BPS BPS - 3 months ago 7
Python Question

How to display line numbers in tkinter.Text widget?

I'm writing my own code editor and I want it to have numbered lines on left side. Based on this answer I wrote this sample code:

#!/usr/bin/env python3

import tkinter


class CodeEditor(tkinter.Frame):
def __init__(self, root):
tkinter.Frame.__init__(self, root)
# Line numbers widget
self.__line_numbers_canvas = tkinter.Canvas(self, width=40, bg='#555555', highlightbackground='#555555', highlightthickness=0)
self.__line_numbers_canvas.pack(side=tkinter.LEFT, fill=tkinter.Y)

self.__text = tkinter.Text(self)
self.__text['insertbackground'] = '#ffffff'
self.__text.pack(side=tkinter.LEFT, fill=tkinter.BOTH, expand=True)

def __update_line_numbers(self):
self.__line_numbers_canvas.delete("all")
i = self.__text.index('@0,0')
self.__text.update() #FIX: adding line
while True:
dline = self.__text.dlineinfo(i)
if dline:
y = dline[1]
linenum = i[0]
self.__line_numbers_canvas.create_text(1, y, anchor="nw", text=linenum, fill='#ffffff')
i = self.__text.index('{0}+1line'.format(i)) #FIX
else:
break

def load_from_file(self, path):
self.__text.delete('1.0', tkinter.END)
f = open(path, 'r')
self.__text.insert('0.0', f.read())
f.close()
self.__update_line_numbers()


class Application(tkinter.Tk):
def __init__(self):
tkinter.Tk.__init__(self)
code_editor = CodeEditor(self)
code_editor.pack(fill=tkinter.BOTH, expand=True)
code_editor.load_from_file(__file__)

def run(self):
self.mainloop()


if __name__ == '__main__':
app = Application()
app.run()


Unfortunately something is wrong inside
__update_line_numbers
. This method should write line numbers from top to bottom on my
Canvas
widget but it prints only the number for the first line (1) and then exits. Why?

Answer

The root problem is that you're calling dlineinfo before returning to the runloop, so the text hasn't been laid out yet.

As the docs explain:

This method only works if the text widget is updated. To make sure this is the case, you can call the update_idletasks method first.

As usual, to get more information, you have to turn to the Tcl docs for the underlying object, which basically tell you that the Text widget may not be correct about which characters are and are not visible until it's updated, in which case it may be returning None not because of any problem, but just because, as far as it's concerned, you're asking for the bbox of something that's off-screen.

A good way to test whether this is the problem is to call self.__text.see(i) before calling dlineinfo(i). If it changes the result of dlineinfo, this was the problem. (Or, if not that, at least something related to that—for whatever reason, Tk thinks everything after line 1 is off-screen.)

But in this case, even calling update_idletasks doesn't work, because it's not just updating the line info that needs to happen, but laying out the text in the first place. What you need to do is explicitly defer this call. For example, add this line to the bottom of load_from_file and now it works:

self.__text.after(0, self.__update_line_numbers)

You could also call self.__text.update() before calling self.__update_line_numbers() inline, and I think that should work.


As a side note, it would really help you to either run this under the debugger, or add a print(i, dline) at the top of the loop, so you can see what you're getting, instead of just guessing.

Also wouldn't it be easier to just increment a linenumber and use '{}.0'.format(linenumber) instead of creating complex indexes like @0,0+1line+1line+1line that (at least for me) don't work. You can call Text.index() to convert any index to canonical format, but why make it so difficult? You know that what you want is 1.0, 2.0, 3.0, etc., right?

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