add unstack method (#350)

* add unstack method

* Update test/test_linear_expression.py

Author: FabianHofmann

doc/release_notes.rst

@@ -6,6 +6,7 @@ Upcoming Version
 
 * The internal handling of `Solution` objects was improved for more consistency. Solution objects created from solver calls now preserve the exact index names from the input file.
 * Multiplication of a linear expression by a constant value may now introduce new dimensions.
+* Added method `unstack` to `LinearExpression`, `Variable` and `Constraint` to unstack a dimension.
 
 Version 0.4.4
 --------------

linopy/constraints.py

@@ -675,6 +675,8 @@ def to_polars(self):
 
     stack = conwrap(Dataset.stack)
 
+    unstack = conwrap(Dataset.unstack)
+
     iterate_slices = iterate_slices
 
 

linopy/expressions.py

@@ -1472,6 +1472,8 @@ def to_polars(self) -> pl.DataFrame:
 
     stack = exprwrap(Dataset.stack)
 
+    unstack = exprwrap(Dataset.unstack)
+
     iterate_slices = iterate_slices
 
 

linopy/variables.py

@@ -1095,6 +1095,8 @@ def equals(self, other: Variable) -> bool:
 
     stack = varwrap(Dataset.stack)
 
+    unstack = varwrap(Dataset.unstack)
+
     iterate_slices = iterate_slices
 
 

test/test_constraint.py

@@ -197,7 +197,7 @@ def test_constraint_wrapped_methods(x, y):
     con.rename({"first": "new_labels"})
     con.rename_dims({"first": "new_labels"})
     con.roll({"first": 1})
-    con.stack(new_dim=("first", "second"))
+    con.stack(new_dim=("first", "second")).unstack("new_dim")
 
 
 def test_anonymous_constraint_sel(x, y):

test/test_linear_expression.py

@@ -669,6 +669,13 @@ def test_variable_stack(v):
     assert result.coord_dims == ("new",)
 
 
+def test_linear_expression_unstack(v):
+    result = v.to_linexpr().expand_dims("new_dim").stack(new=("new_dim", "dim_2"))
+    result = result.unstack("new")
+    assert isinstance(result, LinearExpression)
+    assert result.coord_dims == ("new_dim", "dim_2")
+
+
 def test_linear_expression_diff(v):
     diff = v.to_linexpr().diff("dim_2")
     assert diff.nterm == 2

test/test_variable.py

@@ -267,6 +267,12 @@ def test_variable_stack(x):
     assert result.dims == ("new",)
 
 
+def test_variable_unstack(x):
+    result = x.expand_dims("new_dim").stack(new=("new_dim", "first")).unstack("new")
+    assert isinstance(result, linopy.variables.Variable)
+    assert result.dims == ("new_dim", "first")
+
+
 def test_variable_flat(x):
     result = x.flat
     assert isinstance(result, pd.DataFrame)