python returns variable problem

[wingnut2626]

ok so here is my code:

global a


def determineTypes(arg1, arg2):
    if type(arg1) is not type(arg2):
        print 'WARNING...OBJECTS NOT SIMILAR...ERROR MAY GENERATE...'
        print 'press enter to continue.'
        raw_input('>')
        adder(arg1,arg2)
    else:
        adder(arg1,arg2)


def adder(arg1, arg2):
    if type(arg1) is list:
        arg1.append(arg2)
	return arg1
    elif type(arg1) is str:
        a = arg1 + arg2
        return a
    elif type(arg1) is dict:
        for key in arg2:
            a = arg1
            a[key] = arg2[key]
            return a


    elif type(arg1) is int or float:
        a = arg1 + arg2
        return a


    else:
        #should not happen, just a placeholder
        pass
    


def begin():
    print 'what is item one?'
    item1 = raw_input('>')
    print 'what is item two?'
    item2 = raw_input('>')
    determineTypes(item1,item2)

How come none of the values for a get returned as 'a'. After running the program, say:

adder.determineTypes(3,4)
print adder.a

I get an error, or if i switch the values of arg1 and arg2 i get 'None'. Why is that?

[Bachstelze]

It looks like you expect your module to behave like a class. You can't access a like that, you need something like

class Adder(object):

    def determineTypes(self, arg1, arg2):
        if type(arg1) is not type(arg2):
            print 'WARNING...OBJECTS NOT SIMILAR...ERROR MAY GENERATE...'
            print 'press enter to continue.'
            raw_input('>')
            self.adder(arg1,arg2)
        else:
            self.adder(arg1,arg2)

    def adder(self, arg1, arg2):
        if type(arg1) is list:
            arg1.append(arg2)
            self.a = arg1
        elif type(arg1) is str:
            self.a = arg1 + arg2
        elif type(arg1) is dict:
            for key in arg2:
                self.a = arg1
                self.a[key] = arg2[key]
        elif type(arg1) in [int, float]:
            self.a = arg1 + arg2
        else:
            #should not happen, just a placeholder
            pass

Then

>>> import adder
>>> add = adder.Adder()
>>> add.determineTypes(.5, .3)
>>> add.a
0.8
>>> add.determineTypes(1,2)
>>> add.a
3

P.S.: You should also consider switching to Python 3, there doesn't seem to be anything stopping you.

[nvteighen]

However, if you don't want to write a class, then just importing (import adder) will make your code work without any trouble.

[Bachstelze]

However, if you don't want to write a class, then just importing (import adder) will make your code work without any trouble.

I must be doing something wrong then...

>>> import adder
>>> adder.determineTypes(3,4)
>>> adder.a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'a'

[nvteighen]

I must be doing something wrong then...

>>> import adder
>>> adder.determineTypes(3,4)
>>> adder.a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'module' object has no attribute 'a'

Just when I thought I knew Python really well, it hits me with this one... *facepalm*... globals are module-scoped.

[wingnut2626]

Thanks guys lol. Im just doing it class based now.

[wingnut2626]

how do I mark this thread as complete?

[trent.josephsen]

Edit the original post and add [Solved] to the title.

For posterity, and in case you see something like this again: An assignment within an inner scope masks or "shadows" variables in outer scopes. To prevent this, you need to put a "global" statement (or "nonlocal", in Python 3) in the inner scope, which tells Python "I want to use the `a' in the outer scope, even though I'm assigning to it." This is kind of a gotcha and easy to overlook.

a = "hello"
def greet(target):
    print a, target
greet("world")

will print "hello world", but

a = "hello"
def greet(target):
    print a, target
    a = "hi"
greet("world")

raises an error.

You can fix your original program (well, that particular bug) by moving the "global a" declaration inside the definition of adder().

[alan9800]

according to the programming recommendations contained in the PEP8, you should use:

1. if isinstance(obj, type)

   instead of

   if type(obj) is type(value of the type to be checked)

2. ''.join([string1, string2, ...])

   instead of

   string1 = string1 + string2 + ...