[skip ci] columns

Author: MarcoGorelli

web/pandas/pdeps/0005-no-default-index-mode.md

@@ -153,50 +153,70 @@ In [15]: df.loc[0, 'b']
 IndexError: Cannot use label-based indexing on NoRowIndex!
 ```
 
-### 2. DataFrameFormatter and SeriesFormatter changes
+### Aligning ``NoRowIndex``s
 
-When printing an object with a ``NoIndex``, then the row labels wouldn't be shown:
+To minimise surprises, the rule would be:
+
+  A ``NoRowIndex`` can only be aligned with another ``NoRowIndex`` of the same length.
+  Attempting to align it with anything else would raise.
 
+Example:
 ```python
-In [14]: pd.set_option('mode.no_default_index', True)
+In [1]: ser1 = pd.Series([1,2,3], index=NoRowIndex(3))
 
-In [15]: df = pd.DataFrame({'a': [1,  2, 3], 'b': [4, 5, 6], 'c': [7, 8, 9]})
+In [2]: ser2 = pd.Series([4, 5, 6], index=NoRowIndex(3))
 
-In [16]: df
-Out[16]:
- a  b  c
- 1  4  7
- 2  5  8
- 3  6  9
+In [3]: ser1 + ser2
+Out[3]:
+5
+7
+9
+dtype: int64
+
+In [4]: ser1 + ser2.iloc[1:]
+---------------------------------------------------------------------------
+TypeError: Can't join NoRowIndex of different lengths
 ```
 
-### 3. Nobody should get an index unless they ask for one
+### Columns can't be NoRowIndex
+
+This proposal deals exclusively with letting users not have to think about
+row labels. There's no suggestion to remove the column labels.
+One issue that arises, then, is what to do about ``transpose``, which would swap
+index and columns. Rather than making ``transpose`` break, it could be more
+user-friendly to, within ``transpose`` change a ``NoRowIndex`` index to a
+``RangeIndex`` of the same length before swapping index and columns.
+Note that calling ``transpose`` twice would no longer round-trip.
+
+### DataFrameFormatter and SeriesFormatter changes
+
+When printing an object with a ``NoIndex``, then the row labels wouldn't be shown:
 
-The following would work in the same way:
 ```python
-pivot = (
-    pd.pivot_table(df, values="D", index=["A", "B"], columns=["C"], aggfunc=np.sum)
-).reset_index()
-
-with pd.option_context('mode.no_default_index', True):
-    pivot = (
-        pd.pivot_table(df, values="D", index=["A", "B"], columns=["C"], aggfunc=np.sum)
-    )
+In [15]: df = pd.DataFrame({'a': [1,  2, 3], 'b': [4, 5, 6]}, index=NoRowIndex(3))
+
+In [16]: df
+Out[16]:
+ a  b
+ 1  4
+ 2  5
+ 3  6
 ```
 
-Likewise for ``value_counts``. In ``groupby``, the default would be ``as_index=False``.
+Of the above changes, this may be the only one that would need implementing within
+``DataFrameFormatter`` / ``SerieFormatter``, as opposed to within ``NoRowIndex``.
 
 ## Usage and Impact
 
-Users who like the power of the ``Index`` could continue using pandas exactly as it is,
-without changing anything.
-
-The addition of this mode would enable users who don't want to think about indices to
-not have to.
+By itself, ``NoRowIndex`` would be of limited use. To become useful and user-friendly,
+a mode ``no_default_index`` could be introduced which, if introduced, would change
+the ``default_index`` function to return a ``NoRowIndex`` of the appropriate length.
+In particular, ``.reset_index()`` would result in a ``DataFrame`` with a ``NoRowIndex``.
+Likewise, a ``DataFrame`` constructed without explicitly specifying ``index=``.
 
-The implementation would be quite simple: most of the logic would be handled within the
-``NoIndex`` class, and only some minor adjustments (e.g. to the ``default_index`` function)
-would be needed in core pandas.
+Then, if a user doesn't want to think about row labels, then with ``pd.set_option('no_default_index')``
+set, they wouldn't need to (barring methods such as `.pivot_table` which introduce an index).
+Discussion of such a mode is out-of-scope for this proposal.
 
 ## Implementation
 
@@ -222,11 +242,8 @@ Draft pull request showing proof of concept: https://github.com/pandas-dev/panda
   df.reset_index().set_index('index')
   ```
 
-**Q: Why is it necessary to change the behaviour of ``value_counts``? Isn't the introduction of a ``NoIndex`` object enough?**
+**Q: Why can't a DataFrame columns be ``NoRowIndex``?**
 
-**A:** The objective of this mode is to enable users to not have to think about indices if they don't want to. If they have to call
-  ``.reset_index`` after each ``value_counts`` / ``pivot_table`` call, or remember to pass ``as_index=False`` to each ``groupby``
-  call, then this objective has arguably not quite been reached.
 
 ## PDEP History