Converting string to float using QgsExpression()

Question

I'm trying to convert a string column Rainfall of point layer weather_l to float:

context = QgsExpressionContext()
context.appendScopes(QgsExpressionContextUtils.globalProjectLayerScopes(weather_l))

num_float = QgsExpression('float(fea["Rainfall"])')

with edit(weather_l):
    for fea in weather_l.getFeatures():
        context.setFeature(fea)
        if not fea['Rainfall'].isalpha():
            fea['Rain_D'] = num_float.evaluate(context)
        weather_l.updateFeature(fea)

but num_float.evaluate(context) returns None. How can I correct

num_float = QgsExpression('float(fea["Rainfall"])')

to convert the string to a float?

Answer 1

Your expression is mixing Python and Qgis function syntax.

Try the following expression syntax:

QgsExpression('to_real("Rainfall")')

You need to pass only the field name in double quotes to the to_real function.