output.ToCSV updates context when data has a method_update_context(co…

…ntext). Add ToCSV tests.

lena/output/to_csv.py

@@ -200,6 +200,8 @@ def run(self, flow):
         """Convert values from *flow* to CSV text.
 
         *context.output* is updated with {"filetype": "csv"}.
+        If a data structure has a method *\\_update_context(context)*,
+        it also updates the current context during the transform.
         All not converted data is yielded unchanged.
 
         If *context.output.to_csv* is ``False``, the value is skipped.
@@ -209,11 +211,6 @@ def run(self, flow):
         use :func:`hist1d_to_csv`, :func:`hist2d_to_csv`
         or :func:`iterable_to_table`.
         """
-        def is_writable_hist(val):
-            """Test whether a value from flow can be converted to CSV."""
-            data, context = lena.flow.get_data_context(val)
-            return isinstance(data, lena.structures.histogram)
-
         for val in flow:
             data, context = lena.flow.get_data_context(val)
 
@@ -271,6 +268,9 @@ def is_writable_hist(val):
                     data, row_separator=self._separator, header=self._header
                 )
                 csv = "\n".join(rows)
+                if (hasattr(data, "_update_context") and
+                        callable(data._update_context)):
+                    data._update_context(context)
                 lena.context.update_recursively(context, "output.filetype.csv")
                 yield (csv, context)
                 continue

lena/structures/graph.py

@@ -324,8 +324,10 @@ def _update_context(self, context):
         Existing values are not removed
         from *context.value* and its subcontexts.
 
-        Called on destruction of the graph (for example,
-        in :class:`.ToCSV`).
+        Called on "destruction" of the graph (for example,
+        in :class:`.ToCSV`). By destruction we mean conversion
+        to another structure (like text) in the flow.
+        The graph object is not really destroyed in this process.
         """
         # this method is private, because we encourage users to yield
         # graphs into the flow and process them with ToCSV element

tests/output/test_to_csv.py

@@ -19,20 +19,36 @@ def test_iterable_to_table():
 
 def test_to_csv():
     hist = histogram(edges=[0, 1, 2], bins=[1, 2])
-    gr = graph([[0, 1], [2.5, 3]])
+    gr0 = graph([[0, 1], [2.5, 3]])
     to_csv = ToCSV(separator=",", header=None, duplicate_last_bin=True)
 
     ## histogram and iterables work, other values passed unchanged
     # no context works fine
-    res1 = list(to_csv.run([hist, gr, 3]))
+    res0 = list(to_csv.run([hist, gr0, 3, "a string"]))
     # data parts are correct
-    data_1 = [res1[0][0], res1[1][0], res1[2]]
+    data_0 = [res0[0][0], res0[1][0]]
     # assert list(to_csv.run([hist, gr, 3])) == []
-    assert data_1 == [
+    assert data_0 == [
         '0.000000,1.000000\n1.000000,2.000000\n2.000000,2.000000',
         '0,2.5\n1,3',
-        3
     ]
+    assert res0[2:] == [3, "a string"]
+
+    # no error fields
+    assert res0[1][1] == {'output': {'filetype': 'csv'}}
+
+    # data._update_context is called
+    gr1 = graph([[0, 1], [2.5, 3], [0.1, 0.1]], field_names="x,y,error_y")
+    res1 = list(to_csv.run([gr1]))[0]
+    assert res1[1] == {
+        'output': {'filetype': 'csv'},
+        'error': {'y': {'index': 2}}
+    }
+
+    # context.to_csv.False forbids the transform
+    no_to_csv_context = {"output": {"to_csv": False}}
+    res2 = list(to_csv.run([(gr0, no_to_csv_context)]))[0]
+    assert res2 == (gr0, no_to_csv_context)
 
 
 def test_hist_to_csv():