argument. They behave different and count will not be exposed as a general catch all.

That's why we only expose it on Expr. And pl.len() will be exposed as a catch all. They are different for a reason.

from polars.

Everything conforms to how the documentation specifies.

import polars as pl
from polars import col

df = pl.DataFrame({"a": [1, 2, None]})

df.select(pl.count())              # returns 3 (counts all rows)
df.select(pl.count("a"))           # returns 2 (counts non-nulls only)
df.select(col("a").count())        # returns 2 (counts non-nulls only)
df.group_by(pl.lit(True)).count()  # returns 3 (counts all rows)

The reason we cannot ignore nulls in group_by().count() is because there is no concept of a null row. Is a [0, 1, null] a null row? Is [null] a null row? (it's not the same as an empty row, which would be []). A group_by operation returns a frame, not a row, hence we cannot say how many non-null rows there are, because there is no concept of a null row. Does this make sense?

I agree that it is confusing. It is self-consistent, but it is nonetheless confusing.

from polars.

@ritchie46 @mcrumiller Thanks for the explanations. It seems like I missed the following statement in the doc.

This way of using the function is deprecated. Please use :func:len instead.

from polars.