diff --git a/pandas/_libs/algos.pyx b/pandas/_libs/algos.pyx
index 24828db64c392..6a66e5556e8ba 100644
--- a/pandas/_libs/algos.pyx
+++ b/pandas/_libs/algos.pyx
@@ -409,7 +409,7 @@ def pad(ndarray[algos_t] old, ndarray[algos_t] new, limit=None):
     nleft = len(old)
     nright = len(new)
     indexer = np.empty(nright, dtype=np.int64)
-    indexer.fill(-1)
+    indexer[:] = -1
 
     if limit is None:
         lim = nright
@@ -607,7 +607,7 @@ def backfill(ndarray[algos_t] old, ndarray[algos_t] new, limit=None):
     nleft = len(old)
     nright = len(new)
     indexer = np.empty(nright, dtype=np.int64)
-    indexer.fill(-1)
+    indexer[:] = -1
 
     if limit is None:
         lim = nright
diff --git a/pandas/_libs/algos_rank_helper.pxi.in b/pandas/_libs/algos_rank_helper.pxi.in
index e13f87d15aace..fcb052e8be63b 100644
--- a/pandas/_libs/algos_rank_helper.pxi.in
+++ b/pandas/_libs/algos_rank_helper.pxi.in
@@ -4,9 +4,9 @@ Template for each `dtype` helper function for rank
 WARNING: DO NOT edit .pxi FILE directly, .pxi is generated from .pxi.in
 """
 
-#----------------------------------------------------------------------
+# ----------------------------------------------------------------------
 # rank_1d, rank_2d
-#----------------------------------------------------------------------
+# ----------------------------------------------------------------------
 
 {{py:
 
diff --git a/pandas/_libs/groupby.pyx b/pandas/_libs/groupby.pyx
index c72b4001dcb79..c9c9bddc8ddff 100644
--- a/pandas/_libs/groupby.pyx
+++ b/pandas/_libs/groupby.pyx
@@ -370,7 +370,7 @@ def group_any_all(ndarray[uint8_t] out,
     else:
         raise ValueError("'bool_func' must be either 'any' or 'all'!")
 
-    out.fill(1 - flag_val)
+    out[:] = 1 - flag_val
 
     with nogil:
         for i in range(N):
diff --git a/pandas/_libs/groupby_helper.pxi.in b/pandas/_libs/groupby_helper.pxi.in
index 0917453e3f864..484a4b069305f 100644
--- a/pandas/_libs/groupby_helper.pxi.in
+++ b/pandas/_libs/groupby_helper.pxi.in
@@ -8,9 +8,9 @@ cdef extern from "numpy/npy_math.h":
     double NAN "NPY_NAN"
 _int64_max = np.iinfo(np.int64).max
 
-#----------------------------------------------------------------------
+# ----------------------------------------------------------------------
 # group_add, group_prod, group_var, group_mean, group_ohlc
-#----------------------------------------------------------------------
+# ----------------------------------------------------------------------
 
 {{py:
 
@@ -246,7 +246,7 @@ def group_ohlc_{{name}}(ndarray[{{c_type}}, ndim=2] out,
     if K > 1:
         raise NotImplementedError("Argument 'values' must have only "
                                   "one dimension")
-    out.fill(np.nan)
+    out[:] = np.nan
 
     with nogil:
         for i in range(N):
@@ -629,10 +629,10 @@ def group_max(ndarray[groupby_t, ndim=2] out,
     maxx = np.empty_like(out)
     if groupby_t is int64_t:
         # Note: evaluated at compile-time
-        maxx.fill(-_int64_max)
+        maxx[:] = -_int64_max
         nan_val = iNaT
     else:
-        maxx.fill(-np.inf)
+        maxx[:] = -np.inf
         nan_val = NAN
 
     N, K = (<object>values).shape
@@ -691,10 +691,10 @@ def group_min(ndarray[groupby_t, ndim=2] out,
 
     minx = np.empty_like(out)
     if groupby_t is int64_t:
-        minx.fill(_int64_max)
+        minx[:] = _int64_max
         nan_val = iNaT
     else:
-        minx.fill(np.inf)
+        minx[:] = np.inf
         nan_val = NAN
 
