CODEWITHSUNDEEP
pythonlist
Fei
Fei

Reputation: 1071

List and += operation

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?

Upvotes: 2

Views: 219

Answers (5)

John La Rooy
John La Rooy

Reputation: 304393

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()

Upvotes: 0

Dineshkumar
Dineshkumar

Reputation: 4245

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.

Upvotes: 3

furas
furas

Reputation: 142909

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

Upvotes: 0

Josh Rumbut
Josh Rumbut

Reputation: 2710

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

Upvotes: 1

Ryan
Ryan

Reputation: 1974

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

Upvotes: 1

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