From 5685cb169e5f0afd1b16c057c341163e8fd676f2 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Mon, 2 Nov 2020 11:38:06 +0000
Subject: [PATCH 01/15] refactor core indexes

---
 pandas/core/indexes/api.py          |  5 +-
 pandas/core/indexes/base.py         | 72 +++++++++++------------------
 pandas/core/indexes/category.py     |  2 +-
 pandas/core/indexes/datetimelike.py | 35 +++++++-------
 pandas/core/indexes/datetimes.py    | 16 +++----
 5 files changed, 54 insertions(+), 76 deletions(-)

diff --git a/pandas/core/indexes/api.py b/pandas/core/indexes/api.py
index 18981a2190552..d4f22e482af84 100644
--- a/pandas/core/indexes/api.py
+++ b/pandas/core/indexes/api.py
@@ -282,7 +282,4 @@ def all_indexes_same(indexes):
     """
     itr = iter(indexes)
     first = next(itr)
-    for index in itr:
-        if not first.equals(index):
-            return False
-    return True
+    return all(first.equals(index) for index in itr)
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 18e4540b9a25e..3bdb9d3c5b759 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -115,7 +115,7 @@
     unique="Index",
     duplicated="np.ndarray",
 )
-_index_shared_docs = dict()
+_index_shared_docs = {}
 str_t = str
 
 
@@ -137,10 +137,9 @@ def _new_Index(cls, d):
 
         return _new_PeriodIndex(cls, **d)
 
-    if issubclass(cls, ABCMultiIndex):
-        if "labels" in d and "codes" not in d:
-            # GH#23752 "labels" kwarg has been replaced with "codes"
-            d["codes"] = d.pop("labels")
+    if issubclass(cls, ABCMultiIndex) and "labels" in d and "codes" not in d:
+        # GH#23752 "labels" kwarg has been replaced with "codes"
+        d["codes"] = d.pop("labels")
 
     return cls.__new__(cls, **d)
 
@@ -902,9 +901,7 @@ def __repr__(self) -> str_t:
         if data is None:
             data = ""
 
-        res = f"{klass_name}({data}{prepr})"
-
-        return res
+        return f"{klass_name}({data}{prepr})"
 
     def _format_space(self) -> str_t:
 
@@ -983,7 +980,6 @@ def _format_with_header(
         if is_object_dtype(values.dtype):
             values = lib.maybe_convert_objects(values, safe=1)
 
-        if is_object_dtype(values.dtype):
             result = [pprint_thing(x, escape_chars=("\t", "\r", "\n")) for x in values]
 
             # could have nans
@@ -1584,7 +1580,7 @@ def _drop_level_numbers(self, levnums: List[int]):
         Drop MultiIndex levels by level _number_, not name.
         """
 
-        if len(levnums) == 0:
+        if not levnums:
             return self
         if len(levnums) >= self.nlevels:
             raise ValueError(
@@ -3121,7 +3117,7 @@ def get_indexer(
                 "Reindexing only valid with uniquely valued Index objects"
             )
 
-        if method == "pad" or method == "backfill":
+        if method in ["pad", "backfill"]:
             indexer = self._get_fill_indexer(target, method, limit, tolerance)
         elif method == "nearest":
             indexer = self._get_nearest_indexer(target, limit, tolerance)
@@ -3234,8 +3230,7 @@ def _filter_indexer_tolerance(
         tolerance,
     ) -> np.ndarray:
         distance = abs(self._values[indexer] - target)
-        indexer = np.where(distance <= tolerance, indexer, -1)
-        return indexer
+        return np.where(distance <= tolerance, indexer, -1)
 
     # --------------------------------------------------------------------
     # Indexer Conversion Methods
@@ -3344,9 +3339,7 @@ def _convert_listlike_indexer(self, keyarr):
         keyarr : numpy.ndarray
             Return tuple-safe keys.
         """
-        if isinstance(keyarr, Index):
-            pass
-        else:
+        if not isinstance(keyarr, Index):
             keyarr = self._convert_arr_indexer(keyarr)
 
         indexer = self._convert_list_indexer(keyarr)
@@ -3365,8 +3358,7 @@ def _convert_arr_indexer(self, keyarr):
         -------
         converted_keyarr : array-like
         """
-        keyarr = com.asarray_tuplesafe(keyarr)
-        return keyarr
+        return com.asarray_tuplesafe(keyarr)
 
     def _convert_list_indexer(self, keyarr):
         """
@@ -3738,9 +3730,8 @@ def _join_multi(self, other, how, return_indexers=True):
             other, level, how=how, return_indexers=return_indexers
         )
 
