[Python] pdb.set_trace() fails inside test with monkeypatched open

I have a test monkeypatching builtin open to test a class which expects to read a file. When trying to debug it, I see:

TypeError: test_unique.<locals>.<lambda>() got an unexpected keyword argument 'encoding'


import builtins
import io
import pytest

def count_unique_letters_in_file(path: str):
with open(path) as f:
return len(set(c.lower() for c in f.read() if c.isalpha()))

def test_unique(monkeypatch: pytest.MonkeyPatch):
text = 'Quick frog or something'
monkeypatch.setattr(builtins, 'open', lambda _: io.StringIO(text))
assert count_unique_letters_in_file('blah.txt') == 15


If I put import pdb: pdb.set_trace() anywhere after the monkeypatch line, or run pytest --pdb, then I get the above line. I understand it's because pdb is trying to use open which leads it to my lambda, but how do I work around it? Assume I cannot change the interface of tested function to accept an already open file or its contents.

Traceback is long as it includes all the test framework stuff up to the pdb.set_trace() or failed assert running with --pdb, but ends at:

self = <_pytest.debugging.pytestPDB._get_pdb_wrapper_class.<locals>.PytestPdbWrapper object at 0x123>
completekey = 'tab', stdin = None, stdout = None, skip = None, nosigint = False, readrc = True

def __init__(completekey='tab', stdin=None, stdout=None, skip=None, nosigint=False, readrc=True):
...
if readrc:
try:
with open(os.path.expanduser('~/.pdbrc', encoding='utf-8') as rcFile:

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