DOC: Improved links between expanding and cum* (pandas-dev#12651)

Fixed the PEP8 incompatible line lengths.

Author: adrian-stepien

doc/source/computation.rst

@@ -702,10 +702,10 @@ all accept are:
 
    The output of the ``.rolling`` and ``.expanding`` methods do not return a
    ``NaN`` if there are at least ``min_periods`` non-null values in the current
-   window. This differs from :meth:`~DataFrame.cumsum`, :meth:`~DataFrame.cumprod`,
-   :meth:`~DataFrame.cummax`, and :meth:`~DataFrame.cummin`,
-   which return ``NaN`` in the output wherever a ``NaN``
-   is encountered in the input.
+   window. This differs from :meth:`~DataFrame.cumsum`,
+   :meth:`~DataFrame.cumprod`, :meth:`~DataFrame.cummax`,
+   and :meth:`~DataFrame.cummin`, which return ``NaN`` in the output wherever
+   a ``NaN`` is encountered in the input.
 
    Please see the example below. In order to match the output of ``cumsum``
    with ``expanding``, use :meth:`~DataFrame.fillna`.

pandas/core/generic.py

@@ -5363,7 +5363,8 @@ def compound(self, axis=None, skipna=None, level=None):
 
         cls.cummin = _make_cum_function(
             cls, 'cummin', name, name2, axis_descr, "cumulative minimum",
-            lambda y, axis: np.minimum.accumulate(y, axis), "min", np.inf, np.nan)
+            lambda y, axis: np.minimum.accumulate(y, axis), "min",
+            np.inf, np.nan)
         cls.cumsum = _make_cum_function(
             cls, 'cumsum', name, name2, axis_descr, "cumulative sum",
             lambda y, axis: y.cumsum(axis), "sum", 0., np.nan)
@@ -5372,7 +5373,8 @@ def compound(self, axis=None, skipna=None, level=None):
             lambda y, axis: y.cumprod(axis), "prod", 1., np.nan)
         cls.cummax = _make_cum_function(
             cls, 'cummax', name, name2, axis_descr, "cumulative max",
-            lambda y, axis: np.maximum.accumulate(y, axis), "max", -np.inf, np.nan)
+            lambda y, axis: np.maximum.accumulate(y, axis), "max",
+            -np.inf, np.nan)
 
         cls.sum = _make_stat_function(
             cls, 'sum', name, name2, axis_descr,