# Bug with python mode

**URL:** <https://discourse.processing.org/t/bug-with-python-mode/26533>\
**Category:** Processing.py\
**Created:** [December 27, 2020, 2:58pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533 "2020-12-27T14:58:50Z")\
**Posts on this page:** 11\
**Page:** 1

<div class="post-metadata">

**Author:** ![CodeMasterX](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/codemasterx/32/2942_2.png) [@CodeMasterX](https://discourse.processing.org/u/CodeMasterX)\
**Post date:** [December 27, 2020, 2:58pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/1 "2020-12-27T14:58:50Z")

</div>

![image](https://canada1.discourse-cdn.com/flex036/uploads/processingfoundation1/original/2X/d/d944954040f4b1c8186ebc647610a88083a5877d.png)

```auto
txt = '1';

def setup():
    size(600,600)
    noFill()
    noStroke()

def draw():
    fill('#FF0000') # red quadrant
    rect(width/2,0, width/2,height/2)

    fill('#004477') # blue quadrant
    rect(0,0, width/2,height/2)

    fill('#6633FF') # violet quadrant
    rect(0,height/2, width/2,height/2)

    fill('#FF9900') # orange quadrant
    rect(width/2,height/2, width/2,height/2)
    
    fill(255)
    if mouseX < width/2 and mouseY < height/2:
        #text("BLUE",mouseX,mouseY)
        txt = 'BLUE'
    if mouseX < width/2 and mouseY > height/2:
        #text("PURPLE",mouseX,mouseY)
        txt = 'PURPLE'
    if mouseX > width/2 and mouseY < height/2:
        #text("RED",mouseX,mouseY)
        txt = 'RED'
    if mouseX > width/2 and mouseY > height/2:
        #text("YELLOW",mouseX,mouseY)
        txt = 'YELLOW'
    text(txt,mouseX,mouseY)

```

I don’t know why but after some time the program just crashes

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<div class="post-metadata">

**Author:** ![GoToLoop](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/gotoloop/32/86_2.png) [@GoToLoop](https://discourse.processing.org/u/GoToLoop)\
**Post date:** [December 27, 2020, 3:37pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/2 "2020-12-27T15:37:53Z")

</div>

> **[Globals \\ Language (API)](https://Py.Processing.org/reference/globals.html)**

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<div class="post-metadata">

**Author:** ![Sierra\_Alpha\_Romeo.P](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/sierra_alpha_romeo.p/32/11571_2.png) [@Sierra\_Alpha\_Romeo.P](https://discourse.processing.org/u/Sierra_Alpha_Romeo.P)\
**Post date:** [December 27, 2020, 3:42pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/3 "2020-12-27T15:42:21Z")

</div>

Try using the ‘_global_’ keyword inside the draw function.  
This keyword is to be used if you want to assign the value of an initialized global variable inside a function.

```auto
txt = '1';

def setup():
    size(600,600)
    noFill()
    noStroke()

def draw():
    global txt
    fill('#FF0000') # red quadrant
    rect(width/2,0, width/2,height/2)

    fill('#004477') # blue quadrant
    rect(0,0, width/2,height/2)

    fill('#6633FF') # violet quadrant
    rect(0,height/2, width/2,height/2)

    fill('#FF9900') # orange quadrant
    rect(width/2,height/2, width/2,height/2)
    
    fill(255)
    if mouseX < width/2 and mouseY < height/2:
        #text("BLUE",mouseX,mouseY)
        txt = 'BLUE'
    if mouseX < width/2 and mouseY > height/2:
        #text("PURPLE",mouseX,mouseY)
        txt = 'PURPLE'
    if mouseX > width/2 and mouseY < height/2:
        #text("RED",mouseX,mouseY)
        txt = 'RED'
    if mouseX > width/2 and mouseY > height/2:
        #text("YELLOW",mouseX,mouseY)
        txt = 'YELLOW'
    text(txt,mouseX,mouseY)
```

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<div class="post-metadata">

**Author:** ![villares](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/villares/32/3166_2.png) [@villares](https://discourse.processing.org/u/villares)\
**Post date:** [December 27, 2020, 4:02pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/4 "2020-12-27T16:02:55Z")

</div>

Great question @CodeMasterX, and the answers by @SierraMike and @GoToLoop will help you solve this issue 🙂

Just to give you a bit more context of what is going on… in your code without modifications your local `txt` variable fails to be assigned in some edge case (I would guess it is when the mouse is _exactly_ in the middle of the screen and no `if` clauses are `True`).

