0

As I muddle my way through trying to learn how to code (in python) I've hit the same problem I hit frequently.

How do you pass variables to and from functions properly.

In this example, I want to create a new variable inside the function process, but I don't know how to get it back properly (this method does not work.)

a = "foo"
def func_1():
 b = "bar"
 print a,b
 return (b)
func_1()
print b

I am 'expecting' b to be available after the function call, as I have returned it... I appreciate this me not understanding properly how to implement/manage variables

Thanks.

Improve this question
5
  • 3
    Don't get offended, but I think that what you really need is a Python BeginnersGuide Commented Feb 19, 2012 at 21:47
  • @Rik Poggi - I agree. Don't see why he would be offended though. I feel like he's in for a lot of frustration if he uses a book that assumes the reader already has basic programming knowledge. I remember just starting to learn programming. Its a whole barrel of new concepts, many of which are not intuitive. For those of us who have been programming for a while, its easy to forget all the basic things that we know that we take for granted. Commented Feb 19, 2012 at 21:50
  • 1
    @Rik Poggi - I am not offended, and I agree with DutrowLLC sometimes the basic texts assume a level of semantic knowledge that I don't have. I know roughly what I want to do, I just can't always tie the examples to my view of the world. Its a learning curve, and I'm slowly figuring stuff out. Commented Feb 19, 2012 at 21:53
  • 1
    @Jay Gattuso - I notice you get something that a lot of very good programmers don't get - diplomacy. It looks like you up-voted everyone who took the time to answer and also left a nice comment for them to make them feel good. That skill will take you FAR. Commented Feb 19, 2012 at 21:58
  • "I know roughly what I want to do". Create new variables inside a function? That's a semantic issue that's totally solved by doing a tutorial. You appear to be confused about variable scope rules; a tutorial will help fill the gaps. Commented Feb 19, 2012 at 22:28

7 Answers 7

Reset to default
3

I think this is what you want to do:

a = "foo"
def func_1():
 b = "bar"
 print a,b
 return b     # Took out parenthasis
returned_b = func_1()  # Added variable to hold the returned "b"
print returned_b

It looks like you are just learning to code in general. Your issues look to be with understanding how computer code typically works, not so much with Python in particular.

A couple of other things.

In the line a = "foo", a is a global variable. This is often frowned upon in programming because it can create confusion (although I feel like it may be used more in Python in particular).

It looks like you are having trouble understanding an aspect of how variables work called 'scope'. When you declared the variable b inside of func_1(), it had only function scope. Once the function exited, the variable no longer existed unless you returned the value AND stored it in another variable, that may, coincidentally, also have been called b.

I'm not sure you will be able to learn these concepts out of a normal Python book geared towards people who already know how to program. What you probably need is a book that teaches you how to program in general, and also uses Python as the language of choice. As I mentioned in a comment up top, finding the right learning source is key, otherwise you can be in for a world of frustration.

Here is an example of how a "normal" program might be set up:

def main():                 # Calling main() "runs" the program
   a = add(1,2)
   print a

def add(first, second):     # functions are normally declared outside of other functions, 
   result = first + second  #  that way, they can be used anywhere
   return result
Improve this answer
Sign up to request clarification or add additional context in comments.

6 Comments

Ahhh. So the returned data is an unassigned object, and needs to be assigned to a named object in the main routine to be accessible?
@Jay Gattuso - Not quite. I just added some more to my answer about "scope". "b" no longer existed after the function exited. You needed to assign the returned value of the function to another variable.
Excellent. Thank you. The more I read, the more I understand. I guess then if you have variables the have a multi func scope, it better to have a single func that sets them up, and makes them avail to the main routine, rather than to declare them as globals at the start?
@Jay Gattuso - In short: No. There are some nuances here, but variables that have a "multi function scope" are probably global variables. In your case, if you are just writing a few lines of python at a time to learn, global variables like "a='foo'" are fine. Normally everything inside "real" programs happen inside of functions. So where you declared "a='foo'", would have been inside some kind of main function anyway. I probably shouldn't have mentioned the global variable thing because it is confusing.
@Jay Gattuso - I did notcie that someone below declared a variable to be global inside a function using 'global b'. That is definitely something I would NOT do in your case because it is not a 'normal' thing to do and will inhibit you from getting a feel for how programs are normally written.
|
1

You've returned it, but not assigned it to anything:

def func_1():
    b = "bar"
    a = 2
    return a, b

a, b = func_1()
print b
Improve this answer

Comments

1

Here is your example modified:

a = "foo"
def func_1():
    b = "bar"
    print a,b
    return b
b = func_1()
print b

b inside the function is a local variable, so it's not available outside of the function, but since you return it, you can assign the returned value to a new variable (which I also called b, but it's not the same one).

