changing global variables within a function in python

Question

I'm new to python.

I don't quite understand how I need to set variables and change them within a function to be used late. My script needs to get an x and y value from a function which are determined by the size of the chart the function creates. These variable need to be passed to a print command later in the script to output html. So let's say I have global variables:

    originx_pct = 0.125
    originy_pct = 0.11

But these will need to change when the funtion is run...

    def makeplot(temp, entropy,preq):
        originx_pct = origin.get_points()[0][0]
        originy_pct = origin.get_points()[0][1]

Then printed within the javascript of the html page that is written later...

    print('var originx_pct = {};'.format(originx_pct))
    print('var originy_pct = {};'.format(originy_pct))

The 2 variables don't change and I just don't understand what I need to do to update them and be able to print them (outside the function). I'm assuming that the function does not know about the variables so it can't change them. If I feed the function the 2 variables as arguments, how do i get the values back out for the print part of the script?

Try adding the line `global VARIABLENAMEHERE` at the start of your function (one for each variable you wish to refer to) — Max, Jul 27 '18 at 19:52
In general, it's a good idea to avoid global variables if you can — SuperStew, Jul 27 '18 at 19:54
It's much easier to test your functions and reason about their behavior if these values are passed into the function and returned from it changed. — Patrick Haugh, Jul 27 '18 at 20:00

ltd9938 · Accepted Answer · 2018-07-27T19:55:36.457

39

You need to use the global keyword in your function.

originx_pct = 0.125
originy_pct = 0.11

def makeplot(temp, entropy,preq):
    global originx_pct, originy_pct
    originx_pct = origin.get_points()[0][0]
    originy_pct = origin.get_points()[0][1]

You can read more about global here.

edited Jul 27 '18 at 19:55

answered Jul 27 '18 at 19:52

ltd9938

1,444
1
15
29

Luke B · Answer 2 · 2021-10-31T19:30:04.987

In your function, you need to return the values. Change your makeplot to the following:

def makeplot(temp, entropy, preq):
    local_originx_pct = origin.get_points()[0][0]
    local_originy_pct = origin.get_points()[0][1] # the local_ in the names doesn't mean anything, it is just for clarity.
    return local_originx_pct, local_originy_pct

Then, when you call the function, set your variables to its return value.

originx_pct, originy_pct = makeplot(args_and_stuff)

This is considered better practice then directly changing global variables as in ltd9938's answer. It helps to prevent accidentally messing stuff up for other functions. More reasons not to use global

score 3 · Answer 3 · answered Jul 27 '18 at 20:04

You can either declare the global variables in the function with the lines global originx_pct and global originy_pct, or you can return them when you run the function. To do that, you can do

def makeplot(temp, entropy,preq):
    return (origin.get_points()[0][0],origin.get_points()[0][1])

Or

def makeplot(temp, entropy,preq):
    return origin.get_points()[0][0:2]

If origin.get_points()[0] has only two elements, you do just this:

def makeplot(temp, entropy,preq):
    return origin.get_points()[0]

Then, in your main function, put

originx_pct, originy_pct = makeplot(temp, entropy,preq)

Although I'm not clear on why you're passing temp, entropy, and preq to makeplot, since you don't seem to be using them.

changing global variables within a function in python

3 Answers3

Linked

Related