python eval忽略本地，寻找全局

在新的Python 3.4.1 shell窗口中运行此代码时：

>>> def golf(c):
 a,d=[],{"U":"a.append(int(x[5]))","O":'a.pop() if a else 0',"E":'a[-1]'}
 return sum(eval(d[x[1]])or 0 for x in c)or 0

>>> golf(("PUSH 3", "POP", "POP", "PUSH 4", "PEEK", "PUSH 9", "PUSH 0", "PEEK", "POP", "PUSH 1", "PEEK"))

即使我在第二行中声明了a，也会出现这个错误--a，d=[]，{

Traceback (most recent call last):
  File "<pyshell#25>", line 1, in <module>
    golf(("PUSH 3", "POP", "POP", "PUSH 4", "PEEK", "PUSH 9", "PUSH 0", "PEEK", "POP", "PUSH 1", "PEEK"))
  File "<pyshell#24>", line 3, in golf
    return sum(eval(d[x[1]])or 0 for x in c)or 0
  File "<pyshell#24>", line 3, in <genexpr>
    return sum(eval(d[x[1]])or 0 for x in c)or 0
  File "<string>", line 1, in <module>
NameError: name 'a' is not defined

即使a仍然未定义（在shell窗口中），我得到：

为什么eval（）忽略我的本地a？

使用Python习惯用法：

使用Python惯用语：

def golf(c):
 a,d=[],{"U":"a.append(int(x[5]))","O":'a.pop() if a else 0',"E":'a[-1]'}
 for x in c:
  print(eval(d[x[1]])or 0)
 return []

0
3
0
0
4
0
0
0
0
0
1
[]

def good_append(new_item, a_list=None):
    if a_list is None:
        a_list = []
    a_list.append(new_item)
    return a_list