Question

Problem part of the code:

while counter < length:
    if something:
    else:
        for key,value in dictionary.items():
            if something:
            else:
    counter += 1

The structure is a { key:[ {key:value,key1:value1}, {key:value,key1:value1} ] }
which is what the counter (list index) is for.

So I’ve noticed this a few times in some scripts I’ve written while I’ve been teaching myself Python (3.2.3).

I tried searching but no luck (maybe my search terms are wrong, who knows?) and I’ve checked the documentation and what seems to be the problem is that for while statements it says, “The while statement is used for repeated execution as long as an expression is true”. And for For statements it says, “The expression list is evaluated once”.

Now when my script runs it keeps getting a key error because the counter has increased by 1; but the for statements isn’t updating to reflect the new elements from the list which is another dictionary; it’s still using the first element.

I assumed that each iteration in the while loop would cause the For statement to re-evaluate. Apparently I was wrong on that count.

Now I can rewrite the code, to fix this, but what I’d like to know as I’ve ran into this a few times already, is how can I use a For loop in a while statement in this fashion but have it re-evaluate the For statement on each iteration through the while loop.

Thanks!

Updated: I’ve added the actual code below. It wouldn’t surprise me if I’m just doing something wrong. I was watching the values in the debugger, and when count changed from 0 to 1, it started on the next iteration of the while loop. From there everything went as it should, until it hit the For loop “for key,value in item.items():”, and the values for item.items() didn’t change at all.
Updated Update: After trying to figure it out I’m pretty sure it’s the for item in v: that’s causing the problem. Let’s see if I can figure it out!

Fixed! Problem was with the For item in v: that was outside the while loop, solved by moving it into the loop.

def checkDifferences(newFile,oldFile):
    resultDiff = []
    for k,v in newFile.items():
        if newFile[k] != oldFile[k]:
            for item in v:
                count = 0
                lengthList = len(newFile[k])
                newKeys = newFile[k][count].keys()
                oldKeys = oldFile[k][count].keys()                
                while count < lengthList:
                    if newKeys == oldKeys:
                        for key,value in item.items():
                            newValue = newFile[k][count][key] 
                            oldValue = oldFile[k][count][key]
                            if newValue == oldValue:
                                pass
                            else:
                                resultDiff.append((k,count,key, oldValue,newValue))
                    else:
                        newValue = list(newFile[k][count].keys())
                        oldValue = list(oldFile[k][count].keys())
                        keyList = [key for key in newValue if key not in oldValue]
                        print(keyList)
                        for key,value in item.items():
                            if key in keyList:
                                oldValue = oldFile[k][count][oldKey]
                                resultDiff.append((k,count,key, None,newValue))
                            else:
                                oldValue = oldFile[k][count][key]
                            newValue = newFile[k][count][key]
                            if newValue == oldValue:
                                pass
                            else:
                                resultDiff.append((k,count,key, oldValue,newValue))
                            oldKey = key
                    count += 1
     return resultDiff

Answer 1

The expression in the for statement is evaluated once each time the for loop begins. Since you’ve put the for loop into a while loop, each time around the while loop, when the for statement is encountered, the expression will be evaluated.

There is something else wrong with your code. Post a real code sample.