Bug caused by setting the breakpoint

from flask import Flask
import os

app = Flask(__name__)

if __name__ == '__main__':
    for root, dirs, files in os.walk('/Users/test/dir/', topdown=False):
        for filename in files:
            print(filename) #An exception will occur at this breakpoint 
    app.run()

The exception is as follows:

Traceback (most recent call last):
 File "/Applications/PyCharm CE.app/Contents/plugins/python-ce/helpers/pydev/pydevd.py", line 1551, in _exec
   pydev_imports.execfile(file, globals, locals)  # execute the script
 File "/Applications/PyCharm CE.app/Contents/plugins/python-ce/helpers/pydev/_pydev_imps/_pydev_execfile.py", line 18, in execfile
   exec(compile(contents+"\n", file, 'exec'), glob, loc)
 File "/Users/test/inngke/python/ai_util_script/test.py", line 9, in <module>
   print(filename)
NameError: name 'filename' is not defined

If this breakpoint is absent, the program runs normally.