bpo-29435: Allow is_tarfile to take a filelike obj (GH-18090)

`is_tarfile()` now supports `name` being a file or file-like object.

2 files changed, 38 insertions, 1 deletions

diff --git a/Lib/tarfile.py b/Lib/tarfile.py
index 2c06f91..d0b748c 100755
--- a/Lib/tarfile.py
+++ b/Lib/tarfile.py
@@ -2461,9 +2461,14 @@ class TarFile(object):
 def is_tarfile(name):
     """Return True if name points to a tar archive that we
        are able to handle, else return False.
+
+       'name' should be a string, file, or file-like object.
     """
     try:
-        t = open(name)
+        if hasattr(name, "read"):
+            t = open(fileobj=name)
+        else:
+            t = open(name)
         t.close()
         return True
     except TarError:
diff --git a/Lib/test/test_tarfile.py b/Lib/test/test_tarfile.py
index 15324a4..6a90108 100644
--- a/Lib/test/test_tarfile.py
+++ b/Lib/test/test_tarfile.py
@@ -319,6 +319,38 @@ class LzmaListTest(LzmaTest, ListTest):
 
 class CommonReadTest(ReadTest):
 
+    def test_is_tarfile_erroneous(self):
+        with open(tmpname, "wb"):
+            pass
+
+        # is_tarfile works on filenames
+        self.assertFalse(tarfile.is_tarfile(tmpname))
+
+        # is_tarfile works on path-like objects
+        self.assertFalse(tarfile.is_tarfile(pathlib.Path(tmpname)))
+
+        # is_tarfile works on file objects
+        with open(tmpname, "rb") as fobj:
+            self.assertFalse(tarfile.is_tarfile(fobj))
+
+        # is_tarfile works on file-like objects
+        self.assertFalse(tarfile.is_tarfile(io.BytesIO(b"invalid")))
+
+    def test_is_tarfile_valid(self):
+        # is_tarfile works on filenames
+        self.assertTrue(tarfile.is_tarfile(self.tarname))
+
+        # is_tarfile works on path-like objects
+        self.assertTrue(tarfile.is_tarfile(pathlib.Path(self.tarname)))
+
+        # is_tarfile works on file objects
+        with open(self.tarname, "rb") as fobj:
+            self.assertTrue(tarfile.is_tarfile(fobj))
+
+        # is_tarfile works on file-like objects
+        with open(self.tarname, "rb") as fobj:
+            self.assertTrue(tarfile.is_tarfile(io.BytesIO(fobj.read())))
+
     def test_empty_tarfile(self):
         # Test for issue6123: Allow opening empty archives.
         # This test checks if tarfile.open() is able to open an empty tar