Apply tags in a text widget

Question

I need some help tag-related!

I am writing a simple editor, supporting basic formatting. By using a Text widget (named text), I put a tag 'b' to set the text where this tag applies to bold.

This is not a problem when I apply the bold to a selection:

text.tag_add('b', SEL_FIRST,SEL_LAST)

I have instead two problems when I just want to switch on/off the bold while typing. To switch it on the only way I found is this:

text.insert(INSERT, '  ', 'b' )
text.mark_set("insert", INSERT+'-1c')

notice that I have to insert TWO spaces. If I insert one, the bold doesnt apply. If I insert '', my cursor goes back one char!

My second problem is how to switch it off, when I'm writing within a bolded region - and for this I havent the slightest idea...

Thanks for any help!

Bryan Oakley · Accepted Answer

You may not realize it, but you're trying to do something that's very hard in Tkinter. While the text widget tags are a powerful concept, they have some weaknesses when it comes to creating something like a wysywig editor.

You need to change your approach a bit. Instead of inserting spaces, I think the better solution is to add bindings that apply (or remove) your tags each time a character is inserted. This has it's own set of problems, but with enough attention to detail you can overcome them. We can do that though a custom key binding.

When you insert a character into a text widget, this is handled by a binding on the event on the text widget class. So, we could bind to to add the tag. However, if we add a binding to on the widget, this will fire before the class binding, meaning our code will execute before the character inserted rather than after. We'll be trying to modify something that hasn't actually been inserted into the widget yet.

One way to solve that is to bind to a key release rather than a key press, since the character is inserted on the press. However, think of the scenario where a user presses and holds a key down -- multiple characters will be entered but you may only get a single key-up event. So this solution isn't particularly good.

Another solution is to somehow arrange for our custom binding to happen after the default binding. To do that we need to do two things: 1) adjust the "bind tags" for the widget to have an additional tag after the class tag, and 2) add a binding to this new bind tag.

There are downsides to this approach too. Not because of the bind tags, but because there are a lot more events you need to handle besides (for example, control-v to paste isn't handled by the binding, so you'll have to add a special case for pasting).

That being said, this solution might be good enough for you, or at least good enough to help you better understand the problem, and understanding the problem is often the biggest obstacle to finding a solution.

In the following example I have a text widget that has an additional bindtag named "CustomText" that we place after the standard "Text" bind tag. I put a binding on this tag for the event, and in the handler I simply apply the appropriate tags to the just-inserted character.

You'll have to add your own code to handle a clipboard paste, and the problem of dealing with conflicting tags (such as two tags each with their own font). Hopefully, though, this example will act as inspiration

import Tkinter as tk

class SampleApp(tk.Tk):
    def __init__(self, *args, **kwargs):
        tk.Tk.__init__(self, *args, **kwargs)
        self.tag_vars = {
            "underline": tk.IntVar(),
            "red": tk.IntVar(),
            }

        self.text = MyText(self, width=40, height=8)
        self.text.tag_configure("red", foreground="red")
        self.text.tag_configure("underline", underline=True)

        toolbar = tk.Frame(self)
        self.underline = tk.Checkbutton(self, text="Underline", 
                                        onvalue = True, offvalue=False,
                                        variable = self.tag_vars["underline"]
                                        )
        self.red = tk.Checkbutton(self, text="Red", 
                                  onvalue = True, offvalue=False,
                                  variable = self.tag_vars["red"]
                                  )
        self.underline.pack(in_=toolbar, side="left")
        self.red.pack(in_=toolbar, side="left")

        toolbar.pack(side="top", fill="x")
        self.text.pack(side="top", fill="both", expand=True)

class MyText(tk.Text):
    def __init__(self, parent, *args, **kwargs):
        tk.Text.__init__(self, *args, **kwargs)

        self.parent = parent

        # add a new bind tag, "CustomText" so we
        # can have code run after the class binding
        # has done it's work
        bindtags = list(self.bindtags())
        i = bindtags.index("Text")
        bindtags.insert(i+1, "CustomText")
        self.bindtags(tuple(bindtags))

        # set a binding that will fire whenever a 
        # self-inserting key is pressed
        self.bind_class("CustomText", "", self.OnKey)

    def OnKey(self, event):
        # we are assuming this is called whenever 
        # a character is inserted. Apply or remove
        # each tag depending on the state of the checkbutton
        for tag in self.parent.tag_vars.keys():
            use_tag = self.parent.tag_vars[tag].get()
            if use_tag:
                self.tag_add(tag, "insert-1c", "insert")
            else:
                self.tag_remove(tag, "insert-1c", "insert")

if __name__ == "__main__":
    app = SampleApp()
    app.mainloop()

Apply tags in a text widget

Answers (2)

Related Questions