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Question

I have a folder full of files and they don't have an extension. How can I check file types? I want to check the file type and change the filename accordingly. Let's assume a function filetype(x) returns a file type like png. I want to do this:

files = os.listdir(".")
for f in files:
    os.rename(f, f+filetype(f))

How do I do this?

Answer

The Python Magic library provides the functionality you need.

You can install the library with pip install python-magic and use it as follows:

>>> import magic
>>> magic.from_file('iceland.jpg')
'JPEG image data, JFIF standard 1.01'
>>> magic.from_file('iceland.jpg', mime=True)
'image/jpeg'
>>> magic.from_file('greenland.png')
'PNG image data, 600 x 1000, 8-bit colormap, non-interlaced'
>>> magic.from_file('greenland.png', mime=True)
'image/png'

The Python code in this case is calling to libmagic beneath the hood, which is the same library used by the *NIX file command. Thus, this does the same thing as the subprocess/shell-based answers, but without that overhead.