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翻译完成-20180208 Python Global, Local and Nonlocal variables (With Examples)
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Python Global, Local and Nonlocal variables (With Examples)
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======
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### Global Variables
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In Python, a variable declared outside of the function or in global scope is known as global variable. This means, global variable can be accessed inside or outside of the function.
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Let's see an example on how a global variable is created in Python.
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#### Example 1: Create a Global Variable
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```
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x = "global"
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def foo():
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print("x inside :", x)
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foo()
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print("x outside:", x)
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```
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When we run the code, the will output be:
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```
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x inside : global
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x outside: global
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```
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In above code, we created x as a global variable and defined a `foo()` to print the global variable x. Finally, we call the `foo()` which will print the value of x.
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What if you want to change value of x inside a function?
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```
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x = "global"
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def foo():
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x = x * 2
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print(x)
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foo()
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```
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When we run the code, the will output be:
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```
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UnboundLocalError: local variable 'x' referenced before assignment
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```
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The output shows an error because Python treats x as a local variable and x is also not defined inside `foo()`.
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To make this work we use `global` keyword, to learn more visit [Python Global Keyword][1].
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### Local Variables
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A variable declared inside the function's body or in the local scope is known as local variable.
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#### Example 2: Accessing local variable outside the scope
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```
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def foo():
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y = "local"
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foo()
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print(y)
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```
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When we run the code, the will output be:
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```
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NameError: name 'y' is not defined
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```
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The output shows an error, because we are trying to access a local variable y in a global scope whereas the local variable only works inside `foo() `or local scope.
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Let's see an example on how a local variable is created in Python.
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#### Example 3: Create a Local Variable
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Normally, we declare a variable inside the function to create a local variable.
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```
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def foo():
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y = "local"
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print(y)
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foo()
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```
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When we run the code, it will output:
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```
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local
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```
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Let's take a look to the earlier problem where x was a global variable and we wanted to modify x inside `foo()`.
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### Global and local variables
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Here, we will show how to use global variables and local variables in the same code.
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#### Example 4: Using Global and Local variables in same code
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```
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x = "global"
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def foo():
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global x
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y = "local"
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x = x * 2
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print(x)
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print(y)
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foo()
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```
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When we run the code, the will output be:
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```
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global global
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local
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```
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In the above code, we declare x as a global and y as a local variable in the `foo()`. Then, we use multiplication operator `*` to modify the global variable x and we print both x and y.
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After calling the `foo()`, the value of x becomes `global global` because we used the `x * 2` to print two times `global`. After that, we print the value of local variable y i.e `local`.
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#### Example 5: Global variable and Local variable with same name
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```
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x = 5
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def foo():
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x = 10
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print("local x:", x)
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foo()
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print("global x:", x)
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```
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When we run the code, the will output be:
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```
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local x: 10
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global x: 5
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```
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In above code, we used same name x for both global variable and local variable. We get different result when we print same variable because the variable is declared in both scopes, i.e. the local scope inside `foo()` and global scope outside `foo()`.
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When we print the variable inside the `foo()` it outputs `local x: 10`, this is called local scope of variable.
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Similarly, when we print the variable outside the `foo()`, it outputs `global x: 5`, this is called global scope of variable.
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### Nonlocal Variables
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Nonlocal variable are used in nested function whose local scope is not defined. This means, the variable can be neither in the local nor the global scope.
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Let's see an example on how a global variable is created in Python.
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We use `nonlocal` keyword to create nonlocal variable.
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#### Example 6: Create a nonlocal variable
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```
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def outer():
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x = "local"
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def inner():
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nonlocal x
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x = "nonlocal"
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print("inner:", x)
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inner()
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print("outer:", x)
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outer()
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```
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When we run the code, the will output be:
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```
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inner: nonlocal
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outer: nonlocal
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```
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In the above code there is a nested function `inner()`. We use `nonlocal` keyword to create nonlocal variable. The `inner()` function is defined in the scope of another function `outer()`.
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Note : If we change value of nonlocal variable, the changes appears in the local variable.
