BUG: groupby with resample using on parameter errors when selecting column to apply function

[discort]

• closes BUG: groupby with resample using on parameter errors when selecting column to apply function #17813
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #19433 into master will decrease coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master   #19433      +/-   ##
==========================================
- Coverage   91.62%   91.59%   -0.03%     
==========================================
  Files         150      150              
  Lines       48714    48720       +6     
==========================================
- Hits        44633    44625       -8     
- Misses       4081     4095      +14

Flag	Coverage Δ
#multiple	89.96% <100%> (-0.03%)	⬇️
#single	41.75% <22.22%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
pandas/core/groupby.py	92.17% <100%> (+0.02%)	⬆️
pandas/plotting/_converter.py	65.22% <0%> (-1.74%)	⬇️
pandas/util/testing.py	83.64% <0%> (-0.21%)	⬇️

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[discort]

@jreback

[jreback]

try with something like this. your patch is just working around the issue and not fixing it at the source. its pretty deep code, but try here.

diff --git a/pandas/core/groupby.py b/pandas/core/groupby.py
index 4c03f3bfa..6a2cd69eb 100644
--- a/pandas/core/groupby.py
+++ b/pandas/core/groupby.py
@@ -466,11 +466,15 @@ class Grouper(object):
                 "The Grouper cannot specify both a key and a level!")
 
         # the key must be a valid info item
-        if self.key is not None and not isinstance(obj, Series):
+        if self.key is not None:
             key = self.key
-            if key not in obj._info_axis:
-                raise KeyError("The grouper name {0} is not found".format(key))
-            ax = Index(obj[key], name=key)
+
+            if getattr(self.grouper, 'name', None) == key:
+                ax = self.grouper
+            else:
+                if key not in obj._info_axis:
+                    raise KeyError("The grouper name {0} is not found".format(key))
+                ax = Index(obj[key], name=key)
 
         else:
             ax = obj._get_axis(self.axis)
diff --git a/pandas/core/resample.py b/pandas/core/resample.py
index 03b522abc..43391061d 100644
--- a/pandas/core/resample.py
+++ b/pandas/core/resample.py
@@ -743,11 +743,6 @@ class _GroupByMixin(GroupByMixin):
 
             return x.apply(f, **kwargs)
 
-        # Patch the index in case we're applying after __getitem__. GH 17813
-        if self.groupby.key:
-            key = self.groupby.key
-            self._groupby.obj.index = Index(self.groupby.obj[key], key=key)
-
         result = self._groupby.apply(func)
         return self._wrap_result(result)
 

[discort]

I applied your diff. The result, in this case, would be:

In [11]: result
Out[11]:
id  date
a   2016-01-01    2
    2016-01-03    0
    2016-01-05    0
b   2016-01-01    2
    2016-01-03    1
    2016-01-05    0
Name: data, dtype: int64

instead of:

In [10]: exp
Out[10]:
id  date
a   2016-01-01    2
b   2016-01-03    2
    2016-01-05    1
Name: data, dtype: int64

But I got your idea. The main challenge is that self.grouper is overriding here. Could you take a look at newest solution? @jreback

[jreback]

change looks good. some comments. ping on green.

[jreback]

passing the on= kwarg, and subsequently using .apply() (not the double backticks on both)

[jreback]

can you add a comment here about what this is doing (IOW the on is already defined).

use ABCSeries rather than Series

[jreback]

can you move this a bit lower down in the file (immediately after the other getitem tests)

[discort]

@jreback

[jreback]

thanks @discort