I want to append list in nested list Python

Change new_expression.append(expression) to new_expression.append(expression[:]) can work. The reason is that if you append expression, each element within new_expression will points to the same object. We need to raise a new object by copying.

expression = ['+', '10', 'x']
variables = {"x": (3,4)}
new_expression = []

for key, value in variables.items():
    key_index = expression.index(key)
    
    for i in range(3,5):
        expression[key_index] = i
        new_expression.append(expression[:])
        
print("new_expression",  new_expression)  

You may want to change for i in range(3,5): to for i in value:, as you already define the values in the variables.

Refined version:

expression_template = ['+', '10', 'x']
variables = {"x": (3,4)}
new_expression = []

for key, value in variables.items():
    key_index = expression_template.index(key)
    
    for i in value:
        expression = expression_template[:]
        expression[key_index] = i
        new_expression.append(expression)
        
print("new_expression",  new_expression)