Horizontal scaling while preserving the italic angle. (by "unskewing" the geometry to "upright", then scaling, then "reskewing" back to the italic angle)

obliqueScale.py

from fontTools.pens.transformPen import TransformPen
from fontTools.pens.pointPen import AbstractPointPen
from fontTools.misc.transform import Transform
import math

from defcon.objects.glyph import Glyph

class TransformPen2(TransformPen):
    # because we don't want to change the transform matrix of the actual components
    def addComponent(self, glyphName, transformation):
        self._outPen.addComponent(glyphName, transformation)
    
class CenterPen(AbstractPointPen):
    def __init__(self):
        self.points = []
    def beginPath(self):
        pass

    def endPath(self):
        """End the current sub path."""
        pass
        
    def addPoint(self, pt, segmentType=None, smooth=False, name=None, **kwargs):
        """Add a point to the current sub path."""
        if segmentType == None: return
        self.points.append(pt)

    def addComponent(self, baseGlyphName, transformation):
        """Add a sub glyph."""
        pass
    
    def getCenter(self):
        if not self.points:
            return 0,0
        dx = sum([x for x, y in self.points])/len(self.points)
        dy = sum([y for x, y in self.points])/len(self.points)
        return dx, dy


def italicScale(factor, src, dst, angle):
    # apply a horizontal scale while preserving the angle
    cp = CenterPen()
    src.drawPoints(cp)
    dx, dy = cp.getCenter()
    # destinationGlyph pen
    s = dst.getPen()
    # deskew
    m = Transform()
    m = m.translate(-dx, -dy)
    m = m.skew(-math.radians(angle))
    m = m.scale(factor, 1)
    m = m.skew(math.radians(angle))
    m = m.translate((1/factor)*dx, dy)
    tp = TransformPen2(s,m)
    src.draw(tp)
    src.width *= factor

f = CurrentFont()
g = CurrentGlyph()

# horizontal scale factor
factor = 0.56

background = g.getLayer('background')
background.clear()

foreground = g.getLayer("foreground")

italicScale(factor, foreground, background, f.info.italicAngle)