Selecting features using an expression with PyQGIS

Nowadays (QGIS v3.x)

1. Get the layer reference:

   layer = iface.activeLayer()

2. Select features by expression:

   layer.selectByExpression( "\"ogc_fid\"=482" )

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Before QGIS 2.16

Follow these steps:

1. Get the layer reference:

   cLayer = iface.mapCanvas().currentLayer()

2. Get a featureIterator from an expression:

   expr = QgsExpression( "\"ogc_fid\"=482" )

   it = cLayer.getFeatures( QgsFeatureRequest( expr ) )

3. Build a list of feature Ids from the result obtained in 2.:

   ids = [i.id() for i in it]

4. Select features with the ids obtained in 3.:

   cLayer.setSelectedFeatures( ids )

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NOTE: If you want to set an expression with a string value, you need to add quotation marks to such value, in this way:

expr = QgsExpression( " \"name\" = 'my string' " )

If your string value comes from a variable, you can do this:

myVariable = 'my string'
expr = QgsExpression( " \"name\" = '{}' ".format(myVariable) )

This worked for me on the QGIS Python Console

layer = qgis.utils.iface.activeLayer()
layer .selectByExpression(" \"ogc_fid\" = '{}' ".format(482))

You only need to test it in the GUI interface: "Select by Expression". If it works, you can paste it in your Python code surrounded by double quotes "".

exp = QgsExpression("ogc_fid=482")

If you compare to a string, you can add single quote ''.

exp = QgsExpression("ogc_fid='482'")

It's the same principle in python, it can make the difference between double quote and single quote.