Pillow/Tests/test_file_gif.py

import unittest
from io import BytesIO

import pytest
from PIL import GifImagePlugin, Image, ImageDraw, ImagePalette

from .helper import (
    PillowTestCase,
    assert_image_equal,
    assert_image_similar,
    hopper,
    is_pypy,
    netpbm_available,
)

try:
    from PIL import _webp

    HAVE_WEBP = True
except ImportError:
    HAVE_WEBP = False

codecs = dir(Image.core)

# sample gif stream
TEST_GIF = "Tests/images/hopper.gif"

with open(TEST_GIF, "rb") as f:
    data = f.read()


class TestFileGif(PillowTestCase):
    def setUp(self):
        if "gif_encoder" not in codecs or "gif_decoder" not in codecs:
            self.skipTest("gif support not available")  # can this happen?

    def test_sanity(self):
        with Image.open(TEST_GIF) as im:
            im.load()
            self.assertEqual(im.mode, "P")
            self.assertEqual(im.size, (128, 128))
            self.assertEqual(im.format, "GIF")
            self.assertEqual(im.info["version"], b"GIF89a")

    @unittest.skipIf(is_pypy(), "Requires CPython")
    def test_unclosed_file(self):
        def open():
            im = Image.open(TEST_GIF)
            im.load()

        pytest.warns(ResourceWarning, open)

    def test_closed_file(self):
        def open():
            im = Image.open(TEST_GIF)
            im.load()
            im.close()

        pytest.warns(None, open)

    def test_context_manager(self):
        def open():
            with Image.open(TEST_GIF) as im:
                im.load()

        pytest.warns(None, open)

    def test_invalid_file(self):
        invalid_file = "Tests/images/flower.jpg"

        self.assertRaises(SyntaxError, GifImagePlugin.GifImageFile, invalid_file)

    def test_optimize(self):
        def test_grayscale(optimize):
            im = Image.new("L", (1, 1), 0)
            filename = BytesIO()
            im.save(filename, "GIF", optimize=optimize)
            return len(filename.getvalue())

        def test_bilevel(optimize):
            im = Image.new("1", (1, 1), 0)
            test_file = BytesIO()
            im.save(test_file, "GIF", optimize=optimize)
            return len(test_file.getvalue())

        self.assertEqual(test_grayscale(0), 800)
        self.assertEqual(test_grayscale(1), 44)
        self.assertEqual(test_bilevel(0), 800)
        self.assertEqual(test_bilevel(1), 800)

    def test_optimize_correctness(self):
        # 256 color Palette image, posterize to > 128 and < 128 levels
        # Size bigger and smaller than 512x512
        # Check the palette for number of colors allocated.
        # Check for correctness after conversion back to RGB
        def check(colors, size, expected_palette_length):
            # make an image with empty colors in the start of the palette range
            im = Image.frombytes(
                "P", (colors, colors), bytes(range(256 - colors, 256)) * colors
            )
            im = im.resize((size, size))
            outfile = BytesIO()
            im.save(outfile, "GIF")
            outfile.seek(0)
            with Image.open(outfile) as reloaded:
                # check palette length
                palette_length = max(
                    i + 1 for i, v in enumerate(reloaded.histogram()) if v
                )
                self.assertEqual(expected_palette_length, palette_length)

                assert_image_equal(im.convert("RGB"), reloaded.convert("RGB"))

        # These do optimize the palette
        check(128, 511, 128)
        check(64, 511, 64)
        check(4, 511, 4)

        # These don't optimize the palette
        check(128, 513, 256)
        check(64, 513, 256)
        check(4, 513, 256)

        # other limits that don't optimize the palette
        check(129, 511, 256)
        check(255, 511, 256)
        check(256, 511, 256)

    def test_optimize_full_l(self):
        im = Image.frombytes("L", (16, 16), bytes(range(256)))
        test_file = BytesIO()
        im.save(test_file, "GIF", optimize=True)
        self.assertEqual(im.mode, "L")

    def test_roundtrip(self):
        out = self.tempfile("temp.gif")
        im = hopper()
        im.save(out)
        with Image.open(out) as reread:

            assert_image_similar(reread.convert("RGB"), im, 50)

    def test_roundtrip2(self):
        # see https://github.com/python-pillow/Pillow/issues/403
        out = self.tempfile("temp.gif")
        with Image.open(TEST_GIF) as im:
            im2 = im.copy()
            im2.save(out)
        with Image.open(out) as reread:

            assert_image_similar(reread.convert("RGB"), hopper(), 50)

    def test_roundtrip_save_all(self):
        # Single frame image
        out = self.tempfile("temp.gif")
        im = hopper()
        im.save(out, save_all=True)
        with Image.open(out) as reread:

            assert_image_similar(reread.convert("RGB"), im, 50)

