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Variables that are created outside of a function (as in all of the examples above) are known as global variables.
Global variables can be used by everyone, both inside of functions and outside.
Create a variable outside of a function, and use it inside the function
If you create a variable with the same name inside a function, this variable will be local, and can only be used inside the function. The global variable with the same name will remain as it was, global and with the original value.
Create a variable inside a function, with the same name as the global variable
The global Keyword
Normally, when you create a variable inside a function, that variable is local, and can only be used inside that function.
To create a global variable inside a function, you can use the global keyword.
If you use the global keyword, the variable belongs to the global scope:
Also, use the global keyword if you want to change a global variable inside a function.
To change the value of a global variable inside a function, refer to the variable by using the global keyword:
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Python function global variables? [duplicate]
I know I should avoid using global variables in the first place due to confusion like this, but if I were to use them, is the following a valid way to go about using them? (I am trying to call the global copy of a variable created in a separate function.)
Does the x that the second function uses have the same value of the global copy of x that func_a uses and modifies? When calling the functions after definition, does order matter?
- 1 be careful also not to assume just because you have a variable assigned in your function that python will treat references before the assignment as such. Until the first assignment, if you used x, it would not be the global one, or the local one. You will get the infamous UnboundLocalError exception in your face :) – osirisgothra Aug 22, 2015 at 1:42
- Python does not copy on assignment. – Karl Knechtel Sep 10, 2022 at 7:46
6 Answers 6
If you want to simply access a global variable you just use its name. However to change its value you need to use the global keyword.
This would change the value of the global variable to 55. Otherwise it would just assign 55 to a local variable.
The order of function definition listings doesn't matter (assuming they don't refer to each other in some way), the order they are called does.
- 2 In the code that I gave, is func_B doing things (1) to the global copy of x (as gotten from func_A), (2) to a local variable x with the same value of the result of func_A, or (3) to a local variable x with no value and (in the eyes of the compiler) no relation to "some value" or the x in func_A? – Akshat Shekhar May 14, 2012 at 18:00
- x in func_B is a local variable that gets its value from the return value of the call to func_A - so I guess that would make it your (2) – Levon May 14, 2012 at 18:03
- ok, let's say x was a random sequence of some kind generated by func_A (i.e. that func_A produced a different x each time it was run.) Would running the program as written make func_b modify a different x than what was originally produced when func_a was called? If so, how can I fix it? – Akshat Shekhar May 14, 2012 at 19:00
- 1 Yes, if func_A changes the global variable during each run and returns it to func_B to use, then func_B will work with a changed value each time. I am not sure about your "how to fix it". You may want to accept the most helpful answer to your current/original question and then consider opening up a different question about what looks like a follow-up question. – Levon May 14, 2012 at 19:13
- 1 Actually it depends what x is. If x is immutable, then the x in func_B will stay in it, because it is declared locally even if they have the same value. This applies to tuples, ints... If it's an instance of a list for example and you do x.append("...") , it's the global variable x that is changed, because the local one references the global one. – jadkik94 May 14, 2012 at 19:15
Within a Python scope, any assignment to a variable not already declared within that scope creates a new local variable unless that variable is declared earlier in the function as referring to a globally scoped variable with the keyword global .
Let's look at a modified version of your pseudocode to see what happens:
In fact, you could rewrite all of func_B with the variable named x_local and it would work identically.
The order matters only as far as the order in which your functions do operations that change the value of the global x. Thus in our example, order doesn't matter, since func_B calls func_A . In this example, order does matter:
Note that global is only required to modify global objects. You can still access them from within a function without declaring global . Thus, we have:
Note the difference between create_locally and access_only -- access_only is accessing the global x despite not calling global , and even though create_locally doesn't use global either, it creates a local copy since it's assigning a value.
The confusion here is why you shouldn't use global variables.
- 3 I don't think this is very confusing in-practice, you just have to understand python's scoping rules . – Casey Kuball May 14, 2012 at 18:21
You can directly access a global variable inside a function. If you want to change the value of that global variable, use "global variable_name". See the following example:
Generally speaking, this is not a good programming practice. By breaking namespace logic, code can become difficult to understand and debug.
As others have noted, you need to declare a variable global in a function when you want that function to be able to modify the global variable. If you only want to access it, then you don't need global .
To go into a bit more detail on that, what "modify" means is this: if you want to re-bind the global name so it points to a different object, the name must be declared global in the function.
