BUG: make .mean() raise an exception for strings

Author: kinshukdua

pandas/core/nanops.py

@@ -43,6 +43,7 @@
     is_numeric_dtype,
     is_object_dtype,
     is_scalar,
+    is_string_dtype,
     is_timedelta64_dtype,
     needs_i8_conversion,
     pandas_dtype,
@@ -691,7 +692,11 @@ def nanmean(
         dtype_count = dtype
 
     count = _get_counts(values.shape, mask, axis, dtype=dtype_count)
-    the_sum = _ensure_numeric(values.sum(axis, dtype=dtype_sum))
+    the_sum = values.sum(axis, dtype=dtype_sum)
+    if isinstance(the_sum, str) or is_string_dtype(the_sum):
+        raise TypeError("cannot find the mean of type 'str'")
+    else:
+        _ensure_numeric(the_sum)
 
     if axis is not None and getattr(the_sum, "ndim", False):
         count = cast(np.ndarray, count)