        # Multiframe image
        with Image.open("Tests/images/dispose_bgnd.gif") as im:

            out = self.tempfile("temp.gif")
            im.save(out, save_all=True)
        with Image.open(out) as reread:

            self.assertEqual(reread.n_frames, 5)

    def test_headers_saving_for_animated_gifs(self):
        important_headers = ["background", "version", "duration", "loop"]
        # Multiframe image
        with Image.open("Tests/images/dispose_bgnd.gif") as im:

            info = im.info.copy()

            out = self.tempfile("temp.gif")
            im.save(out, save_all=True)
        with Image.open(out) as reread:

            for header in important_headers:
                self.assertEqual(info[header], reread.info[header])

    def test_palette_handling(self):
        # see https://github.com/python-pillow/Pillow/issues/513

        with Image.open(TEST_GIF) as im:
            im = im.convert("RGB")

            im = im.resize((100, 100), Image.LANCZOS)
            im2 = im.convert("P", palette=Image.ADAPTIVE, colors=256)

            f = self.tempfile("temp.gif")
            im2.save(f, optimize=True)

        with Image.open(f) as reloaded:

            assert_image_similar(im, reloaded.convert("RGB"), 10)

    def test_palette_434(self):
        # see https://github.com/python-pillow/Pillow/issues/434

        def roundtrip(im, *args, **kwargs):
            out = self.tempfile("temp.gif")
            im.copy().save(out, *args, **kwargs)
            reloaded = Image.open(out)

            return reloaded

        orig = "Tests/images/test.colors.gif"
        with Image.open(orig) as im:

            with roundtrip(im) as reloaded:
                assert_image_similar(im, reloaded, 1)
            with roundtrip(im, optimize=True) as reloaded:
                assert_image_similar(im, reloaded, 1)

            im = im.convert("RGB")
            # check automatic P conversion
            with roundtrip(im) as reloaded:
                reloaded = reloaded.convert("RGB")
                assert_image_equal(im, reloaded)

    @unittest.skipUnless(netpbm_available(), "netpbm not available")
    def test_save_netpbm_bmp_mode(self):
        with Image.open(TEST_GIF) as img:
            img = img.convert("RGB")

            tempfile = self.tempfile("temp.gif")
            GifImagePlugin._save_netpbm(img, 0, tempfile)
            with Image.open(tempfile) as reloaded:
                assert_image_similar(img, reloaded.convert("RGB"), 0)

    @unittest.skipUnless(netpbm_available(), "netpbm not available")
    def test_save_netpbm_l_mode(self):
        with Image.open(TEST_GIF) as img:
            img = img.convert("L")

            tempfile = self.tempfile("temp.gif")
            GifImagePlugin._save_netpbm(img, 0, tempfile)
            with Image.open(tempfile) as reloaded:
                assert_image_similar(img, reloaded.convert("L"), 0)

    def test_seek(self):
        with Image.open("Tests/images/dispose_none.gif") as img:
            framecount = 0
            try:
                while True:
                    framecount += 1
                    img.seek(img.tell() + 1)
            except EOFError:
                self.assertEqual(framecount, 5)

    def test_seek_info(self):
        with Image.open("Tests/images/iss634.gif") as im:
            info = im.info.copy()

            im.seek(1)
            im.seek(0)

            self.assertEqual(im.info, info)

    def test_seek_rewind(self):
        with Image.open("Tests/images/iss634.gif") as im:
            im.seek(2)
            im.seek(1)

            with Image.open("Tests/images/iss634.gif") as expected:
                expected.seek(1)
                assert_image_equal(im, expected)

    def test_n_frames(self):
        for path, n_frames in [[TEST_GIF, 1], ["Tests/images/iss634.gif", 42]]:
            # Test is_animated before n_frames
            with Image.open(path) as im:
                self.assertEqual(im.is_animated, n_frames != 1)

