IO Error using etree in python (anaconda)

Question

I'm trying to run the following code using Spyder in Anaconda 4.1.12 (Python 2.7):

import xml.etree.ElementTree as ET

tree = ET.parse("path/file")
root = tree.getroot()
for report in root.iter('Name'):
    print(report)
    tag = report.find('Name2')
    print(tag)
    for group in report.iter('Name3'):
        for tx in group:
            print('FOUND')
            print(tx)
            tx.append(tag)
with open("path/output",'w') as f:
    f.write(ET.tostring(root))

For some reason I'm getting one of two errors.

1) IOError: [Errno 22] invalid mode ('w') or filename: I've double checked that the path is correct and to my knowledge I have to include ('w') to write the file.

2) The program will halt half way through and I have to reset the console

Does anything look incorrect?