Default value in a function in Python

2018-06-09 12:28:49

This question already has an answer here:

“Least Astonishment” and the Mutable Default Argument 30 answers

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.

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

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.).

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