List and += operation

Question

I'm using the list object by declaring

data = []

in my code, and without initializing it, I wrote

data += data2

Where data2 is another list that contains lots of numbers.

An error happened at this line:

local variable 'data' referenced before assignment

How do I fix this?

show more code. it seems you use ` data += data2` in function but you forgot to use `global data` — furas, Nov 27 '15 at 04:14

score 3 · Answer 1 · edited Nov 27 '15 at 04:34

3

Getting 'referenced before assignment' after you initializing data can't happen if you are in the same scope.

The possibility is that you are creating a function (creating a new scope) so when you say data+=data2 it means

data = data + data2 # so what is data in the right hand side?

So if you want to refer the global variable use already available (global) data you have to explicitly say global data or pass that as a parameter to the funciton.

edited Nov 27 '15 at 04:34

abcd

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answered Nov 27 '15 at 04:21

Dineshkumar

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I accurately did create a new function! So that's where goes wrong. Thanks a lot! – Eumaa Nov 27 '15 at 04:28

score 1 · Answer 2 · answered Nov 27 '15 at 04:16

1

You have to initialize your array first. Here is a working example of how to fix this problem.

data = [0]
data2 = [1,2,3,4,5]
print data
data += data2
print data

answered Nov 27 '15 at 04:16

Ryan

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you can use `data = []` in place of `data=[0]` and still works – furas Nov 27 '15 at 04:18

score 1 · Accepted Answer · edited May 23 '17 at 12:07

1

You may want to try using data.extend (data2) instead of using the operator.

See this question for an extended discussion about adding two lists together: Python: take the content of a list and append it to another list

edited May 23 '17 at 12:07

Community

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answered Nov 27 '15 at 04:18

Josh Rumbut

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this answer doesn't address the question. it makes no sense that using `extend` instead of `+=` would prevent the error. @CHEN has left out important information about where these lines of code have been placed in his script or module. – abcd Nov 27 '15 at 04:31
@dbliss It seems the asker disagrees ;) – Josh Rumbut Nov 27 '15 at 04:33
@dbliss I did misread the question though, however I left the answer because I thought it might point to a more flexible way to merge lists. – Josh Rumbut Nov 27 '15 at 04:34
1

what's "more flexible" about `extend`? all it does is add overhead. i'd be interested to know of any case where using `extend` makes more sense than using `+=`. – abcd Nov 27 '15 at 04:36

score 0 · Answer 4 · answered Nov 27 '15 at 04:28

If you use data += data2 in function you need global data

data = []
data2 = [1,2,3,4]

def add():
    globla data

    data += data2

add()
print data

or send data as argument

data = []
data2 = [1,2,3,4]

def add(some_data):
    some_data += data2

add(data)
print data

score 0 · Answer 5 · answered Nov 27 '15 at 04:49

Your problem is covered in the docs here

https://docs.python.org/3/reference/datamodel.html#object.iadd

For instance, if x is an instance of a class with an __iadd__() method, x += y is equivalent to x = x.__iadd__(y) .

So you see data is being rebound (to itself in this case), but it still counts as an assignment.

Since data += data2 appears inside a function scope where data has not been declared to be global, a local variable called data is assumed.

def foo():
    data.extend(data2)           # ok -- global data variable here

def bar():
    data = data.__iadd__(data2)  # not ok -- local data variable here

def baz():
    data += data2                # not ok -- equivalent to bar()

List and += operation

5 Answers5