     N, K = (<object>values).shape
@@ -747,9 +747,9 @@ def group_cummin(ndarray[groupby_t, ndim=2] out,
     N, K = (<object>values).shape
     accum = np.empty_like(values)
     if groupby_t is int64_t:
-        accum.fill(_int64_max)
+        accum[:] = _int64_max
     else:
-        accum.fill(np.inf)
+        accum[:] = np.inf
 
     with nogil:
         for i in range(N):
@@ -795,9 +795,9 @@ def group_cummax(ndarray[groupby_t, ndim=2] out,
     N, K = (<object>values).shape
     accum = np.empty_like(values)
     if groupby_t is int64_t:
-        accum.fill(-_int64_max)
+        accum[:] = -_int64_max
     else:
-        accum.fill(-np.inf)
+        accum[:] = -np.inf
 
     with nogil:
         for i in range(N):
diff --git a/pandas/_libs/join.pyx b/pandas/_libs/join.pyx
index 04c2f222b14ad..748f3f265dd34 100644
--- a/pandas/_libs/join.pyx
+++ b/pandas/_libs/join.pyx
@@ -212,7 +212,7 @@ def _get_result_indexer(sorter, indexer):
     else:
         # length-0 case
         res = np.empty(len(indexer), dtype=np.int64)
-        res.fill(-1)
+        res[:] = -1
 
     return res
 
diff --git a/pandas/_libs/lib.pyx b/pandas/_libs/lib.pyx
index 5907f76c20853..a9e0fcbc4a826 100644
--- a/pandas/_libs/lib.pyx
+++ b/pandas/_libs/lib.pyx
@@ -347,7 +347,7 @@ def get_reverse_indexer(ndarray[int64_t] indexer, Py_ssize_t length):
         int64_t idx
 
     rev_indexer = np.empty(length, dtype=np.int64)
-    rev_indexer.fill(-1)
+    rev_indexer[:] = -1
     for i in range(n):
         idx = indexer[i]
         if idx != -1:
@@ -1670,7 +1670,7 @@ cdef class TimedeltaValidator(TemporalValidator):
 
 
 # TODO: Not used outside of tests; remove?
-def is_timedelta_array(values: ndarray) -> bint:
+def is_timedelta_array(values: ndarray) -> bool:
     cdef:
         TimedeltaValidator validator = TimedeltaValidator(len(values),
                                                           skipna=True)
@@ -1683,7 +1683,7 @@ cdef class Timedelta64Validator(TimedeltaValidator):
 
 
 # TODO: Not used outside of tests; remove?
-def is_timedelta64_array(values: ndarray) -> bint:
+def is_timedelta64_array(values: ndarray) -> bool:
     cdef:
         Timedelta64Validator validator = Timedelta64Validator(len(values),
                                                               skipna=True)
diff --git a/pandas/_libs/missing.pyx b/pandas/_libs/missing.pyx
index d6786a96871bd..b8791359241ad 100644
--- a/pandas/_libs/missing.pyx
+++ b/pandas/_libs/missing.pyx
@@ -278,14 +278,14 @@ def isnaobj2d_old(ndarray arr):
     return result.view(np.bool_)
 
 
-cpdef bint isposinf_scalar(object val):
+def isposinf_scalar(val: object) -> bool:
     if util.is_float_object(val) and val == INF:
         return True
     else:
         return False
 
 
-cpdef bint isneginf_scalar(object val):
+def isneginf_scalar(val: object) -> bool:
     if util.is_float_object(val) and val == NEGINF:
         return True
     else:
diff --git a/pandas/_libs/sparse.pyx b/pandas/_libs/sparse.pyx
index 67698f1b4c2ca..9cd33c4d69db6 100644
--- a/pandas/_libs/sparse.pyx
+++ b/pandas/_libs/sparse.pyx
@@ -221,7 +221,7 @@ cdef class IntIndex(SparseIndex):
 
         n = len(indexer)
         results = np.empty(n, dtype=np.int32)
-        results.fill(-1)
+        results[:] = -1
 
         if self.npoints == 0:
             return results
@@ -250,9 +250,9 @@ cdef class IntIndex(SparseIndex):
         sinds = self.indices
 
         result = np.empty(other.npoints, dtype=np.float64)
-        result.fill(fill_value)
+        result[:] = fill_value
 