An alternative approach would be to use a global (not local) variable inside the function. To do that you need to declare it as global inside the function:

a = "foo"
def func_1():
    global b
    b = "bar"
    print a,b
func_1()
print b
Improve this answer

1 Comment

Ah, I tried the global method before, and got told in no uncertian terms that it was a bad approach. TO be fair, I was using it fudge my lack of understanding... Thanks.
1

The b inside func_1() is local to the function and not available outside of the function. You need to assign the returned value of the function func_1() to a variable name in the main body.

b = func_1()
print b
Improve this answer

Comments

1

Function calls are expressions. Like any expression they have a value. If you don't assign the value to anything, it is lost.

def add(a, b):
    result = a + b
    return result

four = add(2, 2)

Any local variables of the function (like a, b and result in above case) aren't visible outside of the function, i.e. they are local to the function.

The value of a function call is the value that was passed to the return statement inside the function. In this case it's the value of the local variable result.

Improve this answer

Comments

1

I think you mean, how do I access values that I return from a function.

Say you have a function:

def func_1():
    b = 1
    return b

If you want the return value from that function, set some variable equal to it, like so:

someVariable = func_1() #func_1 executes it's code, and returns the value contained in b

You pass arguments to functions by placing them in the parentheses in the function declaration line:

a = "foo"

def func_1(some_foo): #here some_foo represents a variable you are passing to func_1
    b = "bar"
    print some_foo, b #now you're referencing the variable you passed
    return b

#Now you call the function like this:
func_1(a) #passing a (which equals "foo")

To pass multiple arguments, your function declaration would look like this:

def func_1(some_foo1, some_foo2,...):

It's also important to remember that variables declared/initialized inside of a function are only accessible within that function block. Therefore:

def func_1(some_foo1):
    print some_foo1

a = some_foo1 #Doesn't work. The variable some_foo1 doesn't exist outside of `func_1`
Improve this answer

1 Comment

Yip, thats exact what I was trying to do. Thank you.
0

I am 'expecting' b to be available after the function call, as I have returned it

The fundamental misunderstanding: you aren't returning a variable, you're returning a value. Variables are labels (names) for values. Inside the function, you have a name b for a value "foo". You use this name to refer to the value, so that you can return it.

On the outside, to use this value, you must do something with it. The call to the function - func_1() - is a stand-in, in whatever expression contains it, for the value that gets returned. So a line of code like func_1(), by itself, does nothing visible: you have an expression that evaluates to "foo", but then nothing is done with "foo".

You could, for example, print(func_1()) to display the string. You can assign the returned value to another variable: c = func_1(). Now c is a name for the "foo" value that was returned. You can use the function call as part of a larger expression: c = func_1() + "bar" - now c is a name for a string "foobar", created by joining together the string returned from the function with the directly-specified one.

On the other side: to get information into the function, you should pass it as a parameter. These are the values that go inside the parentheses: when you write the function, you put argument names (another kind of variable) inside the parentheses, and when you call the function, you put the parameters that you are calling the function with (expressions that provide the values). Now, inside the function, each time it is called, the arguments will refer to the parameter values.

Let us work a simple example. We will make a function that appends "bar" to an input string, and call it a few times.

def add_bar(to):
    return to + "bar"

Notice how we can return any expression. When the function is called, the expression is evaluated, joining the passed-in argument string to to the constant "bar", and the resulting value is returned. There is no variable here that represents the entire return value, because again, we are returning a value, not a variable.

Now we may call this. Again, the parameter can be whatever expression - so a constant:

add_bar("foo")

or a variable that we have previously assigned a value:

my_string = "sand"
add_bar(my_string)

or something more complex, maybe even involving another call to the function:

add_bar(add_bar("bar ") + " ")

But again, none of these will actually do anything, except produce the string and then throw it away. So let's fix that:

print("I want a " + add_bar("chocolate") + ".")
Improve this answer

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.