Reorder tests in maybe_downcast_numeric

The comment `# if we have any nulls, then we are done` is not consistent with the test `if isna(arr).any()` because `arr` is constructed only from the first element (`r[0]`) not the full ravel'd list of values.  Moreover, calling `np.array()` on some random type can have surprising consequences.

So instead, do the early-out test as intended, just using `r[0]` without going through `np.array()`.  Then test other things about `r[0]`.  Only then should we test all the values (and if we have any nulls, then we are done).

See pandas-dev#55824

Signed-off-by: Michael Tiemann <[email protected]>

Author: MichaelTiemannOSC

pandas/core/dtypes/cast.py

@@ -357,18 +357,20 @@ def trans(x):
             # if we don't have any elements, just astype it
             return trans(result).astype(dtype)
 
-        # do a test on the first element, if it fails then we are done
         r = result.ravel()
-        arr = np.array([r[0]])
 
-        if isna(arr).any():
-            # if we have any nulls, then we are done
+        if isna(r[0]):
+            # do a test on the first element, if it fails then we are done
             return result
 
         elif not isinstance(r[0], (np.integer, np.floating, int, float, bool)):
             # a comparable, e.g. a Decimal may slip in here
             return result
 
+        if isna(r).any():
+            # if we have any nulls, then we are done
+            return result
+
         if (
             issubclass(result.dtype.type, (np.object_, np.number))
             and notna(result).all()