Correct str/bytes mixup in ContainerIO

Image data is expected to be read in bytes mode, not text mode so
ContainerIO should return bytes in all methods. The passed in file
handler is expected to be opened in bytes mode (as TarIO already does).

Tests/test_file_container.py

@@ -19,7 +19,7 @@ def test_isatty(self):
     def test_seek_mode_0(self):
         # Arrange
         mode = 0
-        with open(TEST_FILE) as fh:
+        with open(TEST_FILE, "rb") as fh:
             container = ContainerIO.ContainerIO(fh, 22, 100)
 
             # Act
@@ -32,7 +32,7 @@ def test_seek_mode_0(self):
     def test_seek_mode_1(self):
         # Arrange
         mode = 1
-        with open(TEST_FILE) as fh:
+        with open(TEST_FILE, "rb") as fh:
             container = ContainerIO.ContainerIO(fh, 22, 100)
 
             # Act
@@ -45,7 +45,7 @@ def test_seek_mode_1(self):
     def test_seek_mode_2(self):
         # Arrange
         mode = 2
-        with open(TEST_FILE) as fh:
+        with open(TEST_FILE, "rb") as fh:
             container = ContainerIO.ContainerIO(fh, 22, 100)
 
             # Act
@@ -57,64 +57,64 @@ def test_seek_mode_2(self):
 
     def test_read_n0(self):
         # Arrange
-        with open(TEST_FILE) as fh:
+        with open(TEST_FILE, "rb") as fh:
             container = ContainerIO.ContainerIO(fh, 22, 100)
 
             # Act
             container.seek(81)
             data = container.read()
 
             # Assert
-            self.assertEqual(data, "7\nThis is line 8\n")
+            self.assertEqual(data, b"7\nThis is line 8\n")
 
     def test_read_n(self):
         # Arrange
-        with open(TEST_FILE) as fh:
+        with open(TEST_FILE, "rb") as fh:
             container = ContainerIO.ContainerIO(fh, 22, 100)
 
             # Act
             container.seek(81)
             data = container.read(3)
 
             # Assert
-            self.assertEqual(data, "7\nT")
+            self.assertEqual(data, b"7\nT")
 
     def test_read_eof(self):
         # Arrange
-        with open(TEST_FILE) as fh:
+        with open(TEST_FILE, "rb") as fh:
             container = ContainerIO.ContainerIO(fh, 22, 100)
 
             # Act
             container.seek(100)
             data = container.read()
 
             # Assert
-            self.assertEqual(data, "")
+            self.assertEqual(data, b"")
 
     def test_readline(self):
         # Arrange
-        with open(TEST_FILE) as fh:
+        with open(TEST_FILE, "rb") as fh:
             container = ContainerIO.ContainerIO(fh, 0, 120)
 
             # Act
             data = container.readline()
 
             # Assert
-            self.assertEqual(data, "This is line 1\n")
+            self.assertEqual(data, b"This is line 1\n")
 
     def test_readlines(self):
         # Arrange
         expected = [
-            "This is line 1\n",
-            "This is line 2\n",
-            "This is line 3\n",
-            "This is line 4\n",
-            "This is line 5\n",
-            "This is line 6\n",
-            "This is line 7\n",
-            "This is line 8\n",
+            b"This is line 1\n",
+            b"This is line 2\n",
+            b"This is line 3\n",
+            b"This is line 4\n",
+            b"This is line 5\n",
+            b"This is line 6\n",
+            b"This is line 7\n",
+            b"This is line 8\n",
         ]
-        with open(TEST_FILE) as fh:
+        with open(TEST_FILE, "rb") as fh:
             container = ContainerIO.ContainerIO(fh, 0, 120)
 
             # Act

docs/handbook/tutorial.rst

@@ -468,17 +468,17 @@ Reading from an open file
     with open("hopper.ppm", "rb") as fp:
         im = Image.open(fp)
 
-To read an image from string data, use the :py:class:`~StringIO.StringIO`
-class:
+To read an image from bytes data, use the :py:class:`io.BytesIO` class:
 
-Reading from a string
-^^^^^^^^^^^^^^^^^^^^^
+Reading from a byte string
+^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 ::
 
+    import io
     from PIL import Image
-    import StringIO
-    im = Image.open(StringIO.StringIO(buffer))
+
+    im = Image.open(io.BytesIO(buffer))
 
 Note that the library rewinds the file (using ``seek(0)``) before reading the
 image header. In addition, seek will also be used when the image data is read

src/PIL/ContainerIO.py

@@ -82,7 +82,7 @@ def read(self, n=0):
         else:
             n = self.length - self.pos
         if not n:  # EOF
-            return ""
+            return b""
         self.pos = self.pos + n
         return self.fh.read(n)
 
@@ -92,13 +92,13 @@ def readline(self):
 
         :returns: An 8-bit string.
         """
-        s = ""
+        s = b""
         while True:
             c = self.read(1)
             if not c:
                 break
             s = s + c
-            if c == "\n":
+            if c == b"\n":
                 break
         return s