What's the batched Refactor Fields tool "field mapping" syntax in QGIS?

Question

In QGIS, there is this nice tool "Refactor Fields" in the Processing Toolbox :

Using it on a single layer is fairly straightforward.

However, when you plan on using it in a batch mode, one can see the "Fields mapping" textbox. I'm looking for an example of the syntax to be used there. I haven't found any specific documentation yet.

Answer 1

It is not very comfortable to do, but the following is the required syntax for that textbox (I write it in several lines only for the sake of clearness):

{'expression': u'type the expression here',
 'length': <type the length>,
 'type': <code of the type>,
 'name': u'type the field name here',
 'precision': <type the precision>}

For example (remeber to use curly brackets):

{'expression': u'"FLAG1"+"FLAG2"',
 'length': 20, 'type': 6,
 'name': u'FLAG1',
 'precision': 5}

You need to write as many lines like the one above as the number of input fields, separated by commas. For example, if your layer contains three fields:

{'expression': u'"FLAG1"+"FLAG2"', 'length': 20, 'type': 6, 'name': u'FLAG1', 'precision': 5}, {'expression': u'100*"FLAG2"', 'length': 20, 'type': 6, 'name': u'FLAG2', 'precision': 5}, {'expression': u'"100-"FLAG2"', 'length': 20, 'type': 6, 'name': u'FLAG3', 'precision': 5}

and it should work (it worked for me).

Please note that you may avoid to use this dictionary-oriented syntax and directly use the required values. For example, with reference to the first example, you may simply use:

{u'"FLAG1"+"FLAG2"', 20, 6, u'FLAG1', 5}

If you don't use this syntax, the following error will be shown:

Missing parameter value: Fields mapping (row 1)

Answer 2

There is another algorithm similar to this whose interface simplifies the batch process better.