Return strings or bytes from ContainerIO according to the file object mode

Tests/test_file_container.py

@@ -61,7 +61,8 @@ def test_seek_mode_2():
 
 def test_read_n0():
     # Arrange
-    with open(TEST_FILE, "rb") as fh:
+    for bytesmode in (True, False):
+        with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
             container = ContainerIO.ContainerIO(fh, 22, 100)
 
             # Act
@@ -69,12 +70,15 @@ def test_read_n0():
             data = container.read()
 
             # Assert
-        assert data == b"7\nThis is line 8\n"
+            if bytesmode:
+                data = data.decode()
+            assert data == "7\nThis is line 8\n"
 
 
 def test_read_n():
     # Arrange
-    with open(TEST_FILE, "rb") as fh:
+    for bytesmode in (True, False):
+        with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
             container = ContainerIO.ContainerIO(fh, 22, 100)
 
             # Act
@@ -82,12 +86,15 @@ def test_read_n():
             data = container.read(3)
 
             # Assert
-        assert data == b"7\nT"
+            if bytesmode:
+                data = data.decode()
+            assert data == "7\nT"
 
 
 def test_read_eof():
     # Arrange
-    with open(TEST_FILE, "rb") as fh:
+    for bytesmode in (True, False):
+        with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
             container = ContainerIO.ContainerIO(fh, 22, 100)
 
             # Act
@@ -95,39 +102,46 @@ def test_read_eof():
             data = container.read()
 
             # Assert
-        assert data == b""
+            if bytesmode:
+                data = data.decode()
+            assert data == ""
 
 
 def test_readline():
     # Arrange
-    with open(TEST_FILE, "rb") as fh:
+    for bytesmode in (True, False):
+        with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
             container = ContainerIO.ContainerIO(fh, 0, 120)
 
             # Act
             data = container.readline()
 
             # Assert
-        assert data == b"This is line 1\n"
+            if bytesmode:
+                data = data.decode()
+            assert data == "This is line 1\n"
 
 
 def test_readlines():
     # Arrange
+    for bytesmode in (True, False):
         expected = [
-        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",
+            "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",
         ]
-    with open(TEST_FILE, "rb") as fh:
+        with open(TEST_FILE, "rb" if bytesmode else "r") as fh:
             container = ContainerIO.ContainerIO(fh, 0, 120)
 
             # Act
             data = container.readlines()
 
             # Assert
-
+            if bytesmode:
+                data = [line.decode() for line in data]
             assert data == expected

src/PIL/ContainerIO.py

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