pydata/pandas — comment on commit 2246fb32d6c72b5df796af77b50229302aceaa95
File: pandas/core/groupby.py line 939
By wesm:

`ngroups` is not the same thing, that is the number of aggregation bins. the number of groupings is the number of grouping keys, which is 1 in the case of a custom grouper (probably). Doesn't matter where this number comes from-- you could add `__len__` to the Grouper class or have a property/attribute (maybe a bit better) which returns this number to be extra explicit