svgPathPen: add option to customize number formatting (e.g. rounding)

Using a RoundingPen as a filter doesn't work for TrueType quadratic splines (qCurveTo), because the rounding would occur too early, before these get split into atomic quadratic Bezier segments (by the superclass BasePen.qCurveTo method), thus leaving unwanted floating-point coordinates in the SVG output.
So here we add a 'ntos' parameter (by default simpli calls str()) that takes a Callable[[float], str] and can be used to customize the formatting of the numbers in SVG path coordinates.

Lib/fontTools/pens/svgPathPen.py

@@ -1,18 +1,36 @@
+from typing import Callable
 from fontTools.pens.basePen import BasePen
 
 
-def pointToString(pt):
-    return " ".join([str(i) for i in pt])
+def pointToString(pt, ntos=str):
+    return " ".join(ntos(i) for i in pt)
 
 
 class SVGPathPen(BasePen):
+    """ Pen to draw SVG path d commands.
 
-    def __init__(self, glyphSet):
+    Example::
+        >>> pen = SVGPathPen(None)
+        >>> pen.moveTo((0, 0))
+        >>> pen.lineTo((1, 1))
+        >>> pen.curveTo((2, 2), (3, 3), (4, 4))
+        >>> pen.closePath()
+        >>> pen.getCommands()
+        'M0 0 1 1C2 2 3 3 4 4Z'
+
+    Args:
+        glyphSet: a dictionary of drawable glyph objects keyed by name
+            used to resolve component references in composite glyphs.
+        ntos: a callable that takes a number and returns a string, to
+            customize how numbers are formatted (default: str).
+    """
+    def __init__(self, glyphSet, ntos: Callable[[float], str] = str):
         BasePen.__init__(self, glyphSet)
         self._commands = []
         self._lastCommand = None
         self._lastX = None
         self._lastY = None
+        self._ntos = ntos
 
     def _handleAnchor(self):
         """
@@ -43,7 +61,7 @@ def _moveTo(self, pt):
         ['M0 10']
         """
         self._handleAnchor()
-        t = "M%s" % (pointToString(pt))
+        t = "M%s" % (pointToString(pt, self._ntos))
         self._commands.append(t)
         self._lastCommand = "M"
         self._lastX, self._lastY = pt
@@ -99,11 +117,11 @@ def _lineTo(self, pt):
         # previous was a moveto
         elif self._lastCommand == "M":
             cmd = None
-            pts = " " + pointToString(pt)
+            pts = " " + pointToString(pt, self._ntos)
         # basic
         else:
             cmd = "L"
-            pts = pointToString(pt)
+            pts = pointToString(pt, self._ntos)
         # write the string
         t = ""
         if cmd:
@@ -122,9 +140,9 @@ def _curveToOne(self, pt1, pt2, pt3):
         ['C10 20 30 40 50 60']
         """
         t = "C"
-        t += pointToString(pt1) + " "
-        t += pointToString(pt2) + " "
-        t += pointToString(pt3)
+        t += pointToString(pt1, self._ntos) + " "
+        t += pointToString(pt2, self._ntos) + " "
+        t += pointToString(pt3, self._ntos)
         self._commands.append(t)
         self._lastCommand = "C"
         self._lastX, self._lastY = pt3
@@ -135,11 +153,16 @@ def _qCurveToOne(self, pt1, pt2):
         >>> pen.qCurveTo((10, 20), (30, 40))
         >>> pen._commands
         ['Q10 20 30 40']
+        >>> from fontTools.misc.roundTools import otRound
+        >>> pen = SVGPathPen(None, ntos=lambda v: str(otRound(v)))
+        >>> pen.qCurveTo((3, 3), (7, 5), (11, 4))
+        >>> pen._commands
+        ['Q3 3 5 4', 'Q7 5 11 4']
         """
         assert pt2 is not None
         t = "Q"
-        t += pointToString(pt1) + " "
-        t += pointToString(pt2)
+        t += pointToString(pt1, self._ntos) + " "
+        t += pointToString(pt2, self._ntos)
         self._commands.append(t)
         self._lastCommand = "Q"
         self._lastX, self._lastY = pt2