local variable 'gr' referenced before assignment

This error occurs when you try to use a local variable before it has been assigned a value. In Python, variables must be assigned a value before they can be used.

For example, consider the following code:

def my_function():
    print(gr)
    gr = 10

my_function()

When this code is executed, it will result in a "local variable 'gr' referenced before assignment" error because the variable gr is referenced before it is assigned a value.

To fix this error, you need to make sure that the variable is assigned a value before it is used. For example:

def my_function():
    gr = 10
    print(gr)

my_function()

In this updated code, the variable gr is assigned a value before it is used, so the error will not occur.