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It can be a bit shorter:

def cc2jl(s):
  return "".join(["_"+l if i and l.isupper() and not s[i-1:i+2].isupper() else l for i, l in enumerate(s)]).lower()

It can be a bit shorter:

def cc2jl(s):
  return "".join(["_"+l if i and l.isupper() and not s[i-1:i+2].isupper() else l for i, l in enumerate(s)]).lower()

Regular Expression Alternative:

rx = re.compile(r"(?<=.)(((?<![A-Z])[A-Z])|([A-Z](?=[a-z])))")
def cc2jl(s):
    return rx.sub("_\\1", s).lower()

It can be a bit shorter:

def cc2jl(s):
  return "".join([("_"+l if l.isupper() else l+"_") 
    if l.isupper() != s[i+1 if 0<i<len(s)-1 else i].isupper() else l for i, l in enumerate(s)]).lower()

It can be a bit shorter:

def cc2jl(s):
  return "".join(["_"+l if i and l.isupper() and not s[i-1:i+2].isupper() else l for i, l in enumerate(s)]).lower()

It can be a bit shorter:

def cc2jl(s):
    return "".join(["_"+l if not l.islower() and s[i+1 if 0<i<len(s)-1 else i].islower() else l for i, l in enumerate(s)]).lower()

It can be a bit shorter:

def cc2jl(s):
  return "".join([("_"+l if l.isupper() else l+"_") 
    if l.isupper() != s[i+1 if 0<i<len(s)-1 else i].isupper() else l for i, l in enumerate(s)]).lower()