One solution would be at the start of `draw()` to assign `txt = '1'` (then you don’t need the global assignment, please remove it from the start of your sketch for clarity!) otherwise you should be using the `global` statement for the global variable strategy as the other answers have indicated.

---

<div class="post-metadata">

**Author:** ![CodeMasterX](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/codemasterx/32/2942_2.png) [@CodeMasterX](https://discourse.processing.org/u/CodeMasterX)\
**Post date:** [December 27, 2020, 6:59pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/5 "2020-12-27T18:59:44Z")

</div>

> [@villares](#):
>
> (I would guess it is when the mouse is _exactly_ in the middle of the screen and no `if` clauses are `True`

You were correct, I fixed it now:

```auto
txt = '1';

def setup():
    size(600,600)
    noFill()
    noStroke()

def draw():
    fill('#FF0000') # red quadrant
    rect(width/2,0, width/2,height/2)

    fill('#004477') # blue quadrant
    rect(0,0, width/2,height/2)

    fill('#6633FF') # violet quadrant
    rect(0,height/2, width/2,height/2)

    fill('#FF9900') # orange quadrant
    rect(width/2,height/2, width/2,height/2)
    
    fill(255)
    if mouseX < width/2 and mouseY < height/2:
        #text("BLUE",mouseX,mouseY)
        txt = 'BLUE'
    elif mouseX < width/2 and mouseY > height/2:
        #text("PURPLE",mouseX,mouseY)
        txt = 'PURPLE'
    elif mouseX > width/2 and mouseY < height/2:
        #text("RED",mouseX,mouseY)
        txt = 'RED'
    elif mouseX > width/2 and mouseY > height/2:
        #text("YELLOW",mouseX,mouseY)
        txt = 'YELLOW'
    else:
        txt = 'NONE'
    text(txt,mouseX,mouseY)

```

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<div class="post-metadata">

**Author:** ![javagar](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/javagar/32/11434_2.png) [@javagar](https://discourse.processing.org/u/javagar)\
**Post date:** [December 27, 2020, 8:16pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/6 "2020-12-27T20:16:25Z")

</div>

Hi, @CodeMasterX,

It is important to note that there are two variables named `txt` in your program, one being global, and the other one local to the `draw` function. Because the `draw` function contains at least one assignment to `txt`, and the variable was not explicitly declared `global` within the function, that name refers to a local variable by that name, wherever used inside the function, even if none of those assignment statements ever gets executed. Of course, your `else` block does guarantee that at least one of the assignments will get executed, but that has no bearing on the scope of that variable.

If you do display the value of the global variable, `txt`, you will find that it is distinct from the local variable by that name. To demonstrate that, add this function to your code, making sure that its header is not indented, and call it from within the `draw` function:

```auto
def print_the_global_txt():
    # txt is not assigned anything within this function, 
    # therefore, we are accessing the global variable, txt
    print(txt)

```

You can add a call to the function as the final line in the `draw` function, indented directly under the call to `text`, as shown here:

```auto
    text(txt,mouseX,mouseY)
    print_the_global_txt()

```

Alternatively, you can place the call within one of the conditional blocks.

Regardless of what happens inside the `draw` function, the `print_the_global_txt` function will always display `1` in the console when it is called, because it is accessing the global variable.

See official documentation at [Python.org: What are the rules for local and global variables in Python?](https://docs.python.org/3/faq/programming.html#what-are-the-rules-for-local-and-global-variables-in-python). It states:

> In Python, variables that are only referenced inside a function are implicitly global. If a variable is assigned a value anywhere within the function’s body, it’s assumed to be a local unless explicitly declared as global.

---

<div class="post-metadata">

**Author:** ![GoToLoop](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/gotoloop/32/86_2.png) [@GoToLoop](https://discourse.processing.org/u/GoToLoop)\
**Post date:** [December 27, 2020, 11:44pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/7 "2020-12-27T23:44:29Z")

</div>

In short, b/c the `global` variable _txt_ isn’t used as such anywhere within your sketch, but only as a `local` variable inside callback **draw()**, you should delete `txt = '1';` for clarity of intention. ☯

BtW, semicolons aren’t needed in Python, unless we want more than 1 statement in the same line. 😉

---

<div class="post-metadata">

**Author:** ![CodeMasterX](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/codemasterx/32/2942_2.png) [@CodeMasterX](https://discourse.processing.org/u/CodeMasterX)\
**Post date:** [December 28, 2020, 1:02pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/8 "2020-12-28T13:02:10Z")

</div>

```auto
ys = 0
g = 0.1
x = 300
y = 50

def setup():
    size(600,600)
    delay(1000)
def draw():
    # global ys
    # global g
    # global x
    # global y
    background(0)
    ys += g
    y += ys
    ellipse(x,y,15,15)

```

![image](https://canada1.discourse-cdn.com/flex036/uploads/processingfoundation1/original/2X/1/1d9993d95c9e9608c2e37d5772f5ab285769a93f.png)

It says here that the variables declared at the top of the code will automatically become global variables. So why does it still need global? Is the tutorial outdated or for a different version of processing.py?