-        if flip_order:
-            if isinstance(result, tuple):
-                return result[0], result[2], result[1]
+        if flip_order and isinstance(result, tuple):
+            return result[0], result[2], result[1]
         return result
 
     @final
@@ -4292,7 +4283,7 @@ def append(self, other):
         to_concat = [self]
 
         if isinstance(other, (list, tuple)):
-            to_concat = to_concat + list(other)
+            to_concat += list(other)
         else:
             to_concat.append(other)
 
@@ -4781,9 +4772,7 @@ def _should_fallback_to_positional(self) -> bool:
         """
         Should an integer key be treated as positional?
         """
-        if self.holds_integer() or self.is_boolean():
-            return False
-        return True
+        return not self.holds_integer() and not self.is_boolean()
 
     def _get_values_for_loc(self, series: "Series", loc, key):
         """
@@ -5176,11 +5165,7 @@ def _validate_indexer(self, form: str_t, key, kind: str_t):
         """
         assert kind in ["getitem", "iloc"]
 
-        if key is None:
-            pass
-        elif is_integer(key):
-            pass
-        else:
+        if key is not None and not is_integer(key):
             self._invalid_indexer(form, key)
 
     def _maybe_cast_slice_bound(self, label, side: str_t, kind):
@@ -5284,11 +5269,11 @@ def get_slice_bound(self, label, side: str_t, kind) -> int:
                 slc = lib.maybe_indices_to_slice(
                     slc.astype(np.intp, copy=False), len(self)
                 )
-            if isinstance(slc, np.ndarray):
-                raise KeyError(
-                    f"Cannot get {side} slice bound for non-unique "
-                    f"label: {repr(original_label)}"
-                )
+        if isinstance(slc, np.ndarray):
+            raise KeyError(
+                f"Cannot get {side} slice bound for non-unique "
+                f"label: {repr(original_label)}"
+            )
 
         if isinstance(slc, slice):
             if side == "left":
@@ -5479,9 +5464,10 @@ def _cmp_method(self, other, op):
         """
         Wrapper used to dispatch comparison operations.
         """
-        if isinstance(other, (np.ndarray, Index, ABCSeries, ExtensionArray)):
-            if len(self) != len(other):
-                raise ValueError("Lengths must match to compare")
+        if isinstance(other, (np.ndarray, Index, ABCSeries, ExtensionArray)) and len(
+            self
+        ) != len(other):
+            raise ValueError("Lengths must match to compare")
 
         if not isinstance(other, ABCMultiIndex):
             other = extract_array(other, extract_numpy=True)
@@ -5803,7 +5789,7 @@ def trim_front(strings: List[str]) -> List[str]:
     Trims zeros and decimal points.
     """
     trimmed = strings
-    while len(strings) > 0 and all(x[0] == " " for x in trimmed):
+    while trimmed and all(x[0] == " " for x in trimmed):
         trimmed = [x[1:] for x in trimmed]
     return trimmed
 
@@ -5873,15 +5859,11 @@ def _maybe_cast_with_dtype(data: np.ndarray, dtype: np.dtype, copy: bool) -> np.
             except ValueError:
                 data = np.array(data, dtype=np.float64, copy=copy)
 
-        elif inferred == "string":
-            pass
-        else:
+        elif inferred != "string":
             data = data.astype(dtype)
     elif is_float_dtype(dtype):
         inferred = lib.infer_dtype(data, skipna=False)
-        if inferred == "string":
-            pass
-        else:
+        if inferred != "string":
             data = data.astype(dtype)
     else:
         data = np.array(data, dtype=dtype, copy=copy)
diff --git a/pandas/core/indexes/category.py b/pandas/core/indexes/category.py
index 2f2836519d847..1315388224713 100644
--- a/pandas/core/indexes/category.py
+++ b/pandas/core/indexes/category.py
@@ -526,7 +526,7 @@ def get_indexer(self, target, method=None, limit=None, tolerance=None):
         if self.is_unique and self.equals(target):
             return np.arange(len(self), dtype="intp")
 
