Python variable scope in functions
In Python, variables defined inside a function are local variables and are only valid within the function; externally defined are global variables that can be read anywhere. 1. Local variables are destroyed as the function is executed; 2. The function can access global variables but cannot be modified directly, so the global keyword is required; 3. If you want to modify outer function variables in nested functions, the nonlocal keyword is required; 4. Variables with the same name do not affect each other in different scopes; 5. Global must be declared when modifying global variables, otherwise an UnboundLocalError error will be raised. Understanding these rules helps avoid bugs and write more reliable functions.
In Python, variables defined inside a function are local variables by default, while variables defined outside a function are global variables. Understanding the scope of variables is very important for writing good functions and avoiding bugs.
Local vs Global Scope
When you define a variable inside a function, like this:
def my_func():
x = 10 Here x is a local variable and can only be accessed inside my_func() . Once the function execution is finished, this variable will be destroyed.
And if you define a variable outside the function, it is a global variable that can be read anywhere (but cannot be modified directly unless the global keyword is used).
For example:
y = 5
def show_y():
print(y) # can print 5 normally
show_y() This means that the function can access global variables, but if you want to modify it in the function, you have to declare it with global y .
Scope in nested functions: the use of nonlocal
Sometimes you will define another function in the function, and this time it involves nested scope. for example:
def outer():
a = "outer"
def inner():
a = "inner"
print(a)
inner()
print(a)The above code will output:
inner outer
Because a in inner() is its own local variable and will not affect a in outer() .
But if you want to modify the variables of the outer function in inner() , you need to use nonlocal :
def outer():
a = "outer"
def inner():
nonlocal a
a = "modified"
inner()
print(a) # output modified Remember: nonlocal can only be used in nested functions, and it is bound to the outer layer variable closest to it.
Use global to modify global variables
If you want to modify global variables in a function, you must add global , otherwise Python will think you are defining a new local variable.
Look at this example:
count = 0
def increment():
Global count
count = 1
Increment()
print(count) # output 1 If global count is not added, an error will be reported during runtime: UnboundLocalError: local variable 'count' referenced before assignment .
So as long as you want to change the global variable, don't forget to declare it global .
A few common precautions
- Functions can read global variables, but cannot be modified directly unless
global - Local variables are only valid in the function that defines it
- In nested functions,
nonlocalmust be used to modify variables of outer functions. - Variables with the same name do not affect each other in different scopes (for example, there is an
xglobally, and there is also anxin the function, and they are not the same)
Basically that's it. The scope does not seem complicated, but it is easy to make mistakes if you are not careful, especially when multiple layers of nesting or multiple functions operate the same variables.
The above is the detailed content of Python variable scope in functions. For more information, please follow other related articles on the PHP Chinese website!
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