id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
606549469,MDU6SXNzdWU2MDY1NDk0Njk=,4002,Remove dangerous default argument,40136154,closed,0,,,16,2020-04-24T19:58:59Z,2020-05-23T17:30:51Z,2020-05-23T17:30:51Z,CONTRIBUTOR,,,,"<!-- A short summary of the issue, if appropriate -->
Hi,
It is good not to use the default arguments while calling a function.

#### MCVE Code Sample
<!-- In order for the maintainers to efficiently understand and prioritize issues, we ask you to post a ""Minimal, Complete and Verifiable Example"" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports -->
In file `xarray/tests/test_conventions.py` 
```python
# Your code here
329    @contextlib.contextmanager
330    def roundtrip(
331        self, data, save_kwargs={}, open_kwargs={}, allow_cleanup_failure=False332    ):
333        store = CFEncodedInMemoryStore()
334        data.dump_to_store(store, **save_kwargs)
```

#### Problem Description
<!-- this should explain why the current behaviour is a problem and why the expected output is a better solution -->
Default arguments in Python are evaluated once when the function definition is executed. This means that the expression is evaluated only once when the function is defined and that the same value is used for each subsequent call. So if you are modifying the mutable default argument — a list, dict, etc., it will be modified for all future calls.
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