-        if method == "pad" or method == "backfill":
+        if method in ["pad", "backfill"]:
             raise NotImplementedError(
                 "method='pad' and method='backfill' not "
                 "implemented yet for CategoricalIndex"
diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 9215fc8994d87..c0e2ce3c754cf 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -239,9 +239,8 @@ def min(self, axis=None, skipna=True, *args, **kwargs):
         i8 = self.asi8
         try:
             # quick check
-            if len(i8) and self.is_monotonic:
-                if i8[0] != iNaT:
-                    return self._data._box_func(i8[0])
+            if len(i8) and self.is_monotonic and i8[0] != iNaT:
+                return self._data._box_func(i8[0])
 
             if self.hasnans:
                 if skipna:
@@ -296,9 +295,8 @@ def max(self, axis=None, skipna=True, *args, **kwargs):
         i8 = self.asi8
         try:
             # quick check
-            if len(i8) and self.is_monotonic:
-                if i8[-1] != iNaT:
-                    return self._data._box_func(i8[-1])
+            if len(i8) and self.is_monotonic and i8[-1] != iNaT:
+                return self._data._box_func(i8[-1])
 
             if self.hasnans:
                 if skipna:
@@ -579,9 +577,12 @@ def _get_delete_freq(self, loc: int):
                     loc = lib.maybe_indices_to_slice(
                         np.asarray(loc, dtype=np.intp), len(self)
                     )
-                if isinstance(loc, slice) and loc.step in (1, None):
-                    if loc.start in (0, None) or loc.stop in (len(self), None):
-                        freq = self.freq
+                if (
+                    isinstance(loc, slice)
+                    and loc.step in (1, None)
+                    and (loc.start in (0, None) or loc.stop in (len(self), None))
+                ):
+                    freq = self.freq
         return freq
 
     def _get_insert_freq(self, loc, item):
@@ -599,7 +600,7 @@ def _get_insert_freq(self, loc, item):
             if self.size:
                 if item is NaT:
                     pass
-                elif (loc == 0 or loc == -len(self)) and item + self.freq == self[0]:
+                elif loc in [0, -len(self)] and item + self.freq == self[0]:
                     freq = self.freq
                 elif (loc == len(self)) and item - self.freq == self[-1]:
                     freq = self.freq
@@ -684,8 +685,7 @@ def _shallow_copy(self, values=None, name: Label = lib.no_default):
 
     @Appender(Index.difference.__doc__)
     def difference(self, other, sort=None):
-        new_idx = super().difference(other, sort=sort)._with_freq(None)
-        return new_idx
+        return super().difference(other, sort=sort)._with_freq(None)
 
     def intersection(self, other, sort=False):
         """
@@ -727,10 +727,9 @@ def intersection(self, other, sort=False):
 
         if not isinstance(other, type(self)):
             result = Index.intersection(self, other, sort=sort)
-            if isinstance(result, type(self)):
-                if result.freq is None:
-                    # TODO: no tests rely on this; needed?
-                    result = result._with_freq("infer")
+            if isinstance(result, type(self)) and result.freq is None:
+                # TODO: no tests rely on this; needed?
+                result = result._with_freq("infer")
             return result
 
         elif not self._can_fast_intersect(other):
@@ -876,13 +875,13 @@ def _union(self, other, sort):
             elif result.freq is None:
                 # TODO: no tests rely on this; needed?
                 result = result._with_freq("infer")
-            return result
         else:
             i8self = Int64Index._simple_new(self.asi8)
             i8other = Int64Index._simple_new(other.asi8)
             i8result = i8self._union(i8other, sort=sort)
             result = type(self)(i8result, dtype=self.dtype, freq="infer")
-            return result
+
+        return result
 
     # --------------------------------------------------------------------
     # Join Methods
diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index aa16dc9752565..51cf0f1d7455f 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -425,12 +425,13 @@ def _maybe_utc_convert(self, other: Index) -> Tuple["DatetimeIndex", Index]:
         this = self
 
         if isinstance(other, DatetimeIndex):
-            if self.tz is not None:
-                if other.tz is None:
-                    raise TypeError("Cannot join tz-naive with tz-aware DatetimeIndex")
-            elif other.tz is not None:
+            if (
+                self.tz is not None
+                and other.tz is None
+                or self.tz is None
+                and other.tz is not None
+            ):
                 raise TypeError("Cannot join tz-naive with tz-aware DatetimeIndex")
-
             if not timezones.tz_compare(self.tz, other.tz):
                 this = self.tz_convert("UTC")
                 other = other.tz_convert("UTC")
@@ -758,8 +759,7 @@ def _get_string_slice(self, key: str):
         freq = getattr(self, "freqstr", getattr(self, "inferred_freq", None))
         parsed, reso = parsing.parse_time_string(key, freq)
         reso = Resolution.from_attrname(reso)
-        loc = self._partial_date_slice(reso, parsed)
-        return loc
+        return self._partial_date_slice(reso, parsed)
 
     def slice_indexer(self, start=None, end=None, step=None, kind=None):
         """
@@ -822,7 +822,7 @@ def slice_indexer(self, start=None, end=None, step=None, kind=None):
     # --------------------------------------------------------------------
 
     def is_type_compatible(self, typ) -> bool:
-        return typ == self.inferred_type or typ == "datetime"
+        return typ in [self.inferred_type, "datetime"]
 
