In Python programming, global is a very important keyword, which is used to declare a variable as a global variable inside a function.
However, due to Python's scoping rules and variable lookup mechanisms, the use of global is not always intuitive. Enjoy the New Year
This article will explain the use of global in Python in detail to help readers better grasp this key concept.
global keyword.
In Python, the global keyword is used to declare a variable as a global variable inside a function.
This means that when we modify this global variable inside the function, the change affects the global variable outside the function.
It is important to note that the global keyword can only be used to declare existing global variables, not to create new ones.
Scope of effect. The scope of global is the entire function body, including all the ** blocks and nested functions of the function.
However, it does not affect global variables with the same name in other functions or modules.
Therefore, when using global, we need to be clear about its scope of action to avoid unnecessary misunderstandings and mistakes.
Usage scenarios. Modify global variables: When we need to modify global variables inside a function, we need to use the global keyword.
Otherwise, Python will think that we have created a new local variable inside the function without affecting the original global variable.
The following example shows how to modify a global variable inside a function using the global keyword.
Nested functions: In nested functions, the inner function cannot directly access the global variables of the external function.
At this point, we can use the global keyword in the intrinsic function to declare this global variable, so that it can be accessed and modified.
The following example shows how to use the global keyword in a nested function to access and modify global variables.
Here are the results:
Functions: In the use case of functions, we often need to pass some external variables to functions.
At this point, we can use the global keyword to declare these external variables as global variables, thus avoiding complicated parameter passing and state management.
The following ** shows what happens if you try to modify a global variable without using the global keyword inside the function.
Common misconceptions. Overuse: Some programmers may overrely on the global keyword and declare almost all variables as global.
This practice reduces readability and maintainability, as global variables are visible throughout the program, potentially leading to unexpected and hard-to-track errors.
Therefore, we should try to limit the use of global variables and only use the global keyword when necessary.
Misunderstanding of scope: Some programmers may mistakenly believe that the global keyword can change the scope of a variable.
In fact, the global keyword simply tells the python interpreter that it is a global variable that is accessed inside the function instead of creating a new local variable.
The scope of a variable is still determined by the position in which it is defined.
Ignore initialization: When declaring a global variable with the global keyword, we need to make sure that the global variable has been initialized outside of the function.
Otherwise, python throws a nameerror exception because we are trying to access a global variable that doesn't exist.
Summary. The global keyword plays a very important role in Python programming, allowing us to modify global variables inside a function and access global variables in other functions or modules.
However, due to Python's scoping rules and variable lookup mechanisms, the use of global is not always intuitive.
Therefore, we need to have a deep understanding of its scope, usage scenarios, and common misunderstandings in order to better grasp this key concept and flexibly apply it to practical programming.