Exercise 1 solution

    import gzip
    import os
    import tempfile
    import zipfile

    SIGNATURES = [
        (b"\x89PNG\r\n\x1a\n", "PNG image"),
        (b"\xff\xd8\xff", "JPEG image"),
        (b"GIF87a", "GIF image"),
        (b"GIF89a", "GIF image"),
        (b"%PDF-", "PDF document"),
        (b"PK\x03\x04", "ZIP archive"),
        (b"\x1f\x8b", "gzip data"),
    ]


    def identify_file(path):
        with open(path, "rb") as f:
            head = f.read(16)
        if not head:
            return "empty file"
        for magic, name in SIGNATURES:
            if head.startswith(magic):
                return name
        return "unknown"


    with tempfile.TemporaryDirectory() as folder:
        def make(name, data):
            path = os.path.join(folder, name)
            with open(path, "wb") as f:
                f.write(data)
            return path

        paths = [
            make("a.png", b"\x89PNG\r\n\x1a\n" + bytes(8)),
            make("b.pdf", b"%PDF-1.4\n"),
            make("c.gif", b"GIF89a" + bytes(10)),
            make("d.dat", b"hello, plain text"),
            make("e.png", b"not really a png"),
            make("f.bin", b""),
        ]
        zpath = os.path.join(folder, "g.zip")
        with zipfile.ZipFile(zpath, "w") as z:
            z.writestr("x.txt", "hi")
        paths.append(zpath)
        gpath = os.path.join(folder, "h.gz")
        with gzip.open(gpath, "wb") as g:
            g.write(b"hi")
        paths.append(gpath)

        for p in paths:
            print(os.path.basename(p), "->", identify_file(p))
    print(os.path.exists(folder))

    # a.png -> PNG image
    # b.pdf -> PDF document
    # c.gif -> GIF image
    # d.dat -> unknown
    # e.png -> unknown
    # f.bin -> empty file
    # g.zip -> ZIP archive
    # h.gz -> gzip data
    # False

(Run as written; the output matched the comments.)

Explanation: a file's name is only a label, so the function trusts the first
bytes instead. It reads at most 16 bytes, which is enough for every signature in
the table, returns "empty file" before trying any match, and falls back to
"unknown" for anything else. The plain-text file called e.png is not reported as
a PNG, because its content does not start with the PNG signature. The ZIP and
gzip files were produced by the standard zipfile and gzip modules, so the
signatures matched real output.