qgis QgsExpression doesn't work corretly

Question

I have to do a script that adds a field and changes attributes values utilising an expression so i use QgsExpression. When i put a expression in QgsExpression where there is only 'field name' it works QgsExpression( """ ((Prof_am*Prof_av)/2)*LargTranch*$length """ ), but when i put in the expression a variable it doesn't work ' (ratio_roche)*((Prof_am*Prof_av)/2)*LargTranch*$length '

below is my script after adding new fields:

Features = layer.getFeatures()
for feat in Features:
    if feat[field_Prof_am_index]<feat[field_ProfRocher_index] and feat[field_Prof_av_index]<feat[field_ProfRocher_index]:
        #terrain 100% meuble
        exp_Deb_meuble = QgsExpression( """ ((Prof_am*Prof_av)/2)*LargTranch*$length """ )
        exp_Deb_meuble.prepare(layer.pendingFields())
        feat[field_Deb_meuble_index] = exp_Deb_meuble.evaluate(feat) 
        feat[field_Deb_roche_index] = 0
        layer.updateFeature(feat)
    else:
        #terrain en partie rocheux
        #ratio de la partie rocheuse
        ratio_roche=0.2
        #evaluer quantite de la partie meuble
        exp_Deb_meuble = QgsExpression( ' (ratio_roche)*((Prof_am*Prof_av)/2)*LargTranch*$length ' )
        exp_Deb_meuble.prepare(layer.pendingFields())
        feat[field_Deb_meuble_index] = exp_Deb_meuble.evaluate(feat) 
        #evaluer quantite de la partie rocheuse
        exp_Deb_roche = QgsExpression( ' (1-ratio_roche)*((Prof_am*Prof_av)/2)*LargTranch*$length ' )
        exp_Deb_roche.prepare(layer.pendingFields())
        feat[field_Deb_roche_index] = exp_Deb_roche.evaluate(feat) 
        layer.updateFeature(feat)

Answer 1

Variables are not supported inside the expression. The doc mentions numbers, strings and column name only.

However you can build the expression with the values read from your variable.

ratio_roche=0.2
exp_Deb_meuble = QgsExpression( str(ratio_roche) + ' *((Prof_am*Prof_av)/2)*LargTranch*$length ' )