Many operations that modify (mutate) an object do not re-bind the global name to point to a different object, and so they are all valid without declaring the name global in the function.
Here is one case that caught me out, using a global as a default value of a parameter.
I had expected param to have a value of 42. Surprise. Python 2.7 evaluated the value of globVar when it first parsed the function func. Changing the value of globVar did not affect the default value assigned to param. Delaying the evaluation, as in the following, worked as I needed it to.
Or, if you want to be safe,
- That reminded me of the problem of assigning an empty list as default value . And, as in the example, use is to check if something is None , instead of the normal comparison == . – berna1111 Mar 4, 2020 at 12:48
You must use the global declaration when you wish to alter the value assigned to a global variable.
You do not need it to read from a global variable. Note that calling a method on an object (even if it alters the data within that object) does not alter the value of the variable holding that object (absent reflective magic).
- 2 This wording is unfortunate. In Python, the value assigned to a variable is a reference, so it is technically correct (and I have no doubt you meant that), but many a reader may interpret "alter the value" as "mutate the object", which is not the case -- xs.append(xs.pop(0)) works just fine without global xs . – user395760 May 14, 2012 at 17:51
- @delnan My answer is carefully worded, but I will clarify. – Marcin May 14, 2012 at 18:01
Not the answer you're looking for? Browse other questions tagged python global-variables or ask your own question .
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Python Global Variables
Updated on: October 21, 2022 | Leave a Comment
In this tutorial, you’ll learn what is a global variable in Python and how to use them effectively.
Goals of this lesson :
- Understand what is a global variable in Python with examples
- Use global variables across multiple functions
- Learn how to use the global keyword to modify the global variables
- Learn to use global variables across Python modules/files
- Understand the use of globals() function
- Use global variables inside a nested function
Table of contents
What is a global variable in python, global variable and local variable with same name, modify a global variable inside a function, create a global variable inside a function, rules of global keyword, global variables across python modules/files, globals() function in python, global variables in nested function.
In Python, a variable declared outside the function or in global scope is known as a global variable. We can use global variables both inside and outside the function.
The scope of a global variable is broad. It is accessible in all functions of the same module .
Let’s understand it with an example.
In this example, we declared a global variable name with the value ‘Jessa’. The same global variable name is accessible to everyone, both inside of functions and outside.
Using Global Variables In Function
We can use global variables across multiple functions of the same module.
Now, let’s see how to use the global variable inside a Python function.
- First, create a global variable x and initialize it to 20. The same global variable x is accessible to everyone, both inside of functions and outside.
- Now, create a function with a combination of local variables and global variables.
- Create a local variable y And initialize it to 30. A local variable is declared inside the function and is not accessible from outside it. The local variable’s scope is limited to that function only where it is declared.
- In the end, add a global variable x and local variable y to calculate the sum of two variables.
Note : If you create a new local variable inside a function with the same name as a global variable, it will not override the value of a global variable. Instead, the new variable will be local and can only be used inside the function. The global variable with the same name will remain unchanged.
global Keyword in Python
The global keyword is used in the following two cases.
- To access and modify a global variable inside a function
- To create a new global variable inside a function
let’s see the below code.
Execute the above code to change the global variable x’s value. You’ll get an UnboundLocalError because Python treats x as a local variable, and x is also not defined inside my_func(). i.e, You cannot change or reassign value to a global variable inside a function just like this.
Use the global keyword to change the value of a global variable inside a function .
in Python, the scope of variables created inside a function is limited to that function. We cannot access the local variables from outside of the function. Because the scope is local, those variables are not visible outside the function.
To overcome this limitation, we can use the global keyword to create a global variable inside a function. This global variable is accessible within and outside the function.
Let us see the rules we need to follow to create and use a global keyword.
- If we create a variable inside a function, it is a local variable by default.
- If we create a variable outside the function, it turns into a global variable, and we don’t have to use the keyword global.
- We use the keyword global to create a global variable inside a function or to change a global variable already declared outside the function.
- Using the global keyword outside the function does not make any difference.
By default, the global variables are accessible across multiple functions of the same module . Now, we’ll see how to share global variables across the modules.
- First, create a special module config.py and create global variables in it.
- Now, import the config module in all application modules, then the module becomes available for a global name.
Let us understand it using an example.
In Python, to create a module, write Python code in the file, and save that file with the .py extension.
