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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?
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UnboundLocalError can be solved by changing the scope of the variable which is complaining. You need to explicitly declare the variable global. Variable x's scope in function printx is global. You can verify the same by printing the value of x in terminal and it will be 6.
local - Assign a local variable in a function qsh uses dynamic scoping, so that if you make the variable alpha local to function foo, which then calls function bar, references to the variable alpha made inside bar will refer to the variable declared inside foo, not to the global variable named alpha.
The global Keyword Normally, when you create a variable inside a function, that variable is local, and can only be used inside that function. To create a global variable inside a function, you can use the global keyword.
Use of “global†keyword to modify global variable inside a function. If your function has a local variable with same name as global variable and you want to modify the global variable inside function then use 'global' keyword before the variable name at start of function i.e.
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.
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