-        for 0 <= i < other.npoints:
+        for i in range(other.npoints):
             while oinds[i] > sinds[j] and j < self.npoints:
                 j += 1
 
@@ -582,7 +582,7 @@ cdef class BlockIndex(SparseIndex):
 
         n = len(indexer)
         results = np.empty(n, dtype=np.int32)
-        results.fill(-1)
+        results[:] = -1
 
         if self.npoints == 0:
             return results
diff --git a/pandas/_libs/tslibs/conversion.pyx b/pandas/_libs/tslibs/conversion.pyx
index 8cf42bf93eb2c..f88671b41a16a 100644
--- a/pandas/_libs/tslibs/conversion.pyx
+++ b/pandas/_libs/tslibs/conversion.pyx
@@ -869,10 +869,10 @@ def tz_localize_to_utc(ndarray[int64_t] vals, object tz, object ambiguous=None,
     """
     cdef:
         ndarray[int64_t] trans
-        int64_t[:] deltas, idx_shifted
+        int64_t[:] deltas, idx_shifted, idx_shifted_left, idx_shifted_right
         ndarray ambiguous_array
         Py_ssize_t i, idx, pos, ntrans, n = len(vals)
-        Py_ssize_t delta_idx_offset, delta_idx
+        Py_ssize_t delta_idx_offset, delta_idx, pos_left, pos_right
         int64_t *tdata
         int64_t v, left, right, val, v_left, v_right, new_local, remaining_mins
         ndarray[int64_t] result, result_a, result_b, dst_hours
@@ -920,15 +920,15 @@ def tz_localize_to_utc(ndarray[int64_t] vals, object tz, object ambiguous=None,
 
     trans, deltas, typ = get_dst_info(tz)
 
-    tdata = <int64_t*> cnp.PyArray_DATA(trans)
+    tdata = <int64_t*>cnp.PyArray_DATA(trans)
     ntrans = len(trans)
 
     # Determine whether each date lies left of the DST transition (store in
     # result_a) or right of the DST transition (store in result_b)
     result_a = np.empty(n, dtype=np.int64)
     result_b = np.empty(n, dtype=np.int64)
-    result_a.fill(NPY_NAT)
-    result_b.fill(NPY_NAT)
+    result_a[:] = NPY_NAT
+    result_b[:] = NPY_NAT
 
     idx_shifted_left = (np.maximum(0, trans.searchsorted(
         vals - DAY_NS, side='right') - 1)).astype(np.int64)
@@ -952,7 +952,7 @@ def tz_localize_to_utc(ndarray[int64_t] vals, object tz, object ambiguous=None,
 
     if infer_dst:
         dst_hours = np.empty(n, dtype=np.int64)
-        dst_hours.fill(NPY_NAT)
+        dst_hours[:] = NPY_NAT
 
         # Get the ambiguous hours (given the above, these are the hours
         # where result_a != result_b and neither of them are NAT)
@@ -1045,8 +1045,10 @@ def tz_localize_to_utc(ndarray[int64_t] vals, object tz, object ambiguous=None,
     return result
 
 
-cdef inline bisect_right_i8(int64_t *data, int64_t val, Py_ssize_t n):
-    cdef Py_ssize_t pivot, left = 0, right = n
+cdef inline Py_ssize_t bisect_right_i8(int64_t *data,
+                                       int64_t val, Py_ssize_t n):
+    cdef:
+        Py_ssize_t pivot, left = 0, right = n
 
