Merge branch 'master' into fix/bounds_encode_2

* master: (31 commits)
  Add quantile method to GroupBy (pydata#2828)
  rolling_exp (nee ewm) (pydata#2650)
  Ensure explicitly indexed arrays are preserved (pydata#3027)
  add back dask-dev tests (pydata#3025)
  ENH: keepdims=True for xarray reductions (pydata#3033)
  Revert cmap fix (pydata#3038)
  Add "errors" keyword argument to drop() and drop_dims() (pydata#2994) (pydata#3028)
  More consistency checks (pydata#2859)
  Check types in travis (pydata#3024)
  Update issue templates (pydata#3019)
  Add pytest markers to avoid warnings (pydata#3023)
  Feature/merge errormsg (pydata#2971)
  More support for missing_value. (pydata#2973)
  Use flake8 rather than pycodestyle (pydata#3010)
  Pandas labels deprecation (pydata#3016)
  Pytest capture uses match, not message (pydata#3011)
  dask-dev tests to allowed failures in travis (pydata#3014)
  Fix 'to_masked_array' computing dask arrays twice (pydata#3006)
  str accessor (pydata#2991)
  fix safe_cast_to_index (pydata#3001)
  ...

.github/ISSUE_TEMPLATE/bug_report.md

@@ -0,0 +1,30 @@
+---
+name: Bug report
+about: Create a report to help us improve
+title: ''
+labels: ''
+assignees: ''
+
+---
+
+#### MCVE Code Sample
+
+In order for the maintainers to efficiently understand and prioritize issues, we ask you post a "Minimal, Complete and Verifiable Example" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports
+
+```python
+# Your code here
+
+```
+
+#### Problem Description
+
+[this should explain **why** the current behavior is a problem and why the expected output is a better solution.]
+
+#### Expected Output
+
+#### Output of ``xr.show_versions()``
+
+<details>
+# Paste the output here xr.show_versions() here
+
+</details>

.pep8speaks.yml

@@ -1,16 +1,6 @@
-# File : .pep8speaks.yml
-
-# This should be kept in sync with the duplicate config in the [pycodestyle]
-# block of setup.cfg.
+# https://github.com/OrkoHunter/pep8speaks for more info
+# pep8speaks will use the flake8 configs in `setup.cfg`
 
 scanner:
-    diff_only: False  # If True, errors caused by only the patch are shown
-
-pycodestyle:
-    max-line-length: 79
-    ignore:  # Errors and warnings to ignore
-        - E402  # module level import not at top of file
-        - E731  # do not assign a lambda expression, use a def
-        - E741  # ambiguous variable name
-        - W503  # line break before binary operator
-        - W504  # line break after binary operator
+    diff_only: False
+    linter: flake8

.travis.yml

@@ -22,6 +22,7 @@ matrix:
   - env: CONDA_ENV=py36-zarr-dev
   - env: CONDA_ENV=docs
   - env: CONDA_ENV=lint
+  - env: CONDA_ENV=typing
   - env: CONDA_ENV=py36-hypothesis
 
   allow_failures:
@@ -30,6 +31,7 @@ matrix:
     - EXTRA_FLAGS="--run-flaky --run-network-tests"
   - env: CONDA_ENV=py36-pandas-dev
   - env: CONDA_ENV=py36-zarr-dev
+  - env: CONDA_ENV=typing
 
 before_install:
   - wget https://repo.anaconda.com/miniconda/Miniconda3-latest-Linux-x86_64.sh -O miniconda.sh;
@@ -40,9 +42,10 @@ before_install:
   - conda info -a
 
 install:
-  - if [[ "$CONDA_ENV" == "docs" ]]; then
+  - |
+    if [[ "$CONDA_ENV" == "docs" ]]; then
       conda env create -n test_env --file doc/environment.yml;
-    elif [[ "$CONDA_ENV" == "lint" ]]; then
+    elif [[ "$CONDA_ENV" == "lint" ]] || [[ "$CONDA_ENV" == "typing" ]] ; then
       conda env create -n test_env --file ci/requirements-py37.yml;
     else
       conda env create -n test_env --file ci/requirements-$CONDA_ENV.yml;
@@ -56,11 +59,14 @@ script:
   - which python
   - python --version
   - python -OO -c "import xarray"
-  - if [[ "$CONDA_ENV" == "docs" ]]; then
+  - |
+    if [[ "$CONDA_ENV" == "docs" ]]; then
       cd doc;
       sphinx-build -n -j auto -b html -d _build/doctrees . _build/html;
     elif [[ "$CONDA_ENV" == "lint" ]]; then
-      pycodestyle xarray ;
+      flake8 ;
+    elif [[ "$CONDA_ENV" == "typing" ]]; then
+      mypy . ;
     elif [[ "$CONDA_ENV" == "py36-hypothesis" ]]; then
       pytest properties ;
     else

asv_bench/benchmarks/__init__.py

@@ -1,6 +1,5 @@
-from __future__ import absolute_import
-from __future__ import division
-from __future__ import print_function
+from __future__ import absolute_import, division, print_function
+
 import itertools
 