     @property
     def inferred_type(self) -> str:

From 7695e66c5d2fafcb8d671b9c9ff0ecfe3200c733 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Mon, 2 Nov 2020 14:58:53 +0000
Subject: [PATCH 02/15] parens

---
 pandas/core/indexes/datetimes.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index 51cf0f1d7455f..0b92a9fbdc413 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -426,10 +426,10 @@ def _maybe_utc_convert(self, other: Index) -> Tuple["DatetimeIndex", Index]:
 
         if isinstance(other, DatetimeIndex):
             if (
-                self.tz is not None
-                and other.tz is None
-                or self.tz is None
-                and other.tz is not None
+                (self.tz is not None)
+                and (other.tz is None)
+                or (self.tz is None)
+                and (other.tz is not None)
             ):
                 raise TypeError("Cannot join tz-naive with tz-aware DatetimeIndex")
             if not timezones.tz_compare(self.tz, other.tz):

From d5d453055be61865f31e385fd0a8b5045605631d Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Mon, 2 Nov 2020 15:00:26 +0000
Subject: [PATCH 03/15] reversions

---
 pandas/core/indexes/base.py | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 3bdb9d3c5b759..b1d82e9918108 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -137,9 +137,10 @@ def _new_Index(cls, d):
 
         return _new_PeriodIndex(cls, **d)
 
-    if issubclass(cls, ABCMultiIndex) and "labels" in d and "codes" not in d:
-        # GH#23752 "labels" kwarg has been replaced with "codes"
-        d["codes"] = d.pop("labels")
+    if issubclass(cls, ABCMultiIndex):
+        if "labels" in d and "codes" not in d:
+            # GH#23752 "labels" kwarg has been replaced with "codes"
+            d["codes"] = d.pop("labels")
 
     return cls.__new__(cls, **d)
 
@@ -5269,11 +5270,11 @@ def get_slice_bound(self, label, side: str_t, kind) -> int:
                 slc = lib.maybe_indices_to_slice(
                     slc.astype(np.intp, copy=False), len(self)
                 )
-        if isinstance(slc, np.ndarray):
-            raise KeyError(
-                f"Cannot get {side} slice bound for non-unique "
-                f"label: {repr(original_label)}"
-            )
+            if isinstance(slc, np.ndarray):
+                raise KeyError(
+                    f"Cannot get {side} slice bound for non-unique "
+                    f"label: {repr(original_label)}"
+                )
 
         if isinstance(slc, slice):
             if side == "left":

From 23be6a5f4120ee2a346d8580e4ed4240cff3206d Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Mon, 2 Nov 2020 19:31:43 +0000
Subject: [PATCH 04/15] make pythonic

---
 pandas/core/indexes/base.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index b1d82e9918108..731de13b475ed 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -5790,7 +5790,7 @@ def trim_front(strings: List[str]) -> List[str]:
     Trims zeros and decimal points.
     """
     trimmed = strings
-    while trimmed and all(x[0] == " " for x in trimmed):
+    while trimmed and all(x.startswith(" ") for x in trimmed):
         trimmed = [x[1:] for x in trimmed]
     return trimmed
 

From b3d1dec54ac29a47dbcc644c98e52402078a7466 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Mon, 2 Nov 2020 19:34:07 +0000
Subject: [PATCH 05/15] try moving outside try

---
 pandas/core/indexes/datetimelike.py | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index c0e2ce3c754cf..f5fe1d9b6e1bf 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -237,11 +237,11 @@ def min(self, axis=None, skipna=True, *args, **kwargs):
             return self._na_value
 
         i8 = self.asi8
-        try:
-            # quick check
-            if len(i8) and self.is_monotonic and i8[0] != iNaT:
-                return self._data._box_func(i8[0])
 
+        # quick check
+        if len(self) and self.is_monotonic and i8[0] != iNaT:
+            return self._data._box_func(i8[0])
+        try:
             if self.hasnans:
                 if skipna:
                     min_stamp = self[~self._isnan].asi8.min()

