Python类正在更改全局变量而未将其声明为全局

Question

I made an error while defining a function within a class, yet it doesn't change how the code operates during runtime.

The error I made was using a global variable when I mean to use an instanced variable.

What I meant to write was:

self._map_data[screen_pos_layer][y][x] = selected_material

Instead, I wrote:

map_data[screen_pos_layer][y][x] = selected_material

However, the intended functionality (change the color of an LED) doesn't change, no matter if it's the instanced variable or the global variable. The function that actually writes the color into the LED is in a different class.

I thought that could only happen if I include global <variable> ? I have very little experience with Python but I'm sure this was true.

class Tools(object):

    def __init__(self, _map_data):
        self._map_data = _map_data

    def paint(self, event):
        if selected_tool == select_paint and selected_color != -1:
            for j in range(cursor_size_y):
                for i in range(cursor_size_x):
                    y = screen_pos_y + cursor_pos_y + j
                    x = screen_pos_x + cursor_pos_x + i

                    map_data[screen_pos_layer][y][x] = selected_material
        else:
            return

        moveCursor(event)

tools = Tools(map_data)

# this is a Tkinter object
window.bind_all("<Control-Up>", tools.paint)

I tried searching for this but I could only find posts about people wanting to use a global variable in a class, I specifically am trying not to.

Answer 1

Evidently your code has previously created a global map_data variable, so the code you're showing is not re-creating the map_data variable, it is only modifying an element of the existing variable. The existing variable is found via regular name lookup, and so in this case it is found in the global context.