            # Test is_animated after n_frames
            with Image.open(path) as im:
                self.assertEqual(im.n_frames, n_frames)
                self.assertEqual(im.is_animated, n_frames != 1)

    def test_eoferror(self):
        with Image.open(TEST_GIF) as im:
            n_frames = im.n_frames

            # Test seeking past the last frame
            self.assertRaises(EOFError, im.seek, n_frames)
            self.assertLess(im.tell(), n_frames)

            # Test that seeking to the last frame does not raise an error
            im.seek(n_frames - 1)

    def test_dispose_none(self):
        with Image.open("Tests/images/dispose_none.gif") as img:
            try:
                while True:
                    img.seek(img.tell() + 1)
                    self.assertEqual(img.disposal_method, 1)
            except EOFError:
                pass

    def test_dispose_background(self):
        with Image.open("Tests/images/dispose_bgnd.gif") as img:
            try:
                while True:
                    img.seek(img.tell() + 1)
                    self.assertEqual(img.disposal_method, 2)
            except EOFError:
                pass

    def test_dispose_previous(self):
        with Image.open("Tests/images/dispose_prev.gif") as img:
            try:
                while True:
                    img.seek(img.tell() + 1)
                    self.assertEqual(img.disposal_method, 3)
            except EOFError:
                pass

    def test_save_dispose(self):
        out = self.tempfile("temp.gif")
        im_list = [
            Image.new("L", (100, 100), "#000"),
            Image.new("L", (100, 100), "#111"),
            Image.new("L", (100, 100), "#222"),
        ]
        for method in range(0, 4):
            im_list[0].save(
                out, save_all=True, append_images=im_list[1:], disposal=method
            )
            with Image.open(out) as img:
                for _ in range(2):
                    img.seek(img.tell() + 1)
                    self.assertEqual(img.disposal_method, method)

        # check per frame disposal
        im_list[0].save(
            out,
            save_all=True,
            append_images=im_list[1:],
            disposal=tuple(range(len(im_list))),
        )

        with Image.open(out) as img:

            for i in range(2):
                img.seek(img.tell() + 1)
                self.assertEqual(img.disposal_method, i + 1)

    def test_dispose2_palette(self):
        out = self.tempfile("temp.gif")

        # 4 backgrounds: White, Grey, Black, Red
        circles = [(255, 255, 255), (153, 153, 153), (0, 0, 0), (255, 0, 0)]

        im_list = []
        for circle in circles:
            img = Image.new("RGB", (100, 100), (255, 0, 0))

            # Red circle in center of each frame
            d = ImageDraw.Draw(img)
            d.ellipse([(40, 40), (60, 60)], fill=circle)

            im_list.append(img)

        im_list[0].save(out, save_all=True, append_images=im_list[1:], disposal=2)

        with Image.open(out) as img:
            for i, circle in enumerate(circles):
                img.seek(i)
                rgb_img = img.convert("RGB")

                # Check top left pixel matches background
                self.assertEqual(rgb_img.getpixel((0, 0)), (255, 0, 0))

                # Center remains red every frame
                self.assertEqual(rgb_img.getpixel((50, 50)), circle)

    def test_dispose2_diff(self):
        out = self.tempfile("temp.gif")

        # 4 frames: red/blue, red/red, blue/blue, red/blue
        circles = [
            ((255, 0, 0, 255), (0, 0, 255, 255)),
            ((255, 0, 0, 255), (255, 0, 0, 255)),
            ((0, 0, 255, 255), (0, 0, 255, 255)),
            ((255, 0, 0, 255), (0, 0, 255, 255)),
        ]

        im_list = []
        for i in range(len(circles)):
            # Transparent BG
            img = Image.new("RGBA", (100, 100), (255, 255, 255, 0))

            # Two circles per frame
            d = ImageDraw.Draw(img)
            d.ellipse([(0, 30), (40, 70)], fill=circles[i][0])
            d.ellipse([(60, 30), (100, 70)], fill=circles[i][1])

            im_list.append(img)

        im_list[0].save(
            out, save_all=True, append_images=im_list[1:], disposal=2, transparency=0
        )

        with Image.open(out) as img:
            for i, colours in enumerate(circles):
                img.seek(i)
                rgb_img = img.convert("RGBA")

