'The Surprising Case Of Mutable Default Arguments' python pythonprogramming
We have a Python function that takes an argument that also has a default value assigned to it.>>> foo [1,2,3,4,5] >>> foo [1,2,3,4,5,6,5]
But, what happens when we do not pass any argument and prefer to go with the default argument..? Let's see it ourselves:Wasn't the output supposed to be a new list with justWhy is it that way..?or a mutable datatype i.e. it can be modified and that's what is happening here.Default arguments are bind to the function as soon as the function is definedTherefore, we have a single list associated with our function instead of a new list being created on each function call.
>>> def eggs: print executed') return [] >>> def foo): # as soon as you hit enter after the definition it outputs 'eggs executed' a.append return a eggs executed Now, that message right after the function definition was proof that default arguments get bind as soon as the function is defined.These are also treated the same way i.e. as mutable default arguments these too are bound to the function just after the definition ends.But, as these are immutable so we get the same results instead of modified once. Let's use the above code again:
>>> def eggs: print executed') return # this time we use a tuple instead as these are immutable >>> def foo): # as soon as you hit enter after the definition it outputs 'eggs executed' a += return a eggs executed >>> foo >>> foo You are now aware of default argument bindings to functions in Python so be careful, don't be amazed if you get a modified output instead of what you expected...
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