Eliminate mutable default parameters from methods

[kurtmckee]

Fixed

• Eliminate mutable default parameters from methods.

  Because the parameters are passed to observers, it is possible for an observer to accidentally -- and permanently -- modify a default argument.

  observers = [lambda x: x.append("bad")]
  
  def demo(groups=[]):
      print(groups)
      for observer in observers:
          observer(groups)
  
  demo()  # []
  demo()  # ["bad"]
  demo()  # ["bad", "bad"]

[coveralls]

coverage: 64.748% (-0.2%) from 64.919%
when pulling 0224bcc on kurtmckee:rm-mutable-parameter-defaults
into 0186f21 on LudovicRousseau:master.

[LudovicRousseau]

Thanks