                # Check left circle is correct colour
                self.assertEqual(rgb_img.getpixel((20, 50)), colours[0])

                # Check right circle is correct colour
                self.assertEqual(rgb_img.getpixel((80, 50)), colours[1])

                # Check BG is correct colour
                self.assertEqual(rgb_img.getpixel((1, 1)), (255, 255, 255, 0))

    def test_dispose2_background(self):
        out = self.tempfile("temp.gif")

        im_list = []

        im = Image.new("P", (100, 100))
        d = ImageDraw.Draw(im)
        d.rectangle([(50, 0), (100, 100)], fill="#f00")
        d.rectangle([(0, 0), (50, 100)], fill="#0f0")
        im_list.append(im)

        im = Image.new("P", (100, 100))
        d = ImageDraw.Draw(im)
        d.rectangle([(0, 0), (100, 50)], fill="#f00")
        d.rectangle([(0, 50), (100, 100)], fill="#0f0")
        im_list.append(im)

        im_list[0].save(
            out, save_all=True, append_images=im_list[1:], disposal=[0, 2], background=1
        )

        with Image.open(out) as im:
            im.seek(1)
            self.assertEqual(im.getpixel((0, 0)), 0)

    def test_iss634(self):
        with Image.open("Tests/images/iss634.gif") as img:
            # seek to the second frame
            img.seek(img.tell() + 1)
            # all transparent pixels should be replaced with the color from the
            # first frame
            self.assertEqual(img.histogram()[img.info["transparency"]], 0)

    def test_duration(self):
        duration = 1000

        out = self.tempfile("temp.gif")
        im = Image.new("L", (100, 100), "#000")

        # Check that the argument has priority over the info settings
        im.info["duration"] = 100
        im.save(out, duration=duration)

        with Image.open(out) as reread:
            self.assertEqual(reread.info["duration"], duration)

    def test_multiple_duration(self):
        duration_list = [1000, 2000, 3000]

        out = self.tempfile("temp.gif")
        im_list = [
            Image.new("L", (100, 100), "#000"),
            Image.new("L", (100, 100), "#111"),
            Image.new("L", (100, 100), "#222"),
        ]

        # duration as list
        im_list[0].save(
            out, save_all=True, append_images=im_list[1:], duration=duration_list
        )
        with Image.open(out) as reread:

            for duration in duration_list:
                self.assertEqual(reread.info["duration"], duration)
                try:
                    reread.seek(reread.tell() + 1)
                except EOFError:
                    pass

        # duration as tuple
        im_list[0].save(
            out, save_all=True, append_images=im_list[1:], duration=tuple(duration_list)
        )
        with Image.open(out) as reread:

            for duration in duration_list:
                self.assertEqual(reread.info["duration"], duration)
                try:
                    reread.seek(reread.tell() + 1)
                except EOFError:
                    pass

    def test_identical_frames(self):
        duration_list = [1000, 1500, 2000, 4000]

        out = self.tempfile("temp.gif")
        im_list = [
            Image.new("L", (100, 100), "#000"),
            Image.new("L", (100, 100), "#000"),
            Image.new("L", (100, 100), "#000"),
            Image.new("L", (100, 100), "#111"),
        ]

        # duration as list
        im_list[0].save(
            out, save_all=True, append_images=im_list[1:], duration=duration_list
        )
        with Image.open(out) as reread:

            # Assert that the first three frames were combined
            self.assertEqual(reread.n_frames, 2)

            # Assert that the new duration is the total of the identical frames
            self.assertEqual(reread.info["duration"], 4500)

    def test_identical_frames_to_single_frame(self):
        for duration in ([1000, 1500, 2000, 4000], (1000, 1500, 2000, 4000), 8500):
            out = self.tempfile("temp.gif")
            im_list = [
                Image.new("L", (100, 100), "#000"),
                Image.new("L", (100, 100), "#000"),
                Image.new("L", (100, 100), "#000"),
            ]

            im_list[0].save(
                out, save_all=True, append_images=im_list[1:], duration=duration
            )
            with Image.open(out) as reread:
                # Assert that all frames were combined
                self.assertEqual(reread.n_frames, 1)

