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

@@ -20,7 +20,7 @@ def test_isatty():
 def test_seek_mode_0():
     # Arrange
     mode = 0
-    with open(TEST_FILE) as fh:
+    with open(TEST_FILE, "rb") as fh:
         container = ContainerIO.ContainerIO(fh, 22, 100)
 
         # Act
@@ -34,7 +34,7 @@ def test_seek_mode_0():
 def test_seek_mode_1():
     # Arrange
     mode = 1
-    with open(TEST_FILE) as fh:
+    with open(TEST_FILE, "rb") as fh:
         container = ContainerIO.ContainerIO(fh, 22, 100)
 
         # Act
@@ -48,7 +48,7 @@ def test_seek_mode_1():
 def test_seek_mode_2():
     # Arrange
     mode = 2
-    with open(TEST_FILE) as fh:
+    with open(TEST_FILE, "rb") as fh:
         container = ContainerIO.ContainerIO(fh, 22, 100)
 
         # Act
@@ -61,68 +61,68 @@ def test_seek_mode_2():
 
 def test_read_n0():
     # 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
-        assert data == "7\nThis is line 8\n"
+        assert data == b"7\nThis is line 8\n"
 
 
 def test_read_n():
     # 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
-        assert data == "7\nT"
+        assert data == b"7\nT"
 
 
 def test_read_eof():
     # 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
-        assert data == ""
+        assert data == b""
 
 
 def test_readline():
     # 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
-        assert data == "This is line 1\n"
+        assert data == b"This is line 1\n"
 
 
 def test_readlines():
     # 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

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