 import numpy as np

ci/requirements-py36.yml

@@ -14,7 +14,7 @@ dependencies:
   - pytest-cov
   - pytest-env
   - coveralls
-  - pycodestyle
+  - flake8
   - numpy>=1.12
   - pandas>=0.19
   - scipy
@@ -24,12 +24,11 @@ dependencies:
   - bottleneck
   - zarr
   - pseudonetcdf>=3.0.1
-  - eccodes
+  - cfgrib>=0.9.2
   - cdms2
   - pynio
   - iris>=1.10
   - pydap
   - lxml
   - pip:
-    - cfgrib>=0.9.2
     - mypy==0.660

ci/requirements-py37.yml

@@ -15,7 +15,7 @@ dependencies:
   - pytest-cov
   - pytest-env
   - coveralls
-  - pycodestyle
+  - flake8
   - numpy>=1.12
   - pandas>=0.19
   - scipy
@@ -25,9 +25,9 @@ dependencies:
   - bottleneck
   - zarr
   - pseudonetcdf>=3.0.1
+  - cfgrib>=0.9.2
   - lxml
-  - eccodes
   - pydap
   - pip:
-    - cfgrib>=0.9.2
     - mypy==0.650
+    - numbagg

conftest.py

@@ -1,9 +1,29 @@
 """Configuration for pytest."""
 
+import pytest
+
 
 def pytest_addoption(parser):
     """Add command-line flags for pytest."""
     parser.addoption("--run-flaky", action="store_true",
                      help="runs flaky tests")
     parser.addoption("--run-network-tests", action="store_true",
                      help="runs tests requiring a network connection")
+
+
+def pytest_collection_modifyitems(config, items):
+
+    if not config.getoption("--run-flaky"):
+        skip_flaky = pytest.mark.skip(
+            reason="set --run-flaky option to run flaky tests")
+        for item in items:
+            if "flaky" in item.keywords:
+                item.add_marker(skip_flaky)
+
+    if not config.getoption("--run-network-tests"):
+        skip_network = pytest.mark.skip(
+            reason="set --run-network-tests option to run tests requiring an"
+            "internet connection")
+        for item in items:
+            if "network" in item.keywords:
+                item.add_marker(skip_network)

doc/api-hidden.rst

@@ -153,3 +153,4 @@
 
    CFTimeIndex.shift
    CFTimeIndex.to_datetimeindex
+   CFTimeIndex.strftime

doc/api.rst

@@ -148,6 +148,7 @@ Computation
    Dataset.groupby
    Dataset.groupby_bins
    Dataset.rolling
+   Dataset.rolling_exp
    Dataset.coarsen
    Dataset.resample
    Dataset.diff
@@ -189,6 +190,7 @@ Computation
 :py:attr:`~core.groupby.DatasetGroupBy.last`
 :py:attr:`~core.groupby.DatasetGroupBy.fillna`
 :py:attr:`~core.groupby.DatasetGroupBy.where`
+:py:attr:`~core.groupby.DatasetGroupBy.quantile`
 
 Reshaping and reorganizing
 --------------------------
@@ -315,6 +317,7 @@ Computation
    DataArray.groupby
    DataArray.groupby_bins
    DataArray.rolling
+   DataArray.rolling_exp
    DataArray.coarsen
    DataArray.dt
    DataArray.resample
@@ -324,6 +327,7 @@ Computation
    DataArray.quantile
    DataArray.differentiate
    DataArray.integrate
+   DataArray.str
 
 **Aggregation**:
 :py:attr:`~DataArray.all`
@@ -359,7 +363,7 @@ Computation
 :py:attr:`~core.groupby.DataArrayGroupBy.last`
 :py:attr:`~core.groupby.DataArrayGroupBy.fillna`
 :py:attr:`~core.groupby.DataArrayGroupBy.where`
-
+:py:attr:`~core.groupby.DataArrayGroupBy.quantile`
 
 Reshaping and reorganizing
 --------------------------
@@ -460,6 +464,7 @@ Dataset methods
    :toctree: generated/
 
    open_dataset
+   load_dataset
    open_mfdataset
    open_rasterio
    open_zarr
@@ -487,6 +492,7 @@ DataArray methods
    :toctree: generated/
 
    open_dataarray
+   load_dataarray
    DataArray.to_dataset
    DataArray.to_netcdf
    DataArray.to_pandas
@@ -532,6 +538,7 @@ Rolling objects
    core.rolling.DatasetRolling
    core.rolling.DatasetRolling.construct
    core.rolling.DatasetRolling.reduce
+   core.rolling_exp.RollingExp
 
 Resample objects
 ================
@@ -555,6 +562,15 @@ Resample objects also implement the GroupBy interface
    core.resample.DatasetResample.nearest
    core.resample.DatasetResample.pad
 
+Accessors
+=========
+
+.. autosummary::
+   :toctree: generated/
+
+   core.accessor_dt.DatetimeAccessor
+   core.accessor_str.StringAccessor
+
 Custom Indexes
 ==============
 .. autosummary::

doc/computation.rst

@@ -45,6 +45,12 @@ Use :py:func:`~xarray.where` to conditionally switch between values:
 
     xr.where(arr > 0, 'positive', 'negative')
 
