fix groupby.std() with integer colum names

[NicolasHug]

• Tests added / passed
• Passes black dask / flake8 dask

Fixes #3560

I tried using the tuples solution proposed in #3560 (comment) but pandas wasn't happy.

Note that the solution in this PR might cause collisions if we have a column 0 and another column '0'.

[mrocklin]

The exception here is interesting. It looks like there are some versions where this approach has some issues.

[jrbourbeau]

Thanks for working on this @NicolasHug! Apologies for the delayed response.

The RuntimeWarning: '<' not supported between instances of 'str' and 'int' error we're seeing here seem to be because in _var_chunk, x still has integer columns:

dask/dask/dataframe/groupby.py

Line 285 in a53d45e

x = g.sum()

while n and x2 have string columns.

On a related note, I suspect we should be able to support the tuples-based solution proposed in #3560 (comment) with the following changes:

diff --git a/dask/dataframe/groupby.py b/dask/dataframe/groupby.py
index f1b03b2a..59ac4955 100644
--- a/dask/dataframe/groupby.py
+++ b/dask/dataframe/groupby.py
@@ -284,11 +284,11 @@ def _var_chunk(df, *index):
     g = _groupby_raise_unaligned(df, by=index)
     x = g.sum()

-    n = g[x.columns].count().rename(columns=lambda c: str(c) + "-count")
+    n = g[x.columns].count().rename(columns=lambda c: (c, "count"))

     df[cols] = df[cols] ** 2
     g2 = _groupby_raise_unaligned(df, by=index)
-    x2 = g2.sum().rename(columns=lambda c: str(c) + "-x2")
+    x2 = g2.sum().rename(columns=lambda c: (c ,"x2"))

     x2.index = x.index
     return concat([x, x2, n], axis=1)
@@ -302,8 +302,8 @@ def _var_agg(g, levels, ddof):
     g = g.groupby(level=levels, sort=False).sum()
     nc = len(g.columns)
     x = g[g.columns[: nc // 3]]
-    x2 = g[g.columns[nc // 3 : 2 * nc // 3]].rename(columns=lambda c: c[:-3])
-    n = g[g.columns[-nc // 3 :]].rename(columns=lambda c: c[:-6])
+    x2 = g[g.columns[nc // 3 : 2 * nc // 3]].rename(columns=lambda c: c[0])
+    n = g[g.columns[-nc // 3 :]].rename(columns=lambda c: c[0])

     # TODO: replace with _finalize_var?
     result = x2 - x ** 2 / n

This approach should be a little more robust. It will, for example, avoid the column 0 vs. column '0' issue you mentioned

[TomAugspurger]

@NicolasHug do you have time to apply @jrbourbeau fix from #5096 (review)?

[NicolasHug]

Sorry I was short on time but I'll get back to it this week!

[NicolasHug]

I think I figured out what was wrong with tuples, let's see what CI says

[TomAugspurger]

I'm not sure this test is valid. The group key should I think be the same length as df.

Can you try something like

df = pd.DataFrame({0: [0, 1], 1: [1, 2]})
ddf = dd.from_pandas(df, npartitions=2)
by = pd.Series([0, 1])

expected = df.groupby(by).std()
result = ddf.groupby(dd.from_pandas(by, 2)).std()
assert_eq(result, expected)

[NicolasHug]

I can confirm that the current test fails on master with the same error as in the original issue.

To make the suggested test pass I need to set the following args:

assert_eq(result, expected, check_meta=False, equal_nan=True)

All entries are Nans so I'm not sure that makes lot of sense to compare the results? Let me know which one you prefer.

[TomAugspurger]

Thanks. I suppose it's all NaN because the Dataframe is small, so the std calculation returns NaN? Can you make a larger dataframe?

[TomAugspurger]

That said, we may be going past the original issue now. Remind me, that issue was specifically about column names, which this test exercises?

[NicolasHug]

yes the original issue was about column name mismatch, not about expected results. Do you still need me to adapt your test?

[TomAugspurger]

Nope, I think we're good. Sorry about the misunderstanding.

[TomAugspurger]

thanks @NicolasHug!

[jrbourbeau]

Thanks for working on this and pushing it through @NicolasHug!