The coiler strikes again

just-say-no.py

#!/usr/bin/env python2.7

def test(x=[]):
  x.append(2)
  return x

print (test())  # returns [2]
print (test())  # returns [2, 2]
print (test())  # returns [2, 2, 2]

# The reason, emphasis mine:
#
# https://docs.python.org/2.0/ref/function.html
#
# ***Default parameter values are evaluated when the function definition is executed.
# This means that the expression is evaluated once, when the function is defined, and
# that that same ``pre-computed'' value is used for each call.*** This is especially
# important to understand when a default parameter is a mutable object, such as a
# list or a dictionary: if the function modifies the object (e.g. by appending an
# item to a list), the default value is in effect modified.  ***This is generally not
# what was intended.***  A way around this is to use None as the default, and
# explicitly test for it in the body of the function ...