From 68d0e0d25b498da14414af6e56e33f55ae4674f1 Mon Sep 17 00:00:00 2001
From: Mathias Hauser <mathause@users.noreply.github.com>
Date: Wed, 19 Aug 2020 22:32:36 +0200
Subject: [PATCH] annotate concat (#4346)

* annotate concat

* fix annotation: _from_temp_dataset

* whats new entry

* Update xarray/core/concat.py

* revert faulty change
---
 doc/whats-new.rst        |   2 +
 xarray/core/concat.py    | 105 +++++++++++++++++++++++++++++----------
 xarray/core/dataarray.py |   2 +-
 3 files changed, 81 insertions(+), 28 deletions(-)

diff --git a/doc/whats-new.rst b/doc/whats-new.rst
index 2652af5d9fd..91e6c8292f2 100644
--- a/doc/whats-new.rst
+++ b/doc/whats-new.rst
@@ -85,6 +85,8 @@ Internal Changes
   By `Guido Imperiale <https://github.com/crusaderky>`_
 - Only load resource files when running inside a Jupyter Notebook
   (:issue:`4294`) By `Guido Imperiale <https://github.com/crusaderky>`_
+- Enable type checking for :py:func:`concat` (:issue:`4238`)
+  By `Mathias Hauser <https://github.com/mathause>`_.
 
 
 .. _whats-new.0.16.0:
diff --git a/xarray/core/concat.py b/xarray/core/concat.py
index fa3fac92277..b238bec40ba 100644
--- a/xarray/core/concat.py
+++ b/xarray/core/concat.py
@@ -1,3 +1,16 @@
+from typing import (
+    TYPE_CHECKING,
+    Dict,
+    Hashable,
+    Iterable,
+    List,
+    Optional,
+    Set,
+    Tuple,
+    Union,
+    overload,
+)
+
 import pandas as pd
 
 from . import dtypes, utils
@@ -7,6 +20,40 @@
 from .variable import IndexVariable, Variable, as_variable
 from .variable import concat as concat_vars
 
+if TYPE_CHECKING:
+    from .dataarray import DataArray
+    from .dataset import Dataset
+
+
+@overload
+def concat(
+    objs: Iterable["Dataset"],
+    dim: Union[str, "DataArray", pd.Index],
+    data_vars: Union[str, List[str]] = "all",
+    coords: Union[str, List[str]] = "different",
+    compat: str = "equals",
+    positions: Optional[Iterable[int]] = None,
+    fill_value: object = dtypes.NA,
+    join: str = "outer",
+    combine_attrs: str = "override",
+) -> "Dataset":
+    ...
+
+
+@overload
+def concat(
+    objs: Iterable["DataArray"],
+    dim: Union[str, "DataArray", pd.Index],
+    data_vars: Union[str, List[str]] = "all",
+    coords: Union[str, List[str]] = "different",
+    compat: str = "equals",
+    positions: Optional[Iterable[int]] = None,
+    fill_value: object = dtypes.NA,
+    join: str = "outer",
+    combine_attrs: str = "override",
+) -> "DataArray":
+    ...
+
 
 def concat(
     objs,
@@ -285,13 +332,15 @@ def process_subset_opt(opt, subset):
 
 
 # determine dimensional coordinate names and a dict mapping name to DataArray
-def _parse_datasets(datasets):
+def _parse_datasets(
+    datasets: Iterable["Dataset"],
+) -> Tuple[Dict[Hashable, Variable], Dict[Hashable, int], Set[Hashable], Set[Hashable]]:
 
-    dims = set()
-    all_coord_names = set()
-    data_vars = set()  # list of data_vars
-    dim_coords = {}  # maps dim name to variable
-    dims_sizes = {}  # shared dimension sizes to expand variables
+    dims: Set[Hashable] = set()
+    all_coord_names: Set[Hashable] = set()
+    data_vars: Set[Hashable] = set()  # list of data_vars
+    dim_coords: Dict[Hashable, Variable] = {}  # maps dim name to variable
+    dims_sizes: Dict[Hashable, int] = {}  # shared dimension sizes to expand variables
 
     for ds in datasets:
         dims_sizes.update(ds.dims)
@@ -307,16 +356,16 @@ def _parse_datasets(datasets):
 
 
 def _dataset_concat(
-    datasets,
-    dim,
-    data_vars,
-    coords,
-    compat,
-    positions,
-    fill_value=dtypes.NA,
-    join="outer",
-    combine_attrs="override",
-):
+    datasets: List["Dataset"],
+    dim: Union[str, "DataArray", pd.Index],
+    data_vars: Union[str, List[str]],
+    coords: Union[str, List[str]],
+    compat: str,
+    positions: Optional[Iterable[int]],
+    fill_value: object = dtypes.NA,
+    join: str = "outer",
+    combine_attrs: str = "override",
+) -> "Dataset":
     """
     Concatenate a sequence of datasets along a new or existing dimension
     """
@@ -356,7 +405,9 @@ def _dataset_concat(
 
     result_vars = {}
     if variables_to_merge:
-        to_merge = {var: [] for var in variables_to_merge}
+        to_merge: Dict[Hashable, List[Variable]] = {
+            var: [] for var in variables_to_merge
+        }
 
         for ds in datasets:
             for var in variables_to_merge:
@@ -427,16 +478,16 @@ def ensure_common_dims(vars):
 
 
 def _dataarray_concat(
-    arrays,
-    dim,
-    data_vars,
-    coords,
-    compat,
-    positions,
-    fill_value=dtypes.NA,
-    join="outer",
-    combine_attrs="override",
-):
+    arrays: Iterable["DataArray"],
+    dim: Union[str, "DataArray", pd.Index],
+    data_vars: Union[str, List[str]],
+    coords: Union[str, List[str]],
+    compat: str,
+    positions: Optional[Iterable[int]],
+    fill_value: object = dtypes.NA,
+    join: str = "outer",
+    combine_attrs: str = "override",
+) -> "DataArray":
     arrays = list(arrays)
 
     if data_vars != "all":
diff --git a/xarray/core/dataarray.py b/xarray/core/dataarray.py
index 426329e6a6e..ea0465e2f2d 100644
--- a/xarray/core/dataarray.py
+++ b/xarray/core/dataarray.py
@@ -422,7 +422,7 @@ def _to_temp_dataset(self) -> Dataset:
         return self._to_dataset_whole(name=_THIS_ARRAY, shallow_copy=False)
 
     def _from_temp_dataset(
-        self, dataset: Dataset, name: Hashable = _default
+        self, dataset: Dataset, name: Union[Hashable, None, Default] = _default
     ) -> "DataArray":
         variable = dataset._variables.pop(_THIS_ARRAY)
         coords = dataset._variables