Nicole_code.py

import os
from bs4 import BeautifulSoup

folder = '/Users/nt/Desktop/letters'
files = os.listdir(folder)
tagnames = ['persName', 'abstract', 'date', 'transcription']

files = [f for f in files if 'xml' in f]

output_strs = []

for fname in files:
    with open(os.path.join(folder, fname), "r") as infile:

        file_data= ""
        content = infile.read()
        soup = BeautifulSoup(content,'xml')
        for tagname in tagnames:
            tag_data = soup.find(tagname)
            tag_text = tag_data.get_text()
            file_data += tag_text
            file_data += '\t'
            tagnames.append(tag_text)
            tagnames_str = "\t".join(tagnames)

        output_strs.append("{}\t{}\n".format(fname, text.replace('\n', ' ')))

print(len(output_strs))
with open('/Users/nt/Desktop/tagdata.dat', 'w') as outfile:
    outfile.writelines(output_strs)
print('yo dude im finito')

Have a think about how many times your persName in persNames loop will execute for each fname in files iteration!

consider using a list

data = []
inside loop:
data.append(tag_text)

after loop:
data_str = "\t".join(data)

error:

Traceback (most recent call last):
File "/Users/nt/Desktop/braaaam.py", line 20, in
tag_text = tag_data.get_text()
AttributeError: 'NoneType' object has no attribute 'get_text'

Ok the code was almost right before but now has turned into a little bit of a mess.
The error means that tag_data is None. So soup.find(tagname) returned None. So the tag is not in the document.
Easiest way to debug this now would be to insert
import pdb; pdb.set_trace() on line 11
When you run code, it'll stop on line 11. You'll then have the choice of pressing 'n' for next, moving one line ahead, or typing in 'p some_expression' to find out the value of some_expression at that point in code.

stepping through you can think about whether each expression returns what you expect it to etc.