changes

Author: topper-123

doc/source/whatsnew/v1.1.0.rst

@@ -909,7 +909,7 @@ Missing
 - :meth:`DataFrame.interpolate` uses the correct axis convention now. Previously interpolating along columns lead to interpolation along indices and vice versa. Furthermore interpolating with methods ``pad``, ``ffill``, ``bfill`` and ``backfill`` are identical to using these methods with :meth:`fillna` (:issue:`12918`, :issue:`29146`)
 - Bug in :meth:`DataFrame.interpolate` when called on a DataFrame with column names of string type was throwing a ValueError. The method is no independing of the type of column names (:issue:`33956`)
 - :class:`NA` will now always work when passed into a format string. Previously a ``ValueError`` was raised if any format parameters were supplied to the format string.
-  For example ``"{:.1f}".format(pd.NA)`` would previously raise a ``ValueError``, but will now return the string ``"<NA>"`` (:issue:`xxxxx`)
+  For example ``"{:.1f}".format(pd.NA)`` would previously raise a ``ValueError``, but will now return the string ``"<NA>"`` (:issue:`34738`)
 
 MultiIndex
 ^^^^^^^^^^

pandas/_libs/missing.pyx

@@ -350,6 +350,12 @@ class NAType(C_NAType):
         return "<NA>"
 
     def __format__(self, format_spec) -> str:
+        # accept same format_spec as np.nan
+        try:
+            format(np.nan, format_spec)
+        except ValueError:
+            raise ValueError("Invalid format specifier")
+        # if a string format_spec use it, else just return the repr
         try:
             return self.__repr__().__format__(format_spec)
         except ValueError:

pandas/tests/scalar/test_na_scalar.py

@@ -23,13 +23,21 @@ def test_repr():
 
 
 def test_format():
+    # GH-34738
     assert format(NA) == "<NA>"
     assert format(NA, ">10") == "      <NA>"
-    assert format(NA, "xxx") == "<NA>"   # accept arbitrary format strings
 
     assert "{}".format(NA) == "<NA>"
     assert "{:>10}".format(NA) == "      <NA>"
-    assert "{:xxx}".format(NA) == "<NA>"  # accept arbitrary format strings
+
+
+def test_format_raises():
+    # GH-34738
+    with pytest.raises(ValueError, match="Invalid format specifier"):
+        format(NA, "xxx")
+
+    with pytest.raises(ValueError, match="Invalid format specifier"):
+        "{:xxx}".format(NA)
 
 
 def test_truthiness():