我试图将.osm文件传递给python函数，但获取NameError

Question

这是我的代码： -

import xml.etree.ElementTree as ET
filename= "kolkata_india.osm"

def count_tags(filename):
    tags = {}
    for event, elem in ET.iterparse(filename):
        if elem.tag in tags:
            tags[elem.tag] += 1
        else:
            tags[elem.tag] = 1

    print tags
if __name__ == "__main__":
	count_tags(filename)

这是错误信息： -

C:\Python27\Udacity\mongodb>python mapparser.py
Traceback (most recent call last):
  File "mapparser.py", line 17, in <module>
    count_tags(filename)
  File "mapparser.py", line 8, in count_tags
    filename = kolkata_india.osm
NameError: global name 'kolkata_india' is not defined

这里我做错了什么？