(I am learning processing.py through [https://tabreturn.github.io/code/processing/python/2018/08/10/processing.py\_in\_ten\_lessons-4.1-\_animation.html](https://tabreturn.github.io/code/processing/python/2018/08/10/processing.py_in_ten_lessons-4.1-_animation.html) )

---

<div class="post-metadata">

**Author:** ![Sierra\_Alpha\_Romeo.P](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/sierra_alpha_romeo.p/32/11571_2.png) [@Sierra\_Alpha\_Romeo.P](https://discourse.processing.org/u/Sierra_Alpha_Romeo.P)\
**Post date:** [December 28, 2020, 3:27pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/9 "2020-12-28T15:27:18Z")

</div>

> [@CodeMasterX](#):
>
> It says here that the variables declared at the top of the code will automatically become global variables. So why does it still need global? Is the tutorial outdated or for a different version of processing.py?

The tutorial written is correct and not obselete.

It says:

 ![Screenshot_2020-12-28-20-40-11](https://canada1.discourse-cdn.com/flex036/uploads/processingfoundation1/original/2X/d/d6be91b6a5b2a325fa9be8ae7806d03f8785c979.png)  
 ![Screenshot_2020-12-28-20-40-19](https://canada1.discourse-cdn.com/flex036/uploads/processingfoundation1/original/2X/4/41c8eee392b16d2deb8a7f4d97557e8cdcf61af4.png)  
 ![Screenshot_2020-12-28-20-40-29](https://canada1.discourse-cdn.com/flex036/uploads/processingfoundation1/original/2X/3/301268ad5fbf0743dc99541dc04d07eec6c5f459.png)

> [@CodeMasterX](#):
>
> So why does it still need global?

You need to include the keyword '_global_ ’ only when you want to reassign that variable inside a function.

> [@CodeMasterX](#):
>
> ```auto
> ys += g
> y += ys
> 
> ```

Here, you are reassigning the value of the global variable inside the _draw()_ function.

If you do not want to include that keyword, another alternative is first overriding the value of the global variables inside the _draw()_ function and then assigning them.

```auto
ys = 0
g = 0.1
x = 300
y = 50

def setup():
    size(600,600)
    delay(1000)
def draw():
    ys = 0
    g = 0.1
    x = 300
    y = 50
    background(0)
    ys += g
    y += ys
    ellipse(x,y,15,15)

```

Hope this helps! 😄

---

<div class="post-metadata">

**Author:** ![javagar](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/javagar/32/11434_2.png) [@javagar](https://discourse.processing.org/u/javagar)\
**Post date:** [December 28, 2020, 5:17pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/10 "2020-12-28T17:17:24Z")

</div>

> [@CodeMasterX](#):
>
> It says here that the variables declared at the top of the code will automatically become global variables.

Yes, that is true. However, if you create another variable with the same name within an inner scope, such as a function definition, the global variable by that name will still exist, but will not be visible within that inner scope, except with the use of some particular coding gymnastics, which are not covered in this post. So while you do have a global variable named `txt`, that name refers only to the local variable when it is used within the `draw` function.

If, however, you declare `txt` as global within the function, the name, whether used within the function or in the global scope, will refer to the global variable by that name, and no local variable by that name will be created within that particular function.

That may seem like an arbitrary rule, at first, but I think it is a good language feature.

---

<div class="post-metadata">

**Author:** ![GoToLoop](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.processing.org/gotoloop/32/86_2.png) [@GoToLoop](https://discourse.processing.org/u/GoToLoop)\
**Post date:** [December 28, 2020, 6:55pm UTC](https://discourse.processing.org/t/bug-with-python-mode/26533/11 "2020-12-28T18:55:11Z")

</div>

- In short we can read from existing global variables within a function.
- But once we need to write a value to them, 1st we’ve gotta declare them via keyword `global`.
- Otherwise Python will treat them as local variables, temporarily overshadowing their corresponding global versions within that function.