UnboundLocalError: local variable 'W' referenced before assignment

Question

Following my previous question. I was advised to make a separate python library and then import it.

After reading a bit more on Stackoverflow, I realized that the best way is to write methods and I've taken up that path.

def USB(port):
        activateme = serial.Serial(port,115200)
        #print "starting to monitor"
        for line in activateme:
                #print line
                return line

def USBprocess(line):
        if line.startswith( '#d'):
                fields = line.split(',')
                if len(fields) > 5:
                        W = fields[1]
                        V = fields[2]
                        A = fields[3]
                print "monitoring"
        return W,V,A

op = USB(port)
w,v,a = USBprocess(op)

and I get the error:

UnboundLocalError: local variable 'W' referenced before assignment

what is it that I am doing wrong?

The error will be thrown when the if condition is false because then there will be no variable `W` to return — Ankur Ankan, Jun 14 '13 at 10:47
Clearly, the line did not strat with `#d`.. What do you think should happen in that was the case? — Martijn Pieters, Jun 14 '13 at 10:48
BTW, you can do `W, V, A = fields[1:4]`, or simply `return fields[1:4]` — Elazar, Jun 14 '13 at 10:51

Óscar López · Accepted Answer · 2013-06-14T11:02:25.203

6

You should provide values for W, V, A at the start of the function, in case the first if condition is not True. Something like this, perhaps (change for default values appropriate for your problem):

def USBprocess(line):
    W, V, A = '0', '0', '0'
    if line.startswith('#d'):
        # etc.

edited Jun 14 '13 at 11:02

answered Jun 14 '13 at 10:47

Óscar López

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3

for example `W = A = V = None` – oleg Jun 14 '13 at 10:47

score 0 · Answer 2 · answered Jun 14 '13 at 10:49

0

In case this expression is not true:

 line.startswith( '#d')

variables W, V and A will not be initialized in your USBprocess function, so return will fail.

Fix it by initializing all local variables before the if statement.

answered Jun 14 '13 at 10:49

piokuc

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UnboundLocalError: local variable 'W' referenced before assignment

2 Answers2