bpo-29435: Allow is_tarfile to take a filelike obj

Doc/library/tarfile.rst

@@ -159,7 +159,10 @@ Some facts and figures:
 .. function:: is_tarfile(name)
 
    Return :const:`True` if *name* is a tar archive file, that the :mod:`tarfile`
-   module can read.
+   module can read. *name* may be a file or file-like object too.
+
+   .. versionchanged:: 3.9
+      Support for file and file-like objects.
 
 
 The :mod:`tarfile` module defines the following exceptions:

Lib/tarfile.py

@@ -2461,9 +2461,14 @@ def __exit__(self, type, value, traceback):
 def is_tarfile(name):
     """Return True if name points to a tar archive that we
        are able to handle, else return False.
+
+       The name argument may be a 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:

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

Misc/NEWS.d/next/Library/2020-01-20-18-48-00.bpo-29435.qqJ2Ax.rst

@@ -0,0 +1,2 @@
+Allow :func:`tarfile.is_tarfile` to be used with file and file-like
+objects, like :func:`zipfile.is_zipfile`. Patch by William Woodruff.