[GitHub] [arrow] pitrou commented on a diff in pull request #13264: ARROW-16272: [Python] Fix NativeFile.read1()

[GitBox <gi...@apache.org>]

pitrou commented on code in PR #13264:
URL: https://github.com/apache/arrow/pull/13264#discussion_r885005532


##########
python/pyarrow/tests/test_io.py:
##########
@@ -1201,6 +1202,46 @@ def test_native_file_TextIOWrapper(tmpdir):
         assert res == data
 
 
+def test_native_file_TextIOWrapper_perf(tmpdir):
+    # ARROW-16272: TextIOWrapper.readline() shouldn't exhaust a large
+    # Arrow input stream.
+    data = b'foo\nquux\n'
+    path = str(tmpdir / 'largefile.txt')
+    with open(path, 'wb') as f:
+        f.write(data * 100_000)
+
+    binary_file = pa.OSFile(path, mode='rb')
+    with TextIOWrapper(binary_file) as f:
+        assert binary_file.tell() == 0
+        nbytes = 20_000
+        lines = f.readlines(nbytes)
+        assert len(lines) == math.ceil(2 * nbytes / len(data))
+        assert nbytes <= binary_file.tell() <= nbytes * 2

Review Comment:
   Hmm, for some reason this test doesn't fail without the fix. 



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