Local variable referenced before assignment

Question

I am using the PyQt library to take a screenshot of a webpage  then reading through a CSV file of different URLs  I am keeping a variable feed that incremements everytime a URL is processed and therefore should increment to the number of URLs   Here s code   webpage   QWebPage   fo   open  C  Users Romi Desktop result1 txt    w   feed   0 def onLoadFinished result        fo write  column1 feed     column2 feed   urls feed       feed   0    if not result          print  Request failed      fo write column1 feed       fo write          fo write column2 feed       fo write           fo write urls feed       fo write          fo write  404 image not created n       feed   feed   1         sys exit 1          save page webpage  outputs pop 0       pop output name from list and save    if urls          url   urls pop 0      pop next url to fetch from list         webpage mainFrame   load QUrl url       fo write column1 feed    column2 feed  urls feed   200   image created    n       fo write          fo write column2 feed       fo write           fo write urls feed       fo write          fo write  200 image created n       feed   feed   1    else          app quit      exit after last url  webpage connect webpage  SIGNAL  loadFinished bool     onLoadFinished  webpage mainFrame   load QUrl urls pop 0     fo close   sys exit app exec       It gives me the error   local variable feed referenced before the assignment at fo write column1 feed    column2 feed  urls feed   200   image created    n     Any idea why

User · Accepted Answer

When Python parses the body of a function definition and encounters an assignment such as

feed = ...

Python interprets feed as a local variable by default. If you do not wish for it to be a local variable, you must put

global feed

in the function definition. The global statement does not have to be at the beginning of the function definition, but that is where it is usually placed. Wherever it is placed, the global declaration makes feed a global variable everywhere in the function.

Without the global statement, since feed is taken to be a local variable, when Python executes

feed = feed + 1,

Python evaluates the right-hand side first and tries to look up the value of feed. The first time through it finds feed is undefined. Hence the error.

The shortest way to patch up the code is to add global feed to the beginning of onLoadFinished. The nicer way is to use a class:

class Page(object):
    def __init__(self):
        self.feed = 0
    def onLoadFinished(self, result):
        ...
        self.feed += 1

The problem with having functions which mutate global variables is that it makes it harder to grok your code. Functions are no longer isolated units. Their interaction extends to everything that affects or is affected by the global variable. Thus it makes larger programs harder to understand.

By avoiding mutating globals, in the long run your code will be easier to understand, test and maintain.

User · Answer

Best solution  Don t use globals   gt  gt  gt  test1   0  gt  gt  gt  def test func x           return x   1   gt  gt  gt  test1   test func test1   gt  gt  gt  test1 1

User · Answer

In order for you to modify test1 while inside a function you will need to do define test1 as a global variable  for example   test1   0 def testFunc        global test1      test1    1 testFunc     However  if you only need to read the global variable you can print it without using the keyword global  like so   test1   0 def testFunc         print test1  testFunc     But whenever you need to modify a global variable you must use the keyword global

User · Answer

You have to specify that test1 is global    test1   0 def testFunc        global test1     test1    1 testFunc

User · Answer

Put a global statement at the top of your function and you should be good   def onLoadFinished result       global feed           To demonstrate what I mean  look at this little test   x   0 def t        x    1 t     this blows up with your exact same error where as   x   0 def t        global x     x    1 t     does not   The reason for this is that  inside t  Python thinks that x is a local variable   Furthermore  unless you explicitly tell it that x is global  it will try to use a local variable named x in x    1   But  since there is no x defined in the local scope of t  it throws an error

User · Answer

As the Python interpreter reads the definition of a function  or  I think  even a block of indented code   all variables that are assigned to inside the function are added to the locals for that function  If a local does not have a definition before an assignment  the Python interpreter does not know what to do  so it throws this error   The solution here is to add  global feed   to your function  usually near the top  to indicate to the interpreter that the feed variable is not local to this function

[python] Local variable referenced before assignment?

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