                # Assert that the new duration is the total of the identical frames
                self.assertEqual(reread.info["duration"], 8500)

    def test_number_of_loops(self):
        number_of_loops = 2

        out = self.tempfile("temp.gif")
        im = Image.new("L", (100, 100), "#000")
        im.save(out, loop=number_of_loops)
        with Image.open(out) as reread:

            self.assertEqual(reread.info["loop"], number_of_loops)

    def test_background(self):
        out = self.tempfile("temp.gif")
        im = Image.new("L", (100, 100), "#000")
        im.info["background"] = 1
        im.save(out)
        with Image.open(out) as reread:

            self.assertEqual(reread.info["background"], im.info["background"])

        if HAVE_WEBP and _webp.HAVE_WEBPANIM:
            with Image.open("Tests/images/hopper.webp") as im:
                self.assertIsInstance(im.info["background"], tuple)
                im.save(out)

    def test_comment(self):
        with Image.open(TEST_GIF) as im:
            self.assertEqual(
                im.info["comment"], b"File written by Adobe Photoshop\xa8 4.0"
            )

        out = self.tempfile("temp.gif")
        im = Image.new("L", (100, 100), "#000")
        im.info["comment"] = b"Test comment text"
        im.save(out)
        with Image.open(out) as reread:
            self.assertEqual(reread.info["comment"], im.info["comment"])

        im.info["comment"] = "Test comment text"
        im.save(out)
        with Image.open(out) as reread:
            self.assertEqual(reread.info["comment"], im.info["comment"].encode())

    def test_comment_over_255(self):
        out = self.tempfile("temp.gif")
        im = Image.new("L", (100, 100), "#000")
        comment = b"Test comment text"
        while len(comment) < 256:
            comment += comment
        im.info["comment"] = comment
        im.save(out)
        with Image.open(out) as reread:

            self.assertEqual(reread.info["comment"], comment)

    def test_zero_comment_subblocks(self):
        with Image.open("Tests/images/hopper_zero_comment_subblocks.gif") as im:
            with Image.open(TEST_GIF) as expected:
                assert_image_equal(im, expected)

    def test_version(self):
        out = self.tempfile("temp.gif")

        def assertVersionAfterSave(im, version):
            im.save(out)
            with Image.open(out) as reread:
                self.assertEqual(reread.info["version"], version)

        # Test that GIF87a is used by default
        im = Image.new("L", (100, 100), "#000")
        assertVersionAfterSave(im, b"GIF87a")

        # Test setting the version to 89a
        im = Image.new("L", (100, 100), "#000")
        im.info["version"] = b"89a"
        assertVersionAfterSave(im, b"GIF89a")

        # Test that adding a GIF89a feature changes the version
        im.info["transparency"] = 1
        assertVersionAfterSave(im, b"GIF89a")

        # Test that a GIF87a image is also saved in that format
        with Image.open("Tests/images/test.colors.gif") as im:
            assertVersionAfterSave(im, b"GIF87a")

            # Test that a GIF89a image is also saved in that format
            im.info["version"] = b"GIF89a"
            assertVersionAfterSave(im, b"GIF87a")

    def test_append_images(self):
        out = self.tempfile("temp.gif")

        # Test appending single frame images
        im = Image.new("RGB", (100, 100), "#f00")
        ims = [Image.new("RGB", (100, 100), color) for color in ["#0f0", "#00f"]]
        im.copy().save(out, save_all=True, append_images=ims)

        with Image.open(out) as reread:
            self.assertEqual(reread.n_frames, 3)

        # Tests appending using a generator
        def imGenerator(ims):
            yield from ims

        im.save(out, save_all=True, append_images=imGenerator(ims))

        with Image.open(out) as reread:
            self.assertEqual(reread.n_frames, 3)

        # Tests appending single and multiple frame images
        with Image.open("Tests/images/dispose_none.gif") as im:
            with Image.open("Tests/images/dispose_prev.gif") as im2:
                im.save(out, save_all=True, append_images=[im2])

        with Image.open(out) as reread:
            self.assertEqual(reread.n_frames, 10)

    def test_transparent_optimize(self):
        # from issue #2195, if the transparent color is incorrectly
        # optimized out, gif loses transparency
        # Need a palette that isn't using the 0 color, and one
        # that's > 128 items where the transparent color is actually
        # the top palette entry to trigger the bug.