Example : Share global variables across Python modules.
config.py : The config module stores global variables of school and grade
Now, run the config.py file.
company.py : create a company.py file to import global variables and modify them. In the company.py file, we import the config.py module and modify the values of the name and address.
Now, run the company.py file.
Now, in the employee file, we import both config and company modules to test the values of global variables and whether they are changed.
As you can see in the output, we successfully accessed and modified the global variables across the files or modules.
In this section, we’ll see what the globals() do in Python.
We can also use the globals() function to access and modify the global variables . The globals() function returns the dictionary of the current global symbol table.
The global symbol table stores all information related to the program’s global scope and is accessed using the globals() method.
Function and variables are not part of any class, or functions are stored in a global symbol table.
Example : Modify the global variable using the globals() function.
Now, Let’s see how to use a global variable in a nested function. Global variables can be used in a nested function using global or nonlocal keywords.
The difference between nonlocal and global is that global is used to change global variables, while nonlocal is used to change variables outside the function. Let us illustrate this with an example.
Example : Access global variables in nested functions using the global keyword
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Python Global variables are those which are not defined inside any function and have a global scope whereas Python local variables are those which are defined inside a function and their scope is limited to that function only. In other words, we can say that local variables are accessible only inside the function in which it was initialized whereas the global variables are accessible throughout the program and inside every function.
Python Local Variables
Local variables in Python are those which are initialized inside a function and belong only to that particular function. It cannot be accessed anywhere outside the function. Let’s see how to create a local variable.
Creating local variables in Python
Defining and accessing local variables
Can a local variable be used outside a function?
If we will try to use this local variable outside the function then let’s see what will happen.
Python Global Variables
These are those which are defined outside any function and which are accessible throughout the program, i.e., inside and outside of every function. Let’s see how to create a Python global variable.
Create a global variable in Python
Defining and accessing Python global variables.
The variable s is defined as the global variable and is used both inside the function as well as outside the function.
Note: As there are no locals, the value from the globals will be used but make sure both the local and the global variables should have same name.
Why do we use Local and Global variables in Python?
Now, what if there is a Python variable with the same name initialized inside a function as well as globally? Now the question arises, will the local variable will have some effect on the global variable or vice versa, and what will happen if we change the value of a variable inside of the function f()? Will it affect the globals as well? We test it in the following piece of code:
If a variable with the same name is defined inside the scope of the function as well then it will print the value given inside the function only and not the global value.
Now, what if we try to change the value of a global variable inside the function? Let’s see it using the below example.
To make the above program work, we need to use the “global” keyword in Python. Let’s see what this global keyword is.
The global Keyword
We only need to use the global keyword in a function if we want to do assignments or change the global variable. global is not needed for printing and accessing. Python “assumes” that we want a local variable due to the assignment to s inside of f(), so the first statement throws the error message. Any variable which is changed or created inside of a function is local if it hasn’t been declared as a global variable. To tell Python, that we want to use the global variable, we have to use the keyword “global” , as can be seen in the following example:
Example 1: Using Python global keyword
Now there is no ambiguity.
Example 2: Using Python global and local variables
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Python Variable Scope
In this tutorial, we'll learn about Python Global variables, Local variables, and Nonlocal variables with the help of examples.
Video: Python Local and Global Variables
In Python, we can declare variables in three different scopes: local scope, global, and nonlocal scope.
A variable scope specifies the region where we can access a variable. For example,
Here, the sum variable is created inside the function, so it can only be accessed within it (local scope). This type of variable is called a local variable.
Based on the scope, we can classify Python variables into three types:
- Local Variables
- Global Variables
- Nonlocal Variables
Python Local Variables
When we declare variables inside a function, these variables will have a local scope (within the function). We cannot access them outside the function.
These types of variables are called local variables. For example,
Here, the message variable is local to the greet() function, so it can only be accessed within the function.
That's why we get an error when we try to access it outside the greet() function.
To fix this issue, we can make the variable named message global.
Python Global Variables
In Python, a variable declared outside of the function or in global scope is known as a global variable. This means that a global variable can be accessed inside or outside of the function.
Let's see an example of how a global variable is created in Python.
This time we can access the message variable from outside of the greet() function. This is because we have created the message variable as the global variable.
Now, message will be accessible from any scope (region) of the program.
