PyQGIS gives TypeError: QgsExpression.prepare(): argument 1 has unexpected type 'QgsFields'?

Question

This question is very related to the unanswered Retrieve features from QgsFieldExpressionWidget pyqgis

I have a layer and I've build a QgsExpression that involves a field of that layer. What I'm trying to do is to evaluate the expression and get the results as a single list.

The problem is that I'm trying to do this with QGIS master (so the future 3 version) and the instruction of the PyQGIS cookbook are outdated because some API are changed.

Just to have an example:

vl = iface.activeLayer()
exp = QgsExpression('PH > 7') # where PH is the name of a field
exp.prepare(vl.pendingFields())


# then I get this error

Traceback (most recent call last):
  File "/usr/lib/python3.5/code.py", line 91, in runcode
    exec(code, self.locals)
  File "<input>", line 1, in <module>
TypeError: QgsExpression.prepare(): argument 1 has unexpected type 'QgsFields'

Someone has already successfully used the QgsExpression for QGIS master?

Answer 1

I used your code in my QGIS Python Console (with a shapefile with 'PH' field; only one value > 7) and I got a result (see next image).

So, I printed 'vl.pendingFields()' to find out what kind of object is:

>>>vl.pendingFields()
<qgis._core.QgsFields object at 0x17E10E40>

and afterward, a help for QgsExpression.prepare for corroborating wich parameters it uses:

help(QgsExpression.prepare)
Help on built-in function prepare:

prepare(...)
    QgsExpression.prepare(QgsFields) -> bool
    QgsExpression.prepare(QgsExpressionContext) -> bool

In my case, used parameter is correct and the result was as expected. So, I suggest that you use 'help' command with your QGIS future 3 version to find out the new kind of parameters to use.

Answer 2

In QGIS 3.0, you may need to use a combination of the QgsExpressionContext and QgsExpressionContextScope classes:

vl = iface.activeLayer()
features = [feat for feat in vl.getFeatures()]

context = QgsExpressionContext()
scope = QgsExpressionContextScope()
context.appendScope(scope)
listOfResults = []

for feat in features:
    scope.setFeature(feat)
    exp = QgsExpression('PH > 7')
    listOfResults.append(exp.evaluate(context))

The output is boolean, either 0 (false) or 1 (true), and stored in a list:

>>> [0, 0, 0, 1, 1, 1]

Answer 3

    for name, layer in layers.iteritems():
        if(layer.name()=="[layer name]"):

            lay=layer

    expr = QgsExpression("\"REFCAT\"=22162C50309009")

    expr = QgsExpression(expresion)
    selection= lay.getFeatures(QgsFeatureRequest(expr))
    ids = [s.id() for s in selection]
    lay.setSelectedFeatures(ids)
    iface.legendInterface().setCurrentLayer(lay)
    iface.mapCanvas().zoomToSelected()

Is a code used by me, try changing the format of the expression with double quotation marks.