        data = bytes(range(1, 254))
        palette = ImagePalette.ImagePalette("RGB", list(range(256)) * 3)

        im = Image.new("L", (253, 1))
        im.frombytes(data)
        im.putpalette(palette)

        out = self.tempfile("temp.gif")
        im.save(out, transparency=253)
        with Image.open(out) as reloaded:

            self.assertEqual(reloaded.info["transparency"], 253)

    def test_rgb_transparency(self):
        out = self.tempfile("temp.gif")

        # Single frame
        im = Image.new("RGB", (1, 1))
        im.info["transparency"] = (255, 0, 0)
        pytest.warns(UserWarning, im.save, out)

        with Image.open(out) as reloaded:
            self.assertNotIn("transparency", reloaded.info)

        # Multiple frames
        im = Image.new("RGB", (1, 1))
        im.info["transparency"] = b""
        ims = [Image.new("RGB", (1, 1))]
        pytest.warns(UserWarning, im.save, out, save_all=True, append_images=ims)

        with Image.open(out) as reloaded:
            self.assertNotIn("transparency", reloaded.info)

    def test_bbox(self):
        out = self.tempfile("temp.gif")

        im = Image.new("RGB", (100, 100), "#fff")
        ims = [Image.new("RGB", (100, 100), "#000")]
        im.save(out, save_all=True, append_images=ims)

        with Image.open(out) as reread:
            self.assertEqual(reread.n_frames, 2)

    def test_palette_save_L(self):
        # generate an L mode image with a separate palette

        im = hopper("P")
        im_l = Image.frombytes("L", im.size, im.tobytes())
        palette = bytes(im.getpalette())

        out = self.tempfile("temp.gif")
        im_l.save(out, palette=palette)

        with Image.open(out) as reloaded:

            assert_image_equal(reloaded.convert("RGB"), im.convert("RGB"))

    def test_palette_save_P(self):
        # pass in a different palette, then construct what the image
        # would look like.
        # Forcing a non-straight grayscale palette.

        im = hopper("P")
        palette = bytes([255 - i // 3 for i in range(768)])

        out = self.tempfile("temp.gif")
        im.save(out, palette=palette)

        with Image.open(out) as reloaded:
            im.putpalette(palette)
            assert_image_equal(reloaded, im)

    def test_palette_save_ImagePalette(self):
        # pass in a different palette, as an ImagePalette.ImagePalette
        # effectively the same as test_palette_save_P

        im = hopper("P")
        palette = ImagePalette.ImagePalette("RGB", list(range(256))[::-1] * 3)

        out = self.tempfile("temp.gif")
        im.save(out, palette=palette)

        with Image.open(out) as reloaded:
            im.putpalette(palette)
            assert_image_equal(reloaded, im)

    def test_save_I(self):
        # Test saving something that would trigger the auto-convert to 'L'

        im = hopper("I")

        out = self.tempfile("temp.gif")
        im.save(out)

        with Image.open(out) as reloaded:
            assert_image_equal(reloaded.convert("L"), im.convert("L"))

    def test_getdata(self):
        # test getheader/getdata against legacy values
        # Create a 'P' image with holes in the palette
        im = Image._wedge().resize((16, 16), Image.NEAREST)
        im.putpalette(ImagePalette.ImagePalette("RGB"))
        im.info = {"background": 0}

        passed_palette = bytes([255 - i // 3 for i in range(768)])

        GifImagePlugin._FORCE_OPTIMIZE = True
        try:
            h = GifImagePlugin.getheader(im, passed_palette)
            d = GifImagePlugin.getdata(im)

            import pickle

            # Enable to get target values on pre-refactor version
            # with open('Tests/images/gif_header_data.pkl', 'wb') as f:
            #    pickle.dump((h, d), f, 1)
            with open("Tests/images/gif_header_data.pkl", "rb") as f:
                (h_target, d_target) = pickle.load(f)

            self.assertEqual(h, h_target)
            self.assertEqual(d, d_target)
        finally:
            GifImagePlugin._FORCE_OPTIMIZE = False

    def test_lzw_bits(self):
        # see https://github.com/python-pillow/Pillow/issues/2811
        with Image.open("Tests/images/issue_2811.gif") as im:
            self.assertEqual(im.tile[0][3][0], 11)  # LZW bits
            # codec error prepatch
            im.load()

    def test_extents(self):
        with Image.open("Tests/images/test_extents.gif") as im:
            self.assertEqual(im.size, (100, 100))
            im.seek(1)
            self.assertEqual(im.size, (150, 150))