BUG: Fix pd.json_normalize to not skip the first element of a generator input

doc/source/whatsnew/v1.3.0.rst

@@ -258,6 +258,8 @@ I/O
 - Allow custom error values for parse_dates argument of :func:`read_sql`, :func:`read_sql_query` and :func:`read_sql_table` (:issue:`35185`)
 - Bug in :func:`to_hdf` raising ``KeyError`` when trying to apply
   for subclasses of ``DataFrame`` or ``Series`` (:issue:`33748`).
+- Bug in :func:`json_normalize` resulting in the first element of a generator object not being included in the returned ``DataFrame`` (:issue:`35923`)
+
 
 Period
 ^^^^^^

pandas/io/json/_normalize.py

@@ -1,7 +1,7 @@
 # ---------------------------------------------------------------------
 # JSON normalization routines
 
-from collections import defaultdict
+from collections import abc, defaultdict
 import copy
 from typing import Any, DefaultDict, Dict, Iterable, List, Optional, Union
 
@@ -262,6 +262,11 @@ def _pull_records(js: Dict[str, Any], spec: Union[List, str]) -> List:
     if isinstance(data, list) and not data:
         return DataFrame()
 
+    if isinstance(data, abc.Iterator):
+        # GH35923 Fix pd.json_normalize to not skip the first element of a
+        # generator input
+        data = list(data)
+
     # A bit of a hackjob
     if isinstance(data, dict):
         data = [data]

pandas/tests/io/json/test_normalize.py

@@ -518,6 +518,17 @@ def test_meta_non_iterable(self):
         )
         tm.assert_frame_equal(result, expected)
 
+    def test_generator(self, state_data):
+        # GH35923 Fix pd.json_normalize to not skip the first element of a
+        # generator input
+        def generator_data():
+            yield from state_data[0]["counties"]
+
+        result = json_normalize(generator_data())
+        expected = DataFrame(state_data[0]["counties"])
+
+        tm.assert_frame_equal(result, expected)
+
 
 class TestNestedToRecord:
     def test_flat_stays_flat(self):