Mystery of Mutable Default Arguments in Python
int
, float
, bool
, str
, tuple
, Unicode
) are immutable. Objects of built-in types like (list
, set
, dict
) are mutable.
A mutable object can change its state or contents and immutable objects cannot.
This is a very simple definition of a Mutable and Immutable object in Python. Now coming to the interesting part which is Mutable Default Object in function definitions.
I have written a very simple function as below -
def foobar(element, data=[]):
data.append(element)
return data
data
to a list(Mutable object) as a default argument. Now let’s execute this function -
>>> print(foobar(12))
[12]
>>> print(foobar(22))
[12, 22]
>>> print(foobar(32))
[12, 22, 32]
Why is this happening?
Python’s default arguments are evaluated once when the function is defined, not each time the function is called. When Python encounters it, the first thing it will do is compile it in order to create a code object for this function. While this compilation step is done, Python evaluates and then stores the default arguments in the function object itself. So, let’s do some introspection, to clear the confusions, I have taken a few lines of code as below-- Function Before Execution -
def foo(l=[]):
l.append(10)
__defaults__
.
About __defaults__
: A tuple containing default argument values for those arguments that have defaults, or None if no arguments have a default value.
>>> foo.__defaults__
([],)
__defaults__
- Function After Execution-
>>> foo()
>>> foo.__defaults__
([10],)
>>> foo()
>>> foo()
>>> foo()
>>> foo.__defaults__
([10, 10, 10, 10],)
The Solution
Now, the question is how to codefoobar
in order to get the behavior that we expected.
Fortunately, the solution is straightforward. The mutable objects used as defaults are replaced by None, and then the arguments are tested for None.
def foobar(element, data=None):
if data is None:
data = []
data.append(element)
return data
>>> foobar(12)
[12]
Good Use of Mutable Default Argument
However, Mutable Default Argument have some good use-case also as following-- Binding local variable to the current value of the outer variable in a callback
- Cache/Memoization
About Author
Prashant Sharma
An Engineering professional with 3+ years of experience in Software development. He is a Post-graduate in CS from National Institute of Technology Calicut. He loves learning new stuff and writing about it.Original Source: Original Post
Please share your Feedback:
Did you enjoy reading or think it can be improved? Don’t forget to leave your thoughts in the comments section below! If you liked this article, please share it with your friends, and read a few more!