From e0d36eb95727de06705552be42555c5d7488a57d Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Wed, 4 Nov 2020 19:02:22 +0000
Subject: [PATCH 06/15] move out of try-except

---
 pandas/core/indexes/base.py         |  6 +++---
 pandas/core/indexes/datetimelike.py | 16 ++++++++--------
 2 files changed, 11 insertions(+), 11 deletions(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 5ae627bc24a69..c6196d1dc7b42 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -5457,9 +5457,9 @@ def _cmp_method(self, other, op):
         """
         Wrapper used to dispatch comparison operations.
         """
-        if isinstance(other, (np.ndarray, Index, ABCSeries, ExtensionArray)) and len(
-            self
-        ) != len(other):
+        if isinstance(other, (np.ndarray, Index, ABCSeries, ExtensionArray)) and (
+            len(self) != len(other)
+        ):
             raise ValueError("Lengths must match to compare")
 
         if not isinstance(other, ABCMultiIndex):
diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 15030e8b36b5d..63ce59335a332 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -239,16 +239,16 @@ def min(self, axis=None, skipna=True, *args, **kwargs):
         i8 = self.asi8
 
         # quick check
-        if len(self) and self.is_monotonic and i8[0] != iNaT:
+        if self.is_monotonic and i8[0] != iNaT:
             return self._data._box_func(i8[0])
-        try:
-            if self.hasnans:
-                if skipna:
-                    min_stamp = self[~self._isnan].asi8.min()
-                else:
-                    return self._na_value
+        if self.hasnans:
+            if skipna:
+                min_stamp = self[~self._isnan].asi8.min()
             else:
-                min_stamp = i8.min()
+                return self._na_value
+        else:
+            min_stamp = i8.min()
+        try:
             return self._data._box_func(min_stamp)
         except ValueError:
             return self._na_value

From 26c419e3a9650b4d0cd4c9a5b1b084d0146b1046 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Thu, 5 Nov 2020 07:53:29 +0000
Subject: [PATCH 07/15] keep asi8.min() inside try-except

---
 pandas/core/indexes/datetimelike.py | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 63ce59335a332..a374890efc0b3 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -241,17 +241,17 @@ def min(self, axis=None, skipna=True, *args, **kwargs):
         # quick check
         if self.is_monotonic and i8[0] != iNaT:
             return self._data._box_func(i8[0])
-        if self.hasnans:
-            if skipna:
+        try:
+            if self.hasnans and skipna:
                 min_stamp = self[~self._isnan].asi8.min()
+            elif not self.hasnans:
+                min_stamp = i8.min()
             else:
                 return self._na_value
-        else:
-            min_stamp = i8.min()
-        try:
-            return self._data._box_func(min_stamp)
         except ValueError:
             return self._na_value
+        else:
+            return self._data._box_func(min_stamp)
 
     def argmin(self, axis=None, skipna=True, *args, **kwargs):
         """

From 445f2e944c4351e7f1f6335c8b0748e2cd436618 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Sun, 8 Nov 2020 08:10:04 +0000
Subject: [PATCH 08/15] empty check

---
 pandas/core/indexes/datetimelike.py | 16 ++++++----------
 1 file changed, 6 insertions(+), 10 deletions(-)

diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 1a064493fc6f3..041143277f496 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -241,17 +241,13 @@ def min(self, axis=None, skipna=True, *args, **kwargs):
         # quick check
         if self.is_monotonic and i8[0] != iNaT:
             return self._data._box_func(i8[0])
-        try:
-            if self.hasnans and skipna:
-                min_stamp = self[~self._isnan].asi8.min()
-            elif not self.hasnans:
-                min_stamp = i8.min()
-            else:
-                return self._na_value
-        except ValueError:
-            return self._na_value
+        if self.hasnans and skipna and not self[~self._isnan].empty:
+            min_stamp = self[~self._isnan].asi8.min()
+        elif not self.hasnans:
+            min_stamp = i8.min()
         else:
-            return self._data._box_func(min_stamp)
+            return self._na_value
+        return self._data._box_func(min_stamp)
 
     def argmin(self, axis=None, skipna=True, *args, **kwargs):
         """

From e8f117c181edad7add436dd483e7d931afaf0155 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Sun, 8 Nov 2020 08:14:57 +0000
Subject: [PATCH 09/15] similar simplification in max

