AttributeError Problem

[Skip Montanaro]

> numberOfVertices = int(infile.readline().decode()) # Read the first line from the file
> AttributeError: 'str' object has no attribute 'readline'
...
>     infile = filedialog.askopenfilename()

This is just returning a filename.  You need to open it to get a file
object.  For example:

    infile = filedialog.askopenfilename()
    fd = open(infile)
    ...
    numberOfVertices = int(fd.readline().decode())

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