import itertools
test = 'a1.jpg, a2.jpg, a3.jpg, b1.jpg, b2.jpg, b3.jpg, b4.jpg'
l = test.split(', ')
result = [list(g) for k, g in itertools.groupby(l, key=lambda x:x[0])]
print(result)
aW1wb3J0IGl0ZXJ0b29scwoKdGVzdCA9ICdhMS5qcGcsIGEyLmpwZywgYTMuanBnLCBiMS5qcGcsIGIyLmpwZywgYjMuanBnLCBiNC5qcGcnCmwgPSB0ZXN0LnNwbGl0KCcsICcpCgpyZXN1bHQgPSBbbGlzdChnKSBmb3IgaywgZyBpbiBpdGVydG9vbHMuZ3JvdXBieShsLCBrZXk9bGFtYmRhIHg6eFswXSldCnByaW50KHJlc3VsdCk=