python2.7 - python split method AttributeError

# this program is used to catch the acronym

result = ''
inputdata = []

# input the phrase
inputdata = raw_input('please input the phrase ').upper().split()
for i in inputdata:
    result += i[0]
print result

#刚才随手删掉了 inputdata = []一行结果报错：
#AttributeError: 'builtin_function_or_method' object has no attribute 'split'。

原因是什么？没看懂

多谢...脑袋有点不清楚，可能确实是误操作了...