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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def load_fis(dir=None):
 """Load fits images in a directory"""

    if dir is None:
        import tkFileDialog
        try:
            dir=tkFileDialog.askdirectory()
        except:
            return
    if dir is None:
        return None
    from os.path import walk
    walk(dir,fits_list,"*.fits")