Allow for custom attributes

CHANGES.md

@@ -2,15 +2,34 @@
 
 ### Enhancements
 
+* The configuration setting  `attrs` can now be used to define dynamically 
+  computed dataset attributes using the syntax `{{ expression }}`. [#60]
+
+  Example:
+  ```yaml
+  permit_eval: true
+  attrs:
+    title: HROC Ocean Colour Monthly Composite
+    time_coverage_start: {{ lower_bound(ds.time) }}
+    time_coverage_end: {{ upper_bound(ds.time) }}
+  ```
+
+* Introduced new configuration setting `attrs_update_mode` that controls 
+  how dataset attributes are updated. [#59]
+
 * Simplified logging to console. You can now set configuration setting `logging`
   to a log level which will implicitly enable console logging with given log
   level. [#64]
 
 * Added a section in the notebook `examples/zappend-demo.ipynb`
   that demonstrates transaction rollbacks.
 
+
 * Added CLI option `--traceback`. [#57]
 
+* Added a section in the notebook `examples/zappend-demo.ipynb`
+  that demonstrates transaction rollbacks.
+
 
 ## Version 0.4.1 (from 2024-02-13)
 

docs/config.md

@@ -143,6 +143,36 @@ Variable metadata.
     Type _object_.
     Arbitrary variable metadata attributes.
 
+## `attrs`
+
+Type _object_.
+Arbitrary dataset attributes. If `permit_eval` is set to `true`, string values may include Python expressions enclosed in `{{` and `}}` to dynamically compute attribute values; in the expression, the current dataset  is named `ds`. Refer to the user guide for more information.
+
+## `attrs_update_mode`
+
+The mode used update target attributes from slice attributes. Independently of this setting, extra attributes configured by the `attrs` setting will finally be used to update the resulting target attributes.
+Must be one of the following:
+
+  * Use attributes from first slice dataset and keep them.
+    Its value is `"keep"`.
+
+  * Replace existing attributes by attributes of last slice dataset.
+    Its value is `"replace"`.
+
+  * Update existing attributes by attributes of last slice dataset.
+    Its value is `"update"`.
+
+  * Ignore attributes from slice datasets.
+    Its value is `"ignore"`.
+
+Defaults to `"keep"`.
+
+## `permit_eval`
+
+Type _boolean_.
+Allow for dynamically computed values in dataset attributes `attrs` using the syntax `{{ expression }}`.  Executing arbitrary Python expressions is a security risk, therefore this must be explicitly enabled. Refer to the user guide for more information.
+Defaults to `false`.
+
 ## `target_dir`
 
 Type _string_.

docs/guide.md

@@ -84,7 +84,9 @@ This remainder of this guide explains the how to use the various `zappend`
     variables. A variable comprises the actual data array as well as metadata describing 
     the data dimensions, units, and encoding, such as chunking and compression.
 
-## Dataset Outline
+## Dataset Metadata
+
+### Outline
 
 If no further configuration is supplied, then the target dataset's outline and data 
 encoding is fully prescribed by the first slice dataset provided. By default, the
@@ -152,6 +154,85 @@ Often, it is easier to specify which variables should be excluded:
     "excluded_variables": ["GridCellId"]
 }
 ```
+### Attributes
+
+The target dataset should exploit information about itself using global 
+metadata attributes.
+There are three choices to update the global attributes of the target 
+dataset from slices. The configuration setting `attrs_update_mode` 
+controls how this is done:
+
+* `"keep"` - use attributes from first slice dataset and keep them (default);
+* `"replace"` - replace existing attributes by attributes of last slice dataset;
+* `"update"` - update existing attributes by attributes of last slice dataset;
+* `"ignore"` - ignore attributes from slice datasets.
+
+Extra attributes can be added using the optional configuration setting `attrs`:
+
+```json
+{
+    "attrs_update_mode": "keep",
+    "attrs": {
+        "Conventions": "CF-1.10",
+        "title": "SMOS Level 2C Soil Moisture 2-Days Composite"
+    }
+}
+```
+
+Independently of the `attrs_update_mode` setting, extra attributes configured 
+by the `attrs` setting will always be used to update the resulting target 
+attributes. 
+
+Attribute values in the `attrs` setting may also be computed dynamically using 
+the syntax `{{ expression }}`, where `expression` is an arbitrary Python 
+expression. For this to work, the setting `permit_eval` must be explicitly 
+set for security reasons:
+
+```json
+{
+    "permit_eval": true,
+    "attrs_update_mode": "keep",
+    "attrs": {
+        "time_coverage_start": "{{ ds.time[0] }}", 
+        "time_coverage_end": "{{ ds.time[-1] }}"
+    }
+}
+```
+
+Currently, the only variable accessible from expressions is `ds` which is 
+a reference to the current state of the target dataset after the last slice 
+append. It is of type 
+[xarray.Dataset](https://docs.xarray.dev/en/stable/generated/xarray.Dataset.html).
+
+!!! danger "Evil eval()"
+    The expressions in `{{ expression }}` are evaluated using the Python 
+    [eval() function](https://docs.python.org/3/library/functions.html#eval).
+    This can pose a threat to your application and environment. 
+    Although `zappend` does not allow you to directly access Python built-in 
+    functions via expressions, it should be used judiciously and with extreme 
+    caution if used as part of a web service where configuration is injected 
+    from the outside of your network.
+
+The following utility functions can be used as well and are handy if you need 
+to store the upper and lower bounds of coordinates as attribute values:
+
+* `lower_bound(array, ref: "lower"|"upper"|"center" = "lower")`:
+  Return the lower bound of a one-dimensional (coordinate) array `array`.
+* `upper_bound(array, ref: "lower"|"upper"|"center" = "lower")`:
+  Return the upper bound of a one-dimensional (coordinate) array `array`.
+
+The `ref` value specifies the reference within an array element that is used 
+as a basis for the boundary computation. E.g., if coordinate labels refer to 
+array element centers, pass `ref="center"`.
+
+```json
+{
+    "attrs": {
+        "time_coverage_start": "{{ lower_bound(ds.time, 'center') }}", 
+        "time_coverage_end": "{{ upper_bound(ds.time, 'center') }}"
+    }
+}
+```
 
 ## Variable Metadata