Add min periods parameter to Window

[emilyreff7]

We want to set min_periods=1 on rolling windows to avoid defaulting to min_periods equal to the window size n, thereby eliminating the necessity of NaNs in the first n-1 rows for trailing window operations.

[cpcloud]

@emilyreff7 It looks like you have a lint error: https://circleci.com/gh/ibis-project/ibis/7540?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

I'm going to looking into adding pep8speaks to ibis so that these kinds of failures are immediately surfaced rather than having to hunt through CI.

You can also see these failures on the command line by running make lint.

[cpcloud]

@emilyreff7 Another thing that will prevent lint issues in a PR is to setup the commit hooks using pre-commit. Once you've created your environment you can run pip install pre-commit and then inside your ibis clone run pre-commit install. Then when you run git commit the lint and style checks will run and will prevent committing unless they pass.

[cpcloud]

Looks good, just a couple of comments.

[emilyreff7]

@emilyreff7 Another thing that will prevent lint issues in a PR is to setup the commit hooks using pre-commit. Once you've created your environment you can run pip install pre-commit and then inside your ibis clone run pre-commit install. Then when you run git commit the lint and style checks will run and will prevent committing unless they pass.

I ran the above but got an error once I got to the git commit step:

Errors:
Traceback (most recent call last):
File "/home/emily/anaconda3/envs/ibis-dev/lib/python3.6/site-packages/virtualenv.py", line 2580, in
main()
File "/home/emily/anaconda3/envs/ibis-dev/lib/python3.6/site-packages/virtualenv.py", line 831, in main
symlink=options.symlink,
File "/home/emily/anaconda3/envs/ibis-dev/lib/python3.6/site-packages/virtualenv.py", line 1123, in create_environment
install_wheel(to_install, py_executable, search_dirs, download=download)
File "/home/emily/anaconda3/envs/ibis-dev/lib/python3.6/site-packages/virtualenv.py", line 973, in install_wheel
_install_wheel_with_search_dir(download, project_names, py_executable, search_dirs)
File "/home/emily/anaconda3/envs/ibis-dev/lib/python3.6/site-packages/virtualenv.py", line 1060, in _install_wheel_with_search_dir
call_subprocess(cmd, show_stdout=False, extra_env=env, stdin=script)
File "/home/emily/anaconda3/envs/ibis-dev/lib/python3.6/site-packages/virtualenv.py", line 924, in call_subprocess
raise OSError("Command {} failed with error code {}".format(cmd_desc, proc.returncode))
OSError: Command /home/emily/.cache/p...hon3.6/bin/python3.6 - setuptools pip wheel failed with error code 2

I re-installed pre-commit using conda (conta install -c conda-forge pre_commit) and it resolved the issue.

Also now just waiting on the checks to run...have been running/queued up for 1+ hours now.

[cpcloud]

@emilyreff7 Sometimes the checks take awhile if you haven't enabled the CI services on your fork. If you enable CircleCI and Azure Pipelines on your fork then the CI will run with a separate set of containers than the ones that are used to run CI for the ibis-project user. I can help you set that up if you want.

[cpcloud]

In any event, this LGTM.