---
 pandas/core/indexes/datetimelike.py | 24 ++++++++++--------------
 1 file changed, 10 insertions(+), 14 deletions(-)

diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 041143277f496..7ddbb858add93 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -289,21 +289,17 @@ def max(self, axis=None, skipna=True, *args, **kwargs):
             return self._na_value
 
         i8 = self.asi8
-        try:
-            # quick check
-            if len(i8) and self.is_monotonic and i8[-1] != iNaT:
-                return self._data._box_func(i8[-1])
-
-            if self.hasnans:
-                if skipna:
-                    max_stamp = self[~self._isnan].asi8.max()
-                else:
-                    return self._na_value
-            else:
-                max_stamp = i8.max()
-            return self._data._box_func(max_stamp)
-        except ValueError:
+
+        # quick check
+        if self.is_monotonic and i8[-1] != iNaT:
+            return self._data._box_func(i8[-1])
+        if self.hasnans and skipna and not self[~self._isnan].empty:
+            max_stamp = self[~self._isnan].asi8.max()
+        elif not self.hasnans:
+            max_stamp = i8.max()
+        else:
             return self._na_value
+        return self._data._box_func(max_stamp)
 
     def argmax(self, axis=None, skipna=True, *args, **kwargs):
         """

From a58de562ed6b20e62851e2648538179d195e7370 Mon Sep 17 00:00:00 2001
From: Marco Gorelli <m.e.gorelli@gmail.com>
Date: Mon, 23 Nov 2020 18:55:52 +0000
Subject: [PATCH 10/15] fix merge error

---
 pandas/core/indexes/datetimes.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index 388f5f060dd95..e722b6be7d92b 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -815,7 +815,7 @@ def slice_indexer(self, start=None, end=None, step=None, kind=None):
     # --------------------------------------------------------------------
 
     def is_type_compatible(self, kind: str) -> bool:
-        return typ in [self.inferred_type, "datetime"]
+        return kind in [self.inferred_type, "datetime"]
 
     @property
     def inferred_type(self) -> str:

From be80fb7045d46c9c09243a354185cc85bc13d5a6 Mon Sep 17 00:00:00 2001
From: MarcoGorelli <m.e.gorelli@gmail.com>
Date: Tue, 24 Nov 2020 14:31:28 +0000
Subject: [PATCH 11/15] wip

---
 pandas/core/common.py               | 4 ++++
 pandas/core/indexes/base.py         | 9 +++++----
 pandas/core/indexes/datetimelike.py | 5 ++---
 3 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/pandas/core/common.py b/pandas/core/common.py
index 24680fc855b0d..7b4763038cb70 100644
--- a/pandas/core/common.py
+++ b/pandas/core/common.py
@@ -302,6 +302,10 @@ def is_full_slice(obj, line) -> bool:
     )
 
 
+def is_null_slice(obj) -> bool:
+    return isinstance(obj, slice) and obj.start is None and obj.stop is None and obj.step is None
+
+
 def get_callable_name(obj):
     # typical case has name
     if hasattr(obj, "__name__"):
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 3ec54ffa32964..d3ba9eb56270d 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -5833,10 +5833,11 @@ def trim_front(strings: List[str]) -> List[str]:
     """
     Trims zeros and decimal points.
     """
-    trimmed = strings
-    while trimmed and all(x.startswith(" ") for x in trimmed):
-        trimmed = [x[1:] for x in trimmed]
-    return trimmed
+    if not strings:
+        return strings
+    while all(x[0] == ' ' for x in strings):
+        strings = [x[1:] for x in strings]
+    return strings
 
 
 def _validate_join_method(method: str):
diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 7dd188d6c4084..3aca5aa90576c 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -546,10 +546,9 @@ def _get_delete_freq(self, loc: int):
                         np.asarray(loc, dtype=np.intp), len(self)
                     )
                 if (
-                    isinstance(loc, slice)
-                    and loc.step in (1, None)
-                    and (loc.start in (0, None) or loc.stop in (len(self), None))
+                    com.is_full_slice(loc, len(self)) or com.is_null_slice(loc)
                 ):
+                    breakpoint()
                     freq = self.freq
         return freq
 

From dc0b84ebb5372c6da86a9a82a203a982009c19aa Mon Sep 17 00:00:00 2001
From: MarcoGorelli <m.e.gorelli@gmail.com>
Date: Tue, 24 Nov 2020 15:15:19 +0000
Subject: [PATCH 12/15] early return

