Evaluate Python expression in json_map reader

pynxtools/dataconverter/readers/json_map/README.md

@@ -33,7 +33,7 @@ This file is designed to let you fill in the requirements of a NeXus Application
 The mapping files will always be based on the Template the dataconverter generates. See above on how to generate a mapping file.
 The right hand side values of the Template keys are what you can modify.
 
-Here are the three different ways you can fill the right hand side of the Template keys:
+Here are the different ways you can fill the right hand side of the Template keys:
 * Write the nested path in your datafile. This is indicated by a leading `/` before the word `entry` to make `/entry/data/current_295C` below. 
 Example:
 
@@ -97,6 +97,25 @@ The `datetime`, `dateutil` and `timestamp` properties are mutually exclusive.
 The resulting string replaces the mapped value (dictionary) in the mapping dictionary.
 If date parsing is enabled, the resulting string is ISO-formatted as required by the Nexus standard.
 
+* Python expression.
+The following entry creates an axis array from scalar values in the input file.
+
+```json
+  "/ENTRY[entry]/DATA[image]/angular0": {
+    "eval": "np.linspace(arg0[0], arg1[0], int(arg2[0]))",
+    "arg0": "/scan1/attrs/ScientaSliceBegin",
+    "arg1": "/scan1/attrs/ScientaSliceEnd",
+    "arg2": "/scan1/attrs/ScientaNumSlices"
+  },
+```
+
+The properties of the mapping declare the expression and its arguments.
+
+    "eval": (required) Python expression to be evaluated by the `eval` built-in.
+        The expression can use the built-in and numpy (as np) namespaces
+        as well as the datasets declared by the `argXxx` values.
+    "argXxx", where Xxx is an integer number: (optional) path of dataset to read from the input data
+        and to be used in the expression under the same name.
 
 ## Contact person in FAIRmat for this reader
 Sherjeel Shabih

pynxtools/dataconverter/readers/json_map/reader.py

@@ -238,6 +238,47 @@ def parse_strings(mapping, data):
         mapping[key] = value
 
 
+def eval_expressions(mapping, data):
+    """
+    Evaluate Python expressions in mapping.
+
+    If a mapping entry contains a dictionary with a `eval` key,
+    the `eval` expression is evaluated using the Python built-in `eval`.
+    The expression can use built-in functions, numpy functions in namespace `np`,
+    and argXxx variables that are defined in the mapping and can refer to dataset paths.
+
+    The result of the expression replaces the value of the mapping.
+
+    :param mapping: Mapping dictionary
+    :param data: Data dictionary
+    :return: None
+    """
+
+    for key in mapping:
+        eval_args = mapping[key]
+
+        try:
+            expression = eval_args["eval"]
+        except (KeyError, TypeError):
+            continue
+
+        args = {}
+        for arg, value in eval_args.items():
+            if arg[0:3] == "arg":
+                if is_path(value):
+                    value = get_val_nested_keystring_from_dict(value[1:], data)
+                else:
+                    try:
+                        value = float(value)
+                    except TypeError:
+                        pass
+
+                args[arg] = value
+
+        value = eval(expression, {"np": np}, args)
+        mapping[key] = value
+
+
 class JsonMapReader(BaseReader):
     """A reader that takes a mapping json file and a data file/object to return a template."""
 
@@ -304,6 +345,7 @@ def read(
 
         new_template = Template()
         parse_strings(mapping, data)
+        eval_expressions(mapping, data)
         convert_shapes_to_slice_objects(mapping)
 
         fill_documented(new_template, mapping, template, data)