     assert n >= 1
 
diff --git a/pandas/_libs/tslibs/frequencies.pyx b/pandas/_libs/tslibs/frequencies.pyx
index c555fce9dd007..fff4d04399481 100644
--- a/pandas/_libs/tslibs/frequencies.pyx
+++ b/pandas/_libs/tslibs/frequencies.pyx
@@ -154,8 +154,7 @@ cpdef get_freq_code(freqstr):
         freqstr = (freqstr.rule_code, freqstr.n)
 
     if isinstance(freqstr, tuple):
-        if (is_integer_object(freqstr[0]) and
-                is_integer_object(freqstr[1])):
+        if is_integer_object(freqstr[0]) and is_integer_object(freqstr[1]):
             # e.g., freqstr = (2000, 1)
             return freqstr
         else:
@@ -171,7 +170,7 @@ cpdef get_freq_code(freqstr):
             return code, stride
 
     if is_integer_object(freqstr):
-        return (freqstr, 1)
+        return freqstr, 1
 
     base, stride = _base_and_stride(freqstr)
     code = _period_str_to_code(base)
@@ -183,6 +182,11 @@ cpdef _base_and_stride(freqstr):
     """
     Return base freq and stride info from string representation
 
+    Returns
+    -------
+    base : str
+    stride : int
+
     Examples
     --------
     _freq_and_stride('5Min') -> 'Min', 5
@@ -201,7 +205,7 @@ cpdef _base_and_stride(freqstr):
 
     base = groups.group(2)
 
-    return (base, stride)
+    return base, stride
 
 
 cpdef _period_str_to_code(freqstr):
diff --git a/pandas/_libs/tslibs/offsets.pyx b/pandas/_libs/tslibs/offsets.pyx
index 78e1269aa5363..8f5887754e40d 100644
--- a/pandas/_libs/tslibs/offsets.pyx
+++ b/pandas/_libs/tslibs/offsets.pyx
@@ -532,7 +532,8 @@ cdef inline int month_add_months(npy_datetimestruct dts, int months) nogil:
     New month number after shifting npy_datetimestruct
     number of months.
     """
-    cdef int new_month = (dts.month + months) % 12
+    cdef:
+        int new_month = (dts.month + months) % 12
     return 12 if new_month == 0 else new_month
 
 
diff --git a/pandas/_libs/tslibs/timestamps.pyx b/pandas/_libs/tslibs/timestamps.pyx
index 094b48920fc46..d5bd2e90af3a7 100644
--- a/pandas/_libs/tslibs/timestamps.pyx
+++ b/pandas/_libs/tslibs/timestamps.pyx
@@ -177,7 +177,8 @@ def round_nsint64(values, mode, freq):
 
     # if/elif above should catch all rounding modes defined in enum 'RoundTo':
     # if flow of control arrives here, it is a bug
-    assert False, "round_nsint64 called with an unrecognized rounding mode"
+    raise AssertionError("round_nsint64 called with an unrecognized "
+                         "rounding mode")
 
 
 # This is PITA. Because we inherit from datetime, which has very specific
diff --git a/pandas/_libs/writers.pyx b/pandas/_libs/writers.pyx
index 9af12cbec1e9c..4a0d1a7620fc5 100644
--- a/pandas/_libs/writers.pyx
+++ b/pandas/_libs/writers.pyx
@@ -128,16 +128,16 @@ def max_len_string_array(pandas_string[:] arr) -> Py_ssize_t:
     """ return the maximum size of elements in a 1-dim string array """
     cdef:
         Py_ssize_t i, m = 0, l = 0, length = arr.shape[0]
-        pandas_string v
+        pandas_string val
 
     for i in range(length):
-        v = arr[i]
-        if isinstance(v, str):
-            l = PyString_GET_SIZE(v)
-        elif isinstance(v, bytes):
-            l = PyBytes_GET_SIZE(v)
-        elif isinstance(v, unicode):
-            l = PyUnicode_GET_SIZE(v)
+        val = arr[i]
+        if isinstance(val, str):
+            l = PyString_GET_SIZE(val)
+        elif isinstance(val, bytes):
+            l = PyBytes_GET_SIZE(val)
+        elif isinstance(val, unicode):
+            l = PyUnicode_GET_SIZE(val)
 
         if l > m:
             m = l