---
 pandas/core/common.py               |  4 ----
 pandas/core/indexes/base.py         |  2 +-
 pandas/core/indexes/datetimelike.py | 27 ++++++++++++++-------------
 3 files changed, 15 insertions(+), 18 deletions(-)

diff --git a/pandas/core/common.py b/pandas/core/common.py
index 7b4763038cb70..24680fc855b0d 100644
--- a/pandas/core/common.py
+++ b/pandas/core/common.py
@@ -302,10 +302,6 @@ def is_full_slice(obj, line) -> bool:
     )
 
 
-def is_null_slice(obj) -> bool:
-    return isinstance(obj, slice) and obj.start is None and obj.stop is None and obj.step is None
-
-
 def get_callable_name(obj):
     # typical case has name
     if hasattr(obj, "__name__"):
diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index d3ba9eb56270d..4576270b431a3 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -5835,7 +5835,7 @@ def trim_front(strings: List[str]) -> List[str]:
     """
     if not strings:
         return strings
-    while all(x[0] == ' ' for x in strings):
+    while all(x[0] == " " for x in strings):
         strings = [x[1:] for x in strings]
     return strings
 
diff --git a/pandas/core/indexes/datetimelike.py b/pandas/core/indexes/datetimelike.py
index 3aca5aa90576c..57f6a8ea0cca5 100644
--- a/pandas/core/indexes/datetimelike.py
+++ b/pandas/core/indexes/datetimelike.py
@@ -243,7 +243,7 @@ def min(self, axis=None, skipna=True, *args, **kwargs):
 
         i8 = self.asi8
 
-        if self.is_monotonic_increasing:
+        if len(i8) and self.is_monotonic_increasing:
             # quick check
             if i8[0] != iNaT:
                 return self._data._box_func(i8[0])
@@ -300,7 +300,7 @@ def max(self, axis=None, skipna=True, *args, **kwargs):
 
         i8 = self.asi8
 
-        if self.is_monotonic:
+        if len(i8) and self.is_monotonic:
             # quick check
             if i8[-1] != iNaT:
                 return self._data._box_func(i8[-1])
@@ -545,11 +545,9 @@ def _get_delete_freq(self, loc: int):
                     loc = lib.maybe_indices_to_slice(
                         np.asarray(loc, dtype=np.intp), len(self)
                     )
-                if (
-                    com.is_full_slice(loc, len(self)) or com.is_null_slice(loc)
-                ):
-                    breakpoint()
-                    freq = self.freq
+                if isinstance(loc, slice) and loc.step in (1, None):
+                    if loc.start in (0, None) or loc.stop in (len(self), None):
+                        freq = self.freq
         return freq
 
     def _get_insert_freq(self, loc, item):
@@ -567,7 +565,7 @@ def _get_insert_freq(self, loc, item):
             if self.size:
                 if item is NaT:
                     pass
-                elif loc in [0, -len(self)] and item + self.freq == self[0]:
+                elif (loc == 0 or loc == -len(self)) and item + self.freq == self[0]:
                     freq = self.freq
                 elif (loc == len(self)) and item - self.freq == self[-1]:
                     freq = self.freq
@@ -650,7 +648,8 @@ def is_type_compatible(self, kind: str) -> bool:
 
     @Appender(Index.difference.__doc__)
     def difference(self, other, sort=None):
-        return super().difference(other, sort=sort)._with_freq(None)
+        new_idx = super().difference(other, sort=sort)._with_freq(None)
+        return new_idx
 
     def intersection(self, other, sort=False):
         """
@@ -692,9 +691,10 @@ def intersection(self, other, sort=False):
 
         if not isinstance(other, type(self)):
             result = Index.intersection(self, other, sort=sort)
-            if isinstance(result, type(self)) and result.freq is None:
-                # TODO: no tests rely on this; needed?
-                result = result._with_freq("infer")
+            if isinstance(result, type(self)):
+                if result.freq is None:
+                    # TODO: no tests rely on this; needed?
+                    result = result._with_freq("infer")
             return result
 
         elif not self._can_fast_intersect(other):
@@ -843,6 +843,7 @@ def _union(self, other, sort):
             elif result.freq is None:
                 # TODO: no tests rely on this; needed?
                 result = result._with_freq("infer")
+            return result
         else:
             i8self = Int64Index._simple_new(self.asi8)
             i8other = Int64Index._simple_new(other.asi8)
@@ -852,7 +853,7 @@ def _union(self, other, sort):
             result = type(self)(
                 i8result, dtype=self.dtype, freq="infer"  # type: ignore[call-arg]
             )
-        return result
+            return result
 
