Syntax error when using 'global' keyword in eval

[Bauklotz]

So, recently I've been experimenting with a telnet server, and a 5250 terminal emutator and I thought of making a field system, for easier graphical (well, not quite) displaying.

This basically lets you set the value of a X and Y position on the screen and it automatically updating it, and I needed to make the field_[X & Y coordinates] variables global in order to use them in all the functions. The screen size variables work fine, but since it has many diffirent field_* variables which have diffirent names, I had to use eval, but this throws a syntax error.

The code and error message is located below:

from os import system
 
def set_screen_size(x,y):
    global screen_x
    global screen_y
    screen_x = str(x)
    screen_y = str(y)
    system('mode con cols=' + screen_x + ' lines=' + screen_y)
 
def set_field(x,y,val):
    global screen_x
    global screen_y
    x = str(x)
    y = str(y)
    val = val[0:1]
    if(x > screen_x or x < 0 or y > screen_y or x < 0):
        on_error('Field outside screen given. set_field('+x+','+y+','+val+')')
    eval('global field_' + x + y)
    eval('field_' + x + y + ' = ' + val)
    update_screen()
 
def update_screen():
    global screen_x
    global screen_y
    for x in screen_x:
        line = ""
        for y in screen_y:
            eval('global field_' + x + y)
            eval('val = field_' + x + y)
            line = line + val
        print line
 

Traceback:

Traceback (most recent call last):
  File "G:\FieldScreen\fieldscreen.py", line 34, in <module>
    set_field(0,0,"H")
  File "G:\FieldScreen\fieldscreen.py", line 18, in set_field
    eval('global field_' + x + y)
  File "<string>", line 1
    global field_00
         ^
SyntaxError: invalid syntax
 

[woooee]

Start with this

if(x > screen_x or x < 0 or y > screen_y or x < 0):
if(x > screen_x or x < 0 or y > screen_y or y < 0): <=====