How can you mark a portion of a text widget as readonly?

How can I mark a portion of a tkinter text widget as readonly? That is, I want to be able to allow editing only in certain parts of the widget. For example, I want to allow editing only after a prompt but not before, to simulate a console.

The most bullet-proof solution is to intercept the low-level insert and delete commands, and put logic in there to prevent insertions and deletions based on some sort of criteria. For example, you could disallow edits within any range of text that has the tag "readonly".

Here's an example of this technique. It takes advantage of the fact that all insertions and deletions ultimately call the insert or delete subcommand of the underlying tk widget command, and the fact that the widget command can be replaced with a Tcl proc.

try:
    # python 2.x
    import Tkinter as tk
except ImportError:
    # python 3.x
    import tkinter as tk

class Example(tk.Frame):
    def __init__(self, parent):
        tk.Frame.__init__(self, parent)

        text = ReadonlyText(self)
        sb = tk.Scrollbar(self, orient="vertical", command=text.yview)
        text.configure(yscrollcommand=sb.set)
        sb.pack(side="left", fill="y")
        text.pack(side="right", fill="both", expand=True)

        text.insert("end", "You can edit this line\n")
        text.insert("end", "You cannot edit or delete this line\n", "readonly")
        text.insert("end", "You can edit this, too.")

        text.tag_configure("readonly", foreground="darkgray")

class ReadonlyText(tk.Text):
    '''A text widget that doesn't permit inserts and deletes in regions tagged with "readonly"'''
    def __init__(self, *args, **kwargs):
        tk.Text.__init__(self, *args, **kwargs)

        # this code creates a proxy that will intercept
        # each actual insert and delete. 
        self.tk.eval(WIDGET_PROXY)

        # this code replaces the low level tk widget 
        # with the proxy
        widget = str(self)
        self.tk.eval('''
            rename {widget} _{widget}
            interp alias {{}} ::{widget} {{}} widget_proxy _{widget} 
        '''.format(widget=widget))

WIDGET_PROXY = '''
if {[llength [info commands widget_proxy]] == 0} {
    # Tcl code to implement a text widget proxy that disallows
    # insertions and deletions in regions marked with "readonly"
    proc widget_proxy {actual_widget args} {
        set command [lindex $args 0]
        set args [lrange $args 1 end]
        if {$command == "insert"} {
            set index [lindex $args 0]
            if [_is_readonly $actual_widget $index "$index+1c"] {
                bell
                return ""
            }
        }
        if {$command == "delete"} {
            foreach {index1 index2} $args {
                if {[_is_readonly $actual_widget $index1 $index2]} {
                    bell
                    return ""
                }
            }
        }
        # if we passed the previous checks, allow the command to 
        # run normally
        $actual_widget $command {*}$args
    }

    proc _is_readonly {widget index1 index2} {
        # return true if any text in the range between
        # index1 and index2 has the tag "readonly"
        set result false
        if {$index2 eq ""} {set index2 "$index1+1c"}
        # see if "readonly" is applied to any character in the
        # range. There's probably a more efficient way to do this, but
        # this is Good Enough
        for {set index $index1} \
            {[$widget compare $index < $index2]} \
            {set index [$widget index "$index+1c"]} {
                if {"readonly" in [$widget tag names $index]} {
                    set result true
                    break
                }
            }
        return $result
    }
}
'''

def main():
    root = tk.Tk()
    Example(root).pack(fill="both", expand=True)
    root.mainloop()

if __name__ == "__main__":
    main()