I'm running into an issue where a global variable isn't "remembered" after it's modified in 2 different functions. The variable df is supposed to be a data frame, and it doesn't point to anything until the user loads in the right file. This is similar to something I have [using pandas and tkinter]:
global df
class World:
def __init__[self, master]:
df = None
....
def load[self]:
....
df = pd.read_csv[filepath]
def save[self]:
....
df = df.append[...]
save[]
is always called after load[]. Thing is, when I call save[], I get the error that "df is not defined." I thought df got its initial assignment in init[], and then got "updated" in load[]? What am I doing wrong here?
asked Apr 8, 2018 at 8:15
You have to use global df inside the function that needs to modify the global variable. Otherwise [if writing to it], you are creating a local scoped variable of the same name inside the function and your changes won't be reflected in the global one.
p = "bla"
def func[]:
print["print from func:", p] # works, readonly access, prints global one
def func1[]:
try:
print["print from func:", p] # error, python does not know you mean the global one
p = 22 # because function overrides global with local name
except UnboundLocalError as unb:
print[unb]
def func2[]:
global p
p = "blubb" # modifies the global p
print[p]
func[]
func1[]
print[p]
func2[]
print[p]
Output:
bla # global
print from func: bla # readonly global
local variable 'p' referenced before assignment # same named local var confusion
bla # global
blubb # changed global
answered Apr 8, 2018 at 8:18
Patrick ArtnerPatrick Artner
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You have to use the global keyword inside the function rather than outside. All the df that you have defined inside your function are locally scoped. Here is the right way -
df = pd.DataFrame[] # No need to use global here
def __init__[self, master]:
global df # declare here
df = None
....
def load[self]:
global df # declare here
....
df = pd.read_csv[filepath]
def save[self]:
global df # declare here
....
df = df.append[...]
answered Apr 8, 2018 at 8:18
for anyone coming here using python3 - try using nonlocal instead of global - a new construct introduced in python3 which allows you to mutate and read global variables in local scope
answered Dec 28, 2021 at 4:13
Python also uses variables to hold data. They also have a name and a type; however, in python, you don't have to declare the data type. Instead, you can create a python variable as follows. class_number = 4; In the above example, the variable 'class_number' has the value of 4; it is an integer data type. And unlike other programming languages, you don't need to declare a variable without initializing. Variable scope means the area in which parts of a program can access the variable. There are four variable scopes in python: In this article, you will learn the first two types. You will learn to create python variables with local and global scope. In the programming world, a global variable in Python means having a scope throughout the program, i.e., a global variable value is accessible throughout the program unless shadowed. A global variable in
Python is often declared as the top of the program. In other words, variables that are declared outside of a function are known as global variables. You can access global variables in Python both inside and outside the function. Syntax: X = “sampleGlobalValue” Def fn1[]: To create a global variable in Python, you need to declare the variable outside the function or in a global scope. In the example depicted above, you saw a global variable declared and accessed both inside and outside of the
function. So, you are accessing the value both inside and outside of the function, which is fine, but what happens if you try to modify the global scope variable value inside a function? See the example mentioned below to understand better. As it is evident, this throws an error. When you try to modify the global variable
value inside a function, it will throw UnboundLocalError, because while modifying Python treats x as a local variable, but x is also not defined inside the function [myfunc[]]. That’s where the Global keyword comes into the picture. You will see the usage of Global Keywords in the following sections. A local variable's
scope is a function in which you declared it. To access the variable, you have to call the corresponding function. For example, you can create a local variable as shown below. def superfunc[] #defining a function x = fantastic #defining a local variable print["Python is" + x] #accessing a local variable superfunc[] #calling the function Global keyword is used to modify the global variable outside its current scope and meaning. It is used to make changes in the global variable in a local context. The keyword ‘Global’ is also used to create or declare a global variable inside a function. Usually, when you create a variable inside a function [a local variable], it can only be used within that function. That’s where the
global keyword comes in the play, which helps to create global variables inside the function and which can be accessible in a global scope. Syntax: Def func[]: Global variable Use a global keyword to create a variable in the global scope. Use a global keyword to change the value of the global variable
inside the function. What Does Variable Scope in Python Mean?
What Is the Global Variable In Python?
How to Create Global Variables in Python?
Example:
Output:
How to Access the Global Variable Inside and Outside of the Function?
Example:
Output:
Example:
Output:
How to Create Variables With Local Scope in Python with Examples?
Global Keyword
Example 1:
Output:
Example 2:
Output:
You have seen what ‘global’ keywords are, their examples, and how to use global keywords. But Python has some basic rules to use the ‘global’ keyword.
Let’s see Global in Nested functions.
