Default value in a function in Python [duplicate]

Question

I am noticing the following:

class c:
  def __init__(self, data=[]):
    self._data=data
a=c()
b=c()
a._data.append(1)
print b._data
[1]

Is this the correct behavior?

Answer 1

Yes, it's correct behavior.

However, from your question, it appears that it's not what you expected.

If you want it to match your expectations, be aware of the following:

Rule 1. Do not use mutable objects as default values.

def anyFunction( arg=[] ):

Will not create a fresh list object. The default list object for arg will be shared all over the place.

Similarly

def anyFunction( arg={} ):

will not create a fresh dict object. This default dict will be shared.

class MyClass( object ):
    def __init__( self, arg= None ):
        self.myList= [] if arg is None else arg 

That's a common way to provide a default argument value that is a fresh, empty list object.

Answer 2

This is a classic pitfall. See http://zephyrfalcon.org/labs/python_pitfalls.html, section 5: "Mutable default arguments"

Answer 3

Always make functions like this then:

def __init__ ( self, data = None ):
    if data is None:
       data = []

    self._data = data

Alternatively you could also use data = data or [], but that prevents the user from passing empty parameters ('', 0, False etc.).