为什么会出现此TypeError？

Question

RESOLVED : Okay, you guys probably won't believe this. I did a lot of digging and it turns out that all the files we are loading and using were created incorrectly. The files fail to conform with the code we are writing — the things we want to do in our program are simply not possible based on the current state of the files we load. I am currently working on fixing this. Sorry about the non-question, guys!

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In Python I have code that essentially reads as follows:

partsList = getPartsList() # this function returns a list
for part in partsList:
    ...
bar(partsList)

def bar(partsList):
    for part in partsList:
        ...

But when I run the code I get the following TypeError:

TypeError: iteration over non-sequence

This TypeError is in reference to the noted line:

def bar(partsList):
    for part in partsList: # this is the line with the TypeError
        ...

How can this be? I know that partsList is not a non-sequence because just before my program calls bar(partsList) , I explicitly iterate over partsList .

My function does not modify partsList before interacting with it, and I do not modify partsList when iterating through it prior to calling the function, yet somehow it changes from a list to a non-sequence when the function is called.

I am working entirely within a class so these are all methods actually; I just thought it would be easier to read if I present the code this way.

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The following is in response to the comments:

I wish I could provide you all with the full code, but at the moment the program requires exactly 275 files to run and has 20+ .py files. I will mention that the method in question does employ recursion after iteration through its given list. I thought this may be linked to the error, but when when attempting to print the list itself and its contents, the program gave the same TypeError before making it through the method even once, so I know that this is not due to the recursion; it never even recursed.

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Ok I have inserted print statements as follows (keep in mind these are within methods in a class):

def someMethod(self):
    ...
    partsList = self.getPartsList() # this function returns a list
    for part in partsList:
        ...
    print partsList                      # prints [object(1), object(2)]
    self.bar(partsList)

def bar(self, partsList):
    print partsList        # prints <filename.class instance at 0x04886148>
    for part in partsList: # still gives me the TypeError
        ...

When I say filename.class I don't literally mean filename and class . You guys know what I mean.

Is the second print statement printing <filename.class instance at 0x04886148> because it is pointing to the actual partsList ? I'm not entirely sure how pointers and references work in Python.

Answer 1

You don't define bar correctly; its first argument is a reference to the object that calls it, and the second argument is the list you pass as the explicit argument.

def bar(self, partsList):
    for part in partsList:
        ...

Answer 2

Your answer is there in the print lines.

def bar(self, partsList):
    print partsList        # prints <filename.class instance at 0x04886148>
    for part in partsList: # still gives me the TypeError
        ...

partsList isn't a list going into this method. Here is some tweaked, functioning, example code from your code:

class myClass():
    def someMethod(self):
        partsList=self.getPartsList()
        for part in partsList:
            print part
        self.bar(partsList)

    def bar(self, pList):
        print pList
        for part in pList:
            print part

    def getPartsList(self):
        return ['a', 'b', 'c']

Running this interactively gets me this:

from fake_try import myClass x = myClass() x.someMethod() abc ['a', 'b', 'c'] abc

You'll notice that when I called "print pList" I received a pretty print output of the list. You are receiving an object of your class type.

I understand and empathize with your situation. Having a large, complex program throwing errors can be quite painful to debug. Unfortunately without seeing your entire code I don't think anyone here will be able to debug your issue because my guess is that you are calling 'someMethod' in a way that is unexpected in the actual code(or in an unexpected place) which is causing you to have issues.

There are a couple of ways you can debug this.

1. I am assuming that everything ran UNTIL you added the someMethod functionality? Revert your code to a state prior to the error and add lines on at a time(with dummy functions if neccesary) to find exactly where the unexpected value is coming from. If you cannot revert my first step would be to simplify all logic surrounding this issue. You have a function 'getPartsList()' that's supposed to return a list. It looks like it is here, but make it even easier to check. Make a dummy function that simply returns a fake list and see what the behavior is. Change things one step at a time until you iron out where the issue is.

2. You may not be familiar with the inspect module. Try importing inspect in your module and using inspect.getmember(x) with x being the object you want more information about. I would probably use this in place of you print partsList in the bar method( something like inspect.getmember(partsList) ) I would guess that you're somehow passing a class there instead of the list, this should tell you what that class has been instantiated as.