The event object

A callback bound with bind receives one argument: an event object whose attributes describe what happened. tkinter fills these from the underlying Tk event; an attribute that does not apply to the event type holds the string "??" (or a meaningless value), so read only the ones relevant to the event you bound.

Attribute	Tk field	Meaning	Populated for
widget	%W	The widget instance the event fired on.	all events
x, y	%x %y	Pointer position relative to the widget’s top-left.	pointer & key events
x_root, y_root	%X %Y	Pointer position relative to the whole screen.	pointer & key events
num	%b	Button number (1–5).	Button events
delta	%D	Wheel rotation amount and direction.	MouseWheel
char	%A	The character produced ("" for non-printing keys).	key events
keysym	%K	The key symbol name ("Left", "a").	key events
keysym_num	%N	The key symbol as a number.	key events
keycode	%k	The hardware key code.	key events
state	%s	Modifier/button state as a bitmask integer (see below).	pointer, key & other events
time	%t	Server timestamp in milliseconds.	most events
width, height	%w %h	New size of the widget.	Configure, Expose
type	%T	The event type as an EventType enum (see below).	all events
focus	%f	Whether the pointer’s window has focus.	Enter, Leave
send_event	%E	True if the event was synthesized.	all events
serial	%#	The event’s serial number.	all events

event.type

event.type is an EventType enum member, not a bare string. Read its name or compare against the enum:

import tkinter as tk

def handler(event):
    print(event.type.name)              # -> "ButtonPress"
    if event.type == tk.EventType.KeyPress:
        ...

event.state

state is a bitmask of the modifiers and buttons held when the event fired (Shift = 0x0001, Lock = 0x0002, Control = 0x0004, Button1 = 0x0100, …). Decoding it by hand is rarely worth it — prefer naming the modifier in the pattern (<Shift-Button-1>) so tkinter matches it for you.

Fields tkinter does not expose

Tk defines several event fields that tkinter’s event object omits — notably the detail / user-data field (%d) of a virtual event. This means a value passed to event_generate(..., data="…") is not readable on the Python event; to pass information alongside a virtual event, keep it somewhere both sides can see (an attribute, a variable) rather than on the event. The X11-internal fields (%a, %c, %m, %o, %p, %B, %P, %R, %S, %i) are likewise not exposed.