+Use `@` to perform matrix multiplication:
+
+.. ipython:: python
+
+    arr @ arr
+
 Data arrays also implement many :py:class:`numpy.ndarray` methods:
 
 .. ipython:: python
@@ -143,20 +149,35 @@ name of the dimension as a key (e.g. ``y``) and the window size as the value
 
     arr.rolling(y=3)
 
-The label position and minimum number of periods in the rolling window are
-controlled by the ``center`` and ``min_periods`` arguments:
+Aggregation and summary methods can be applied directly to the ``Rolling``
+object:
 
 .. ipython:: python
 
-    arr.rolling(y=3, min_periods=2, center=True)
+    r = arr.rolling(y=3)
+    r.reduce(np.std)
+    r.mean()
 
-Aggregation and summary methods can be applied directly to the ``Rolling`` object:
+Aggregation results are assigned the coordinate at the end of each window by
+default, but can be centered by passing ``center=True`` when constructing the
+ ``Rolling`` object:
 
 .. ipython:: python
 
-    r = arr.rolling(y=3)
+    r = arr.rolling(y=3, center=True)
+    r.mean()
+
+As can be seen above, aggregations of windows which overlap the border of the
+array produce ``nan``s.  Setting ``min_periods`` in the call to ``rolling``
+changes the minimum number of observations within the window required to have
+a value when aggregating:
+
+.. ipython:: python
+
+    r = arr.rolling(y=3, min_periods=2)
+    r.mean()
+    r = arr.rolling(y=3, center=True, min_periods=2)
     r.mean()
-    r.reduce(np.std)
 
 Note that rolling window aggregations are faster when bottleneck_ is installed.
 
@@ -169,6 +190,22 @@ We can also manually iterate through ``Rolling`` objects:
    for label, arr_window in r:
       # arr_window is a view of x
 
+.. _comput.rolling_exp:
+
+While ``rolling`` provides a simple moving average, ``DataArray`` also supports
+an exponential moving average with :py:meth:`~xarray.DataArray.rolling_exp`.
+This is similiar to pandas' ``ewm`` method. numbagg_ is required.
+
+.. _numbagg: https://github.com/shoyer/numbagg
+
+.. code:: python
+
+    arr.rolling_exp(y=3).mean()
+
+The ``rolling_exp`` method takes a ``window_type`` kwarg, which can be ``'alpha'``,
+``'com'`` (for ``center-of-mass``), ``'span'``, and ``'halflife'``. The default is
+``span``.
+
 Finally, the rolling object has a ``construct`` method which returns a
 view of the original ``DataArray`` with the windowed dimension in
 the last position.

doc/conf.py

@@ -13,11 +13,11 @@
 # serve to show the default.
 from __future__ import absolute_import, division, print_function
 
-from contextlib import suppress
 import datetime
 import os
 import subprocess
 import sys
+from contextlib import suppress
 
 import xarray
 

doc/contributing.rst

@@ -351,20 +351,18 @@ the more common ``PEP8`` issues:
   - passing arguments should have spaces after commas, e.g. ``foo(arg1, arg2, kw1='bar')``
 
 :ref:`Continuous Integration <contributing.ci>` will run
-the `pycodestyle <http://pypi.python.org/pypi/pycodestyle>`_ tool
+the `flake8 <http://flake8.pycqa.org/en/latest/>`_ tool
 and report any stylistic errors in your code. Therefore, it is helpful before
-submitting code to run the check yourself::
+submitting code to run the check yourself:
 
-   pycodestyle xarray
+   flake8
 
 Other recommended but optional tools for checking code quality (not currently
 enforced in CI):
 
 - `mypy <http://mypy-lang.org/>`_ performs static type checking, which can
   make it easier to catch bugs. Please run ``mypy xarray`` if you annotate any
   code with `type hints <https://docs.python.org/3/library/typing.html>`_.
-- `flake8 <http://pypi.python.org/pypi/flake8>`_ includes a few more automated
-  checks than those enforced by pycodestyle.
 - `isort <https://github.com/timothycrosley/isort>`_ will highlight
   incorrectly sorted imports. ``isort -y`` will automatically fix them. See
   also `flake8-isort <https://github.com/gforcada/flake8-isort>`_.

doc/examples/_code/weather_data_setup.py

@@ -1,6 +1,6 @@
 import numpy as np
 import pandas as pd
-import seaborn as sns  # pandas aware plotting library
+import seaborn as sns  # noqa, pandas aware plotting library
 
 import xarray as xr
 

doc/installing.rst

@@ -45,6 +45,8 @@ For accelerating xarray
 - `bottleneck <https://github.com/kwgoodman/bottleneck>`__: speeds up
   NaN-skipping and rolling window aggregations by a large factor
   (1.1 or later)
+- `numbagg <https://github.com/shoyer/numbagg>`_: for exponential rolling
+  window operations
 
 For parallel computing
 ~~~~~~~~~~~~~~~~~~~~~~