Compare between ArcGIS and Proj.4 when converting between WGS84 and Mass State Plane.

compare-projection.py

import pyproj
import arcpy

# ArcMap result (meters):
# 306,414.514  854,124.573

x = -70.216908
y = 41.930268

# perform projection using pyproj (PROJ.4)
p1 = pyproj.Proj(init='epsg:4326')   # geographic WGS84
p2 = pyproj.Proj(init='epsg:26986') # Mass State Plane
p_x,p_y = pyproj.transform(p1,p2,x,y)

# perform projection using Arcpy (PE)
sr_wgs84 = arcpy.SpatialReference(4326)
sr_msp = arcpy.SpatialReference(26986)
point = arcpy.Point(x, y)
point_geom = arcpy.PointGeometry(point, sr_wgs84)
proj_point = point_geom.projectAs(sr_msp)
a_x, a_y = (proj_point.firstPoint.X, proj_point.firstPoint.Y)

print """pyproj:\t\tarcpy:
{0}\t{1}
{2}\t{3}""".format(p_x, a_x, p_y, a_y)

$ python compare-projection.py
pyproj:         arcpy:
306414.524181   306414.524181
854124.578684   854124.578684