tox always returns "dictionary changed size during iteration"

Question

I have a fresh install of tox that always returns "RuntimeError: dictionary changed size during iteration". Even when running it without parameters. It looks like it's failing when it attempts to import thing from pyparsing. Any help would be appreciated.

I'm running tox 3.20.0 on MacOS under Python 3.7.3.

Here's the full stack trace:

(py37) thisuser$ tox
Traceback (most recent call last):
  File "/Users/thisuser/.virtualenvs/py37/bin/tox", line 5, in <module>
    from tox import cmdline
  File "/Users/thisuser/.virtualenvs/py37/lib/python3.7/site-packages/tox/__init__.py", line 32, in <module>
    from .session import cmdline  # isort:skip
  File "/Users/thisuser/.virtualenvs/py37/lib/python3.7/site-packages/tox/session/__init__.py", line 22, in <module>
    from tox.config import INTERRUPT_TIMEOUT, SUICIDE_TIMEOUT, TERMINATE_TIMEOUT, parseconfig
  File "/Users/thisuser/.virtualenvs/py37/lib/python3.7/site-packages/tox/config/__init__.py", line 21, in <module>
    from packaging import requirements
  File "/Users/thisuser/.virtualenvs/py37/lib/python3.7/site-packages/packaging/requirements.py", line 9, in <module>
    from pyparsing import stringStart, stringEnd, originalTextFor, ParseException
  File "/Users/thisuser/.virtualenvs/py37/lib/python3.7/site-packages/pyparsing.py", line 4909, in <module>
    _escapedPunc = Word( _bslash, r"\[]-*.$+^?()~ ", exact=2 ).setParseAction(lambda s,l,t:t[0][1])
  File "/Users/thisuser/.virtualenvs/py37/lib/python3.7/site-packages/pyparsing.py", line 1294, in setParseAction
    self.parseAction = list(map(_trim_arity, list(fns)))
  File "/Users/thisuser/.virtualenvs/py37/lib/python3.7/site-packages/pyparsing.py", line 1076, in _trim_arity
    this_line = extract_stack(limit=2)[-1]
  File "/Users/thisuser/.virtualenvs/py37/lib/python3.7/site-packages/pyparsing.py", line 1060, in extract_stack
    frame_summary = traceback.extract_stack(limit=-offset+limit-1)[offset]
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/traceback.py", line 211, in extract_stack
    stack = StackSummary.extract(walk_stack(f), limit=limit)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/traceback.py", line 363, in extract
    f.line
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/traceback.py", line 285, in line
    self._line = linecache.getline(self.filename, self.lineno).strip()
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/linecache.py", line 16, in getline
    lines = getlines(filename, module_globals)
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.7/lib/python3.7/linecache.py", line 48, in getlines
    for mod in sys.modules.values():
RuntimeError: dictionary changed size during iteration

Answer 1

I had the same issue when using Mac OS default Python version (3.7.3). I checked the tox.ini file, and I saw some references to py36, so I used pyenv to switch to Python 3.6.9. This has fixes the RuntimeError: dictionary changed size during iteration error for me, and all tests were successfully triggered.