When you declare a global keyword variable inside the nested function and when you change the global keyword variable inside the nested function, it will reflect outside the local scope, since it is used as a global keyword.
Example:
Let's see an example for global in nested functions.
Output:
You can see the above output for the global in nested functions. But maybe a quick following explanation will help for better understanding.
You have declared the global variable inside the inner[] function, which is nested inside the main[] function.
Before and after calling the inner[], the variable ‘integ’ takes the value of the local variable main i.e. integ = 20. Outside of the main[] function, the variable ‘integ’ takes the value of the global keyword declared inside the inner[] function i.e., integ = 20 as you used the global keyword inside the inner[] function local scope. If you make any changes inside the inner[] function global keyword variable ‘integ’, will reflect outside of the scope, as a behavior of the global keyword.
The fundamental rules of the ‘global’ keyword are as follows:
- When you create a variable inside the function, it is in a local context by default
- When you create or define a variable outside the function, by default it is a global context, there’s no need for a global keyword here
- Global keywords can be used to read or modify the global variable inside the function
- Using a global keyword outside of the function has no use or makes no effect.
How Can You Create Variables Using Global Scope in Python With Examples?
You can create a variable with global scope by initializing outside all the functions in a python program. And you can access the variable from anywhere in the python program.
Creating a global variable is simple; you can do it as follows.
x = "wonderful" | #defining a global variable |
def wonderfunc[]: | #declaring a function |
print["Python is" + x] | #accessing the global variable |
wonderfunc[] | #calling the function |
How to Use Global Keywords in Python With Examples?
If you use a variable inside a function, python thinks you are referring to a local variable. So use the global keyword to change a global variable within a python function.
The following example shows the use of global keywords in a python program.
x = 5 | #initializing a global variable |
def life[] | #defining a function |
global x | #using global keyword |
x = x + 2 | #changing the global variable |
life[] | #calling the function |
print[x] | #accessing the global variable |
Local Variables
The following example shows a mistake.
Example 1: Accessing Local Variable Outside the Scope
def loc[] | #defining loc[] function |
y = "local" | # declaring y locally |
loc[] | # calling the function loc[] |
print[y] | # accessing the variable y |
In the above program, you are trying to access 'y' defined in the function loc[]. And the line print[y] will give you a Name Error: name 'y' is not defined.
The following example shows how to rewrite the above program.
Example 2: Create a Local Variable
def loc[] | #defining the function |
y = "local" | # declaring the local variable |
print[y] | #locally accessing the local variable |
loc[] | #calling a function |
Global and Local Variables
As you can not access a local variable from outside a function, it does not matter if the global and the local variables have the same name. Below you can find an example where there are two variables. One is global, and the other is local. Both have the same name.
Example1: Global Variable and Local Variable With the Same Name
x = 5; | #initializing a global variable |
def man[]: | #defining a function man[] |
x = 4 | #initializing a local variable |
print["local x:", x] | # accessing a local variable |
man[] | #calling the man function |
print["global x:", x] | #accessing a local variable |
In the above example, the print function in the man [] function accesses the local variable x with a value of 4. And the print function outside accesses the local variable with a value of 5.
Difference Between Global and Local Variables
Let us see an example of how global and local variables behave in the same code.
Example:
Output:
Explanation:
Here in the program above, you declared x as a global and y as a local variable in the same program. Then it tried to modify the global variable using the global keyword in the local function and printing both gx and ly.
Once you called function1[], the value of gx became global global. As you tried to modify as gx*2, it printed ‘global’ two times. After this, you printed the local variable ly, which displayed the local variable value i.e., again ‘local’.
Difference Between Global and Nonlocal Variables
When a variable is either in local or global scope, it is called a nonlocal variable. Nonlocal variables are defined in the nested function whose scope is not defined.
Example:
Output:
Explanation:
From the above program, it can be noticed that the nested function is innerfn[]. Inside the innerfn[], you saw the use of a nonlocal keyword to create a nonlocal variable. The innerfn[] is defined in the scope of outerfn[]. If you make changes to the value of a nonlocal variable, they reflect in the local variable.
In conclusion, understanding the scope of python variables is essential for an error-free program. You can access the global variables from anywhere in the program. However, you can only access the local variables from the function. Additionally, if you need to change a global variable from a function, you need to declare that the variable is global. You can do this using the "global" keyword.
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Conclusion
Variables are one of the most basic elements of a programming language. It is an abstraction layer for the memory cells that contain the actual value. Global, Local and Nonlocal types of variables help the programmer to access some values entirely in the program scope, or some values that are limited to within the function.
In this article, you learned what a global variable is in Python, how to define global variables in Python, how to declare a global variable in Python, what is a global keyword, when to use a global keyword, the difference between global, local, and nonlocal variables along with some working examples.
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