'when' tag support in Python API

docs/tutorials/conditional_example.wic

@@ -0,0 +1,10 @@
+steps:
+  toString:
+    in:
+      input: !ii 27
+    out:
+    - output: !& string_int
+  echo:
+    when: '$(inputs.message < "27")'
+    in:
+      message: !* string_int

examples/scripts/when_pyapi.py

@@ -0,0 +1,31 @@
+from wic.api.pythonapi import Step, Workflow
+
+
+def workflow() -> Workflow:
+    # conditional on input
+    # step toString
+    toString = Step(clt_path='../../cwl_adapters/toString.cwl')
+    toString.input = 27
+    # step echo
+    echo = Step(clt_path='../../cwl_adapters/echo.cwl')
+    echo.message = toString.output
+    # add a when clause
+    # alternate js syntax
+    # echo.when = '$(inputs["message"] < 27)'
+    echo.when = '$(inputs.message < 27)'
+    # since the condition is not met the echo step is skipped!
+
+    # arrange steps
+    steps = [toString, echo]
+
+    # create workflow
+    filename = 'when_pyapi_py'  # .yml
+    wkflw = Workflow(steps, filename)
+    return wkflw
+
+
+# Do NOT .run() here
+
+if __name__ == '__main__':
+    when_wic = workflow()
+    when_wic.run()  # .run() here inside main

src/sophios/api/pythonapi.py

@@ -260,6 +260,8 @@ class Step(BaseModel):  # pylint: disable=too-few-public-methods
     # these are not part of 'clt data'
     scatter: list[ProcessInput] = []
     scatterMethod: str = ''
+    # use when tag to enable conditional steps
+    when: str = ''
     _input_names: list[str] = PrivateAttr(default_factory=list)
     _output_names: list[str] = PrivateAttr(default_factory=list)
 
@@ -367,6 +369,11 @@ def __setattr__(self, __name: str, __value: Any) -> Any:
             if not all([isinstance(x, ProcessInput) for x in __value]):
                 raise TypeError("all scatter inputs must be ProcessInput type")
             return super().__setattr__(__name, __value)
+        if __name == "when":
+            if (isinstance(__value, str)) and __value.startswith('$(') and __value.endswith(')'):
+                return super().__setattr__(__name, __value)
+            else:
+                raise ValueError("Invalid input to when.The js string must start with '$(' and end with ')'")
 
         if hasattr(self, "_input_names") and __name in self._input_names:
             set_input_Step_Workflow(self, __name, __value)
@@ -443,6 +450,9 @@ def _yml(self) -> dict:
             if '' == self.scatterMethod:
                 self.scatterMethod = ScatterMethod.dotproduct.value
             d["scatterMethod"] = self.scatterMethod
+        # when operates on step
+        if self.when != '':
+            d["when"] = self.when
         return d
 
     def _save_cwl(self, path: Path) -> None: