Python closure: Why changing dictionary in closure is allowed while changing numbers isn't?

user_15153348 0
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this code works well when I try to change dictionary in closure

def a():
    x = {'a':'b'}
    def b():
        x['a'] = 'd'
        print(x)
    return b
>>> b = a()
>>> b()
{'a':'d'}

output is meeting my expectation. but why the code below doesn't work?

def m():
x = 1
def n():
    x += 1
    print(x)
return n
>>> n = m()
>>> n()

UnboundLocalError: local variable 'x' referenced before assignment

Honestly, I've known that we can use nonlocal x statement to solve this problem
But can anybody explain the reason more deeply for me? what the difference between a dictionary and a number
Thanks!

local python-3.x closures
1 Answers

Python has a great FAQ specifically on this.

In short, when you modify a dictionary, or any mutable object, you modify the same object, so you don't re-assign the variable. In case of an integer, since it's immutable, by doing a += you create a new object and put it into x. Since x is now defined inside the inner function, but you're trying to bring data from the outer function, you have an issue.

You can check if it's the same object using id().

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