这两段代码到底有什么不同？---第一段代码toppings对应的值列表中两种配料间没有加逗号，导致两种配料不会分行显示

**#两种配料间没有加逗号**
pizza = {
'crust':'thick',
'toppings':['mushroom''extra cheese'],
}

#概述所点的比萨
print("You ordered a " + pizza['crust'] + "-crust pizza"+
"with the following toppings:")

for topping in pizza["toppings"]:
print("\t" + topping)

pizza = {
'crust': 'thick',
'toppings': ['mushroom', 'extra cheese']
}
print("you ordered a " + pizza['crust'] + "-crust pizza" +
" with the following toppings")

for topping in pizza['toppings']:
print("\t" + topping)

为什么第一段代码结果里 mushroomextra cheese 不会分行显示？