Allow sinks/sources to have multiple inputs/outputs

There was a warning and a comment that having multiple inputs/outputs
for one Sink/Source is unintended but should be possible.
I think we can remove that "strong reccomendation" as it just works.

To not let beginners fall into the trap of having a dangling Sink
or Source, like the old warning also might have prevented, inputs and
outputs are now explicitly named arguments without a default.

src/oemof/solph/components/_sink.py

@@ -13,10 +13,7 @@
 SPDX-License-Identifier: MIT
 
 """
-from warnings import warn
-
 from oemof.network import Node
-from oemof.tools import debugging
 
 
 class Sink(Node):
@@ -27,6 +24,9 @@ class Sink(Node):
     label : str
         String holding the label of the Sink object.
         The label of each object must be unique.
+    inputs: dict
+        A dictionary mapping input nodes to corresponding inflows
+        (i.e. input values).
 
     Examples
     --------
@@ -39,30 +39,14 @@ class Sink(Node):
     ...    label='el_export',
     ...    inputs={bel: solph.flows.Flow()})
 
-
-    Notes
-    -----
-    It is theoretically possible to use the Sink object with multiple inputs.
-    However, we strongly recommend using multiple Sink objects instead.
     """
 
-    def __init__(self, label=None, inputs=None, custom_attributes=None):
+    def __init__(self, label=None, *, inputs, custom_attributes=None):
         if inputs is None:
             inputs = {}
         if custom_attributes is None:
             custom_attributes = {}
 
-        if len(inputs) != 1:
-            msg = (
-                "A Sink is designed to have one input but you provided {0}."
-                " If this is intended and you know what you are doing you can "
-                "disable the SuspiciousUsageWarning globally."
-            )
-            warn(
-                msg.format(len(inputs)),
-                debugging.SuspiciousUsageWarning,
-            )
-
         super().__init__(
             label=label, inputs=inputs, custom_properties=custom_attributes
         )

src/oemof/solph/components/_source.py

@@ -13,11 +13,7 @@
 SPDX-License-Identifier: MIT
 
 """
-
-from warnings import warn
-
 from oemof.network import Node
-from oemof.tools import debugging
 
 
 class Source(Node):
@@ -28,6 +24,9 @@ class Source(Node):
     label : str
         String holding the label of the Source object.
         The label of each object must be unique.
+    outputs: dict
+        A dictionary mapping input nodes to corresponding outflows
+        (i.e. output values).
 
     Examples
     --------
@@ -48,31 +47,14 @@ class Source(Node):
 
     >>> str(pv_plant.outputs[bel].output)
     'electricity'
-
-    Notes
-    -----
-    It is theoretically possible to use the Source object with multiple
-    outputs. However, we strongly recommend using multiple Source objects
-    instead.
     """
 
-    def __init__(self, label=None, outputs=None, custom_attributes=None):
+    def __init__(self, label=None, *, outputs, custom_attributes=None):
         if outputs is None:
             outputs = {}
         if custom_attributes is None:
             custom_attributes = {}
 
-        if len(outputs) != 1:
-            msg = (
-                "A Source is designed to have one output but you provided {0}."
-                " If this is intended and you know what you are doing you can "
-                "disable the SuspiciousUsageWarning globally."
-            )
-            warn(
-                msg.format(len(outputs)),
-                debugging.SuspiciousUsageWarning,
-            )
-
         super().__init__(
             label=label, outputs=outputs, custom_properties=custom_attributes
         )

tests/test_warnings.py

@@ -28,50 +28,6 @@ def warning_fixture():
     warnings.simplefilter(action="ignore", category=FutureWarning)
 
 
-def test_that_the_sink_errors_actually_get_raised(warning_fixture):
-    """Sink doesn't warn about potentially erroneous usage."""
-    look_out = solph.Bus()
-    with pytest.raises(
-        TypeError, match="got an unexpected keyword argument 'outputs'"
-    ):
-        solph.components.Sink(label="test_sink", outputs={look_out: "A typo!"})
-
-    msg = (
-        "A Sink is designed to have one input but you provided 0."
-        " If this is intended and you know what you are doing you can "
-        "disable the SuspiciousUsageWarning globally."
-    )
-    with warnings.catch_warnings(record=True) as w:
-        solph.components.Sink(
-            label="no input",
-        )
-        assert len(w) == 1
-        assert msg in str(w[-1].message)
-
-
-def test_that_the_source_warnings_actually_get_raised(warning_fixture):
-    """Source doesn't warn about potentially erroneous usage."""
-    look_out = solph.Bus()
-    with pytest.raises(
-        TypeError, match="got an unexpected keyword argument 'inputs'"
-    ):
-        solph.components.Source(
-            label="test_source", inputs={look_out: "A typo!"}
-        )
-
-    msg = (
-        "A Source is designed to have one output but you provided 0."
-        " If this is intended and you know what you are doing you can "
-        "disable the SuspiciousUsageWarning globally."
-    )
-    with warnings.catch_warnings(record=True) as w:
-        solph.components.Source(
-            label="no output",
-        )
-        assert len(w) == 1
-        assert msg in str(w[-1].message)
-
-
 def test_that_the_converter_warnings_actually_get_raised(warning_fixture):
     """Converter doesn't warn about potentially erroneous usage."""
     look_out = solph.Bus()