only replace float-nans with NaN and not other null objects

Author: realead

pandas/_libs/missing.pyx

@@ -248,6 +248,31 @@ cdef bint checknull_with_nat_and_na(object obj):
     return checknull_with_nat(obj) or obj is C_NA
 
 
+@cython.wraparound(False)
+@cython.boundscheck(False)
+def is_float_nan(values: ndarray) -> ndarray:
+    """
+    True for elements which correspond to a float nan
+
+    Returns
+    -------
+    ndarray[bool]
+    """
+    cdef:
+        ndarray[uint8_t] result
+        Py_ssize_t i, N
+        object val
+
+    N = len(values)
+    result = np.zeros(N, dtype=np.uint8)
+
+    for i in range(N):
+        val = values[i]
+        if util.is_nan(val):
+                result[i] = True
+    return result.view(bool)
+
+
 @cython.wraparound(False)
 @cython.boundscheck(False)
 def is_numeric_na(values: ndarray) -> ndarray:

pandas/core/indexes/base.py

@@ -33,6 +33,7 @@
     is_datetime_array,
     no_default,
 )
+from pandas._libs.missing import is_float_nan
 from pandas._libs.tslibs import (
     IncompatibleFrequency,
     OutOfBoundsDatetime,
@@ -1392,7 +1393,7 @@ def _format_with_header(self, header: list[str_t], na_rep: str_t) -> list[str_t]
             result = [pprint_thing(x, escape_chars=("\t", "\r", "\n")) for x in values]
 
             # could have nans
-            mask = isna(values)
+            mask = is_float_nan(values)
             if mask.any():
                 result_arr = np.array(result)
                 result_arr[mask] = na_rep