ENH: ``__repr__`` for NpzFile object (#23357)

Improves the repr to include information about the arrays contained, e.g.:

   >>> npzfile = np.load('arr.npz')
   >>> npzfile
   NpzFile 'arr.npz' with keys arr_0, arr_1, arr_2, arr_3, arr_4...

closes #23319

Co-authored-by: Ross Barnowski <rossbar@berkeley.edu>

1 files changed, 17 insertions, 2 deletions

diff --git a/numpy/lib/tests/test_io.py b/numpy/lib/tests/test_io.py
index 06d6dbf8d..5a68fbc97 100644
--- a/numpy/lib/tests/test_io.py
+++ b/numpy/lib/tests/test_io.py
@@ -321,6 +321,21 @@ class TestSavezLoad(RoundtripTest):
             data.close()
             assert_(fp.closed)
 
+    @pytest.mark.parametrize("count, expected_repr", [
+        (1, "NpzFile {fname!r} with keys: arr_0"),
+        (5, "NpzFile {fname!r} with keys: arr_0, arr_1, arr_2, arr_3, arr_4"),
+        # _MAX_REPR_ARRAY_COUNT is 5, so files with more than 5 keys are
+        # expected to end in '...'
+        (6, "NpzFile {fname!r} with keys: arr_0, arr_1, arr_2, arr_3, arr_4..."),
+    ])
+    def test_repr_lists_keys(self, count, expected_repr):
+        a = np.array([[1, 2], [3, 4]], float)
+        with temppath(suffix='.npz') as tmp:
+            np.savez(tmp, *[a]*count)
+            l = np.load(tmp)
+            assert repr(l) == expected_repr.format(fname=tmp)
+            l.close()
+
 
 class TestSaveTxt:
     def test_array(self):
@@ -597,8 +612,8 @@ class TestSaveTxt:
         # in our process if needed, see gh-16889
         memoryerror_raised = Value(c_bool)
 
-        # Since Python 3.8, the default start method for multiprocessing has 
-        # been changed from 'fork' to 'spawn' on macOS, causing inconsistency 
+        # Since Python 3.8, the default start method for multiprocessing has
+        # been changed from 'fork' to 'spawn' on macOS, causing inconsistency
         # on memory sharing model, lead to failed test for check_large_zip
         ctx = get_context('fork')
         p = ctx.Process(target=check_large_zip, args=(memoryerror_raised,))