     # --------------------------------------------------------------------
     # Join Methods

From 4074a888f0e2bbac54478438f8516a83314335ae Mon Sep 17 00:00:00 2001
From: MarcoGorelli <m.e.gorelli@gmail.com>
Date: Tue, 24 Nov 2020 15:25:07 +0000
Subject: [PATCH 13/15] parens

---
 pandas/core/indexes/datetimes.py | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index e36055b2d4e2a..338cc10a90b99 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -407,11 +407,8 @@ def _maybe_utc_convert(self, other: Index) -> Tuple["DatetimeIndex", Index]:
         this = self
 
         if isinstance(other, DatetimeIndex):
-            if (
-                (self.tz is not None)
-                and (other.tz is None)
-                or (self.tz is None)
-                and (other.tz is not None)
+            if (self.tz is not None and other.tz is None) or (
+                self.tz is None and other.tz is not None
             ):
                 raise TypeError("Cannot join tz-naive with tz-aware DatetimeIndex")
             if not timezones.tz_compare(self.tz, other.tz):

From 00d6be00faea7b9946da910ef8f841b4489caacd Mon Sep 17 00:00:00 2001
From: MarcoGorelli <m.e.gorelli@gmail.com>
Date: Tue, 24 Nov 2020 15:28:09 +0000
Subject: [PATCH 14/15] :art:

---
 pandas/core/indexes/datetimes.py | 1 +
 1 file changed, 1 insertion(+)

diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index 338cc10a90b99..51760ce9f3074 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -411,6 +411,7 @@ def _maybe_utc_convert(self, other: Index) -> Tuple["DatetimeIndex", Index]:
                 self.tz is None and other.tz is not None
             ):
                 raise TypeError("Cannot join tz-naive with tz-aware DatetimeIndex")
+
             if not timezones.tz_compare(self.tz, other.tz):
                 this = self.tz_convert("UTC")
                 other = other.tz_convert("UTC")

From 1b254ae59bff5d4a3b7cf6bb2c7a846d871350f2 Mon Sep 17 00:00:00 2001
From: MarcoGorelli <m.e.gorelli@gmail.com>
Date: Wed, 9 Dec 2020 18:40:19 +0000
Subject: [PATCH 15/15] coverage

---
 pandas/core/indexes/base.py      | 14 ++++++++++++--
 pandas/core/indexes/datetimes.py |  4 +---
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/pandas/core/indexes/base.py b/pandas/core/indexes/base.py
index 75361b45bf10a..1c43aabbd0e04 100644
--- a/pandas/core/indexes/base.py
+++ b/pandas/core/indexes/base.py
@@ -3408,7 +3408,9 @@ def _convert_listlike_indexer(self, keyarr):
         keyarr : numpy.ndarray
             Return tuple-safe keys.
         """
-        if not isinstance(keyarr, Index):
+        if isinstance(keyarr, Index):
+            pass
+        else:
             keyarr = self._convert_arr_indexer(keyarr)
 
         indexer = self._convert_list_indexer(keyarr)
@@ -5925,10 +5927,18 @@ def ensure_has_len(seq):
 def trim_front(strings: List[str]) -> List[str]:
     """
     Trims zeros and decimal points.
+
+    Examples
+    --------
+    >>> trim_front([" a", " b"])
+    ['a', 'b']
+
+    >>> trim_front([" a", " "])
+    ['a', '']
     """
     if not strings:
         return strings
-    while all(x[0] == " " for x in strings):
+    while all(strings) and all(x[0] == " " for x in strings):
         strings = [x[1:] for x in strings]
     return strings
 
diff --git a/pandas/core/indexes/datetimes.py b/pandas/core/indexes/datetimes.py
index f8ab06d8c5ab1..156dbcbed21ab 100644
--- a/pandas/core/indexes/datetimes.py
+++ b/pandas/core/indexes/datetimes.py
@@ -408,9 +408,7 @@ def _maybe_utc_convert(self, other: Index) -> Tuple["DatetimeIndex", Index]:
         this = self
 
         if isinstance(other, DatetimeIndex):
-            if (self.tz is not None and other.tz is None) or (
-                self.tz is None and other.tz is not None
-            ):
+            if (self.tz is None) ^ (other.tz is None):
                 raise TypeError("Cannot join tz-naive with tz-aware DatetimeIndex")
 
             if not timezones.tz_compare(self.tz, other.tz):