Why python does not update an external parameter with assignment?

Question

Can anyone explain the reason why python behaves differently in the following two cases please? Much appreciation.

def modifyNone(x):
    print("B4:"+str(x))
    # x.append(5)
    x=[5]
    print("In:"+str(x))

a = []
modifyNone(a)
print("After:"+str(a))

Output:

B4:[]
In:[5]
After:[]

Method:

def modifyNone(x):
    print("B4:"+str(x))
    x.append(5)
    # x=[5]
    print("In:"+str(x))

a = []
modifyNone(a)
print("After:"+str(a))

Output:

B4:[]
In:[5]
After:[5]

There is nothing inconsistent going on here. What did you expect to happen? — Martijn Pieters, Mar 30 '15 at 21:13
@JeffMercado: They are not suggesting anything. We can *guess* at this, but even their first example still shows `After:5` and not `After:1`. — Martijn Pieters, Mar 30 '15 at 21:22
@MartijnPieters: Oh oops, I had the feeling he was expecting the value to have changed. — Jeff Mercado, Mar 30 '15 at 21:24
Your edit didn't make any more sense, but I suspect you just need to read [How do I pass a variable by reference?](http://stackoverflow.com/q/986006) — Martijn Pieters, Mar 30 '15 at 21:32
@MartijnPieters sorry for the unclear post. I messed up my work space and was misled to wrong conclusion. The post has been updated. — Rui Zheng, Mar 30 '15 at 21:37
@user1858489: your question is now a duplicate of the post I mentioned. — Martijn Pieters, Mar 30 '15 at 21:42
@user1858489: they are references to the same object. All python names are just references. You could see this as `x = a`. — Martijn Pieters, Mar 30 '15 at 21:59
@MartijnPieters I think I got it. My thought began with a totally wrong observation and turned out to be a ridiculous conclusion. Sorry for the unclear post and thank you for your time. — Rui Zheng, Mar 30 '15 at 22:02

score 3 · Answer 1 · answered Mar 30 '15 at 21:16

3

Python is pass by value, so you'll have to reassign a returned value like so:

a = modifyNone(a) # where the function returns a value

You don't seem to understand variable scope as well, try the documentation.

answered Mar 30 '15 at 21:16

Malik Brahimi

1 Answers1