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--------------------------------------------------------------------------------
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via: https://www.programiz.com/python-programming/global-local-nonlocal-variables
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作者：[programiz][a]
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译者：[译者ID](https://github.com/译者ID)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://www.programiz.com/
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[1]:https://www.programiz.com/python-programming/global-keyword
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@ -0,0 +1,187 @@
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Python全局，局部和非局部变量（带示例）
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======
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### 全局变量
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在Python中，在函数之外或在全局范围内声明的变量被称为全局变量。 这意味着，全局变量可以在函数内部或外部访问。
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我们来看一个关于如何在Python中创建一个全局变量的示例。
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#### 示例1：创建全局变量
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```python
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x = "global"
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def foo():
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print("x inside :", x)
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foo()
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print("x outside:", x)
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```
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当我们运行代码时，将会输出：
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```
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x inside : global
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x outside: global
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```
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在上面的代码中，我们创建了x作为全局变量，并定义了一个`foo()`来打印全局变量x。 最后，我们调用`foo()`来打印x的值。
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倘若你想改变一个函数内的x的值该怎么办？
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```python
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x = "global"
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def foo():
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x = x * 2
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print(x)
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foo()
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```
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当我们运行代码时，将会输出：
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```
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UnboundLocalError: local variable 'x' referenced before assignment
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```
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输出显示一个错误，因为Python将x视为局部变量，x也没有`foo()`内部定义。
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为了运行正常，我们使用`global`关键字，访问[PythonGlobal关键字][1]以便了解更多。
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### 局部变量
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在函数体内或局部作用域内声明的变量称为局部变量。
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#### 示例2：访问作用域外的局部变量
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```python
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def foo():
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y = "local"
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foo()
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print(y)
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```
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当我们运行代码时，将会输出：
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```
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NameError: name 'y' is not defined
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```
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输出显示了一个错误，因为我们试图在全局范围内访问局部变量y，而局部变量只能在`foo() `函数内部或局部作用域内有效。
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我们来看一个关于如何在Python中创建一个局部变量的例子。
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#### 示例3：创建一个局部变量
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通常，我们在函数内声明一个变量来创建一个局部变量。
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```python
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def foo():
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y = "local"
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print(y)
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foo()
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```
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当我们运行代码时，将会输出：
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```
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local
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```
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让我们来看看前面的问题，其中x是一个全局变量，我们想修改`foo()`内部的x。
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### 全局变量和局部变量
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在这里，我们将展示如何在同一份代码中使用全局变量和局部变量。
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#### 示例4：在同一份代码中使用全局变量和局部变量
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```python
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x = "global"
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def foo():
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global x
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y = "local"
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x = x * 2
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print(x)
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print(y)
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foo()
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```
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当我们运行代码时，将会输出（译者注：原文中输出结果的两个global有空格，正确的是没有空格）：
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```
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globalglobal
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local
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```
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在上面的代码中，我们将x声明为全局变量，将y声明为`foo()`中的局部变量。 然后，我们使用乘法运算符`*`来修改全局变量x，并打印x和y。
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在调用`foo()`之后，x的值变成`globalglobal`了（译者注：原文同样有空格，正确的是没有空格），因为我们使用`x * 2`打印两次`global`。 之后，我们打印局部变量y的值，即`local`。
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#### 示例5：具有相同名称的全局变量和局部变量
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```python
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x = 5
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def foo():
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x = 10
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print("local x:", x)
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foo()
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print("global x:", x)
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```
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当我们运行代码时，将会输出：
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```
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local x: 10
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global x: 5
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```
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在上面的代码中，我们对全局变量和局部变量使用了相同的名称x。 当我们打印相同的变量时却得到了不同的结果，因为这两个作用域内都声明了变量，即`foo()`内部的局部作用域和`foo()`外面的全局作用域。
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当我们在`foo()`内部打印变量时，它输出`local x: 10`，这被称为变量的局部作用域。
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同样，当我们在`foo()`外部打印变量时，它输出`global x: 5`，这被称为变量的全局作用域。
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### 非局部变量
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非局部变量用于局部作用域未定义的嵌套函数。 这意味着，变量既不能在局部也不能在全局范围内。
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我们来看一个关于如何在Python中创建一个非局部变量的例子。（译者注：原文为创建全局变量，疑为笔误）
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我们使用`nonlocal`关键字来创建非局部变量。
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#### 例6：创建一个非局部变量
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```python
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def outer():
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x = "local"
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def inner():
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nonlocal x
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x = "nonlocal"
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print("inner:", x)
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inner()
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print("outer:", x)
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outer()
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```
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当我们运行代码时，将会输出：
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```
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inner: nonlocal
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outer: nonlocal
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```
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在上面的代码中有一个嵌套函数`inner()`。 我们使用`nonlocal`关键字来创建非局部变量。`inner()`函数是在另一个函数`outer()`的作用域中定义的。
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注意：如果我们改变非局部变量的值，那么变化就会出现在局部变量中。
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--------------------------------------------------------------------------------
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via: https://www.programiz.com/python-programming/global-local-nonlocal-variables
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作者：[programiz][a]
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译者：[Flowsnow](https://github.com/Flowsnow)
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校对：[校对者ID](https://github.com/校对者ID)
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本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
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[a]:https://www.programiz.com/
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[1]:https://www.programiz.com/python-programming/global-keyword
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