For basic operations, you might need numpy and matplotlib. For more complex 3D operations, consider pymesh, vtk, or mayavi. I've attached a Python script to create and visualize a 3D mesh cube for an easy start.

Heng Wu, I presume the cube is named "Concrete Cube," and the database for the model is called "Model-1." In that case, the following command is expected to be effective for your purpose, having surface named "Outer Surface" :

# Define surface from Mesh

p = mdb.models['Model-1'].parts['Concrete Cube']

s = p.faces

side1Faces = s.getSequenceFromMask(mask=('[#3f ]', ), )

p.Surface(side1Faces=side1Faces, name='Outer Surface')

While the Python script ( Link to demonstration: https://youtu.be/l91wtTDBd6I ) is attached below as well, as the code is as given here:

# Import the necessary libraries

from abaqus import *

from abaqusConstants import *

from caeModules import *

from driverUtils import executeOnCaeStartup

# Define the Cube Dimensions

s = mdb.models['Model-1'].ConstrainedSketch(name='__profile__',

sheetSize=200.0)

g, v, d, c = s.geometry, s.vertices, s.dimensions, s.constraints

s.setPrimaryObject(option=STANDALONE)

s.rectangle(point1=(-25.0, 25.0), point2=(25.0, -25.0))

p = mdb.models['Model-1'].Part(name='Concrete Cube', dimensionality=THREE_D,

type=DEFORMABLE_BODY)

p = mdb.models['Model-1'].parts['Concrete Cube']

p.BaseSolidExtrude(sketch=s, depth=50.0)

s.unsetPrimaryObject()

p = mdb.models['Model-1'].parts['Concrete Cube']

# Assemble Cube

a = mdb.models['Model-1'].rootAssembly

a.DatumCsysByDefault(CARTESIAN)

p = mdb.models['Model-1'].parts['Concrete Cube']

a.Instance(name='Concrete Cube-1', part=p, dependent=ON)

# Mesh the Cube

p = mdb.models['Model-1'].parts['Concrete Cube']

p.seedPart(size=5.0, deviationFactor=0.1, minSizeFactor=0.1)

p = mdb.models['Model-1'].parts['Concrete Cube']

p.generateMesh()

# Define surface from Mesh

p = mdb.models['Model-1'].parts['Concrete Cube']

s = p.faces

side1Faces = s.getSequenceFromMask(mask=('[#3f ]', ), )

p.Surface(side1Faces=side1Faces, name='Outer Surface')

I hope it help :)