Python: unusual behaviour dictionary update method

Question

I am writing a script which builds a dictionary, and I observe a strange behaviour.

I will describe it below with a small example.

def test_f(parameter):
    parameter.update({'watermelon':'22'})
    return parameter

fruits = {'apple':20, 'orange': 35, 'guava': 12}
new_fruits = test_f(fruits)

In short, I have a dictionary which I pass to a function, test_f. The function appends a new dictionary to the input and returns it. I capture the output of the function in a variable called new_fruits. This however changes the original variable fruits too.

Why does the original variable fruits changes? Am I using the update method in an incorrect way?

score 5 · Accepted Answer · edited May 23 '17 at 12:34

5

No, you are using it right.

But lists, dictionaries etc are mutable types. And they will get updated even if you change them in a local scope. That because, python actually pass parameters by names, rather by values.

Read more from here.

Solution:

I will suggest you to make a new copy of dictionary and pass it as a parameter of your function. Use copy.deepcopy to copy the dictionary.

Change your function call like below:

import copy 

fruits = {'apple':20, 'orange': 35, 'guava': 12}
new_fruits = test_f(copy.deepcopy(fruits))

edited May 23 '17 at 12:34

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answered Apr 23 '17 at 05:56

Ahsanul Haque

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Glad to know it helped. You can tick the green mark on the left top of my answer to mark it as accepted. – Ahsanul Haque Apr 23 '17 at 06:43

Python: unusual behaviour dictionary update method

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