Python Nonlocal Variables
In Python, nonlocal variables are used in nested functions whose local scope is not defined. This means that the variable can be neither in the local nor the global scope.
We use the nonlocal keyword to create nonlocal variables.For example,
In the above example, there is a nested inner() function. We have used the nonlocal keywords to create a nonlocal variable.
The inner() function is defined in the scope of another function outer() .
Note : If we change the value of a nonlocal variable, the changes appear in the local variable.
Table of Contents
- Global Variables in Python
- Local Variables in Python
- Global and Local Variables Together
- Nonlocal Variables in Python
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How to use a global variable in a Python function?
There are 2 types of variables in python namely Local variables and Global variables. Local variables mean the variables that are declared inside a function or inside a method whose impact or scope is only present inside that specific block and it doesn’t affect the program outside that block.
Global variables mean the variables that are declared outside any function or method and these variables have an impact or scope through the entire program.
We can also instantiate global variables inside a function by using a global keyword, if we want to declare global variables outside a function then we may not need to use a global keyword.
If a variable with the same name globally and locally then inside the function where the local variable is declared the local value is used and the global value is used elsewhere.
Let us see an example for a global variable in python −
Following is another example for this −
In the following example we are defining two global variables after the function −
Now let us let us try to create a global variable with in a function using the “ global ” keyword −
Following example shows how the global variable is accessed both inside and outside the function sample.
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Global Variable in Python – Non-Local Python Variables
In Python and most programming languages, variables declared outside a function are known as global variables. You can access such variables inside and outside of a function, as they have global scope.
Here's an example of a global variable:
The variable x in the code above was declared outside a function: x = 10 .
Using the showX() function, we were still able to access x because it was declared in a global scope.
Let's take a look at another example that shows what happens when we declare a variable inside a function and try to access it elsewhere.
In the example above, we declared x inside a function and tried to access it in another function. This resulted in a NameError because x was not defined globally.
Variables defined inside functions are called local variables. Their value can only be used within the function where they are declared.
You can change the scope of a local variable using the global keyword – which we'll discuss in the next section.
What is the global Keyword Used for in Python?
The global keyword is mostly used for two reasons:
- To modify the value of a global variable.
- To make a local variable accessible outside the local scope.
Let's look at some examples for each scenario to help you understand better.
Example #1 - Modifying a Global Variable Using the global Keyword
In the last section where we declared a global variable, we did not try to change the value of the variable. All we did was access and print its value in a function.
Let's try and change the value of a global variable and see what happens:
As you can see above, when we tried to add 2 to the value of x , we got an error. This is because we can only access but not modify x .
To fix that, we use the global variable. Here's how:
Using the global keyword in the code above, we were able to modify x and add 2 to its initial value.
Example #2 - How to Make a Local Variable Accessible Outside the Local Scope Using the global Keyword
When we created a variable inside a function, it wasn't possible to use its value inside another function because the compiler did not recognize the variable.
Here's how we can fix that using the global keyword:
To make it possible for x to be accessible outside its local scope, we declared it using the global keyword: global x .
After that, we assigned a value to x . We then called the function we used to declare it: X()
When we called the showX() function, which prints the value of x declared in the X() function, we did not get an error because x has a global scope.
In this article, we talked about global and local variables in Python.
The examples showed how to declare both global and local variables.
We also talked about the global keyword which lets you modify the value of a global variable or make a local variable accessible outside its scope.
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Normally, when you create a variable inside a function, that variable is local, and can only be used inside that function. To create a global variable inside a
If you want to simply access a global variable you just use its name. However to change its value you need to use the global keyword. E.g.
How to Use Global Keywords in Python With Examples? ; x = 5. #initializing a global variable ; def life(). #defining a function ; global x. #using global keyword.
Using Global Variables In Function · First, create a global variable x and initialize it to 20. · Now, create a function with a combination of
A global keyword is a keyword that allows a user to modify a variable outside the current scope. It is used to create global variables in Python
Global and Local Variables in Python · Global variables are those which are not defined inside any function and have a global scope whereas local
Python Global Variables ... In Python, a variable declared outside of the function or in global scope is known as a global variable. This means that a global
Global variables mean the variables that are declared outside any function or method and these variables have an impact or scope through the
When you define a variable outside a function, like at the top of the file, it has a global scope and it is known as a global variable. A global
In Python and most programming languages, variables declared outside a function are known as global variables. You can access such variables