default_disaster.py

#!/usr/bin/python
from __future__ import print_function

def disaster_function(a_default=[]):
  # BEWARE: the next line, if a_default is not provided, modifies the default
  # object, and **not a copy** of the default object! This is because you are
  # modifying in place a "name" pointing to the default, by effectively changing
  # the default value for all future function calls
  a_default.append("a string")
  print("disaster_function(): %s" % a_default)

def prevent_disaster(a_default=None):
  # Prevent the disaster by setting defaults to None, and using it as a
  # placeholder to trigger the creation to a unique, per-function call copy of
  # the intended default
  if a_default is None:
    a_default = []
  a_default.append("a string")
  print("prevent_disaster(): %s" % a_default)

disaster_function()  # expect: ['a string']
disaster_function()  # naively expecting: ['a string'], getting ['a string', 'a string']
prevent_disaster()   # expect: ['a string']
prevent_disaster()   # getting as naively expected: ['a string']

@mconcas devil's in the detail