Merge pull request #7659 from lujun9972/add-MjAxODAyMDggUHl0aG9uIEdsb2JhbCwgTG9jYWwgYW5kIE5vbmxvY2FsIHZhcmlhYmxlcyAoV2l0aCBFeGFtcGxlcykubWQK

选题: Python Global, Local and Nonlocal variables (With Examples)

sources/tech/20180208 Python Global, Local and Nonlocal variables (With Examples).md

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+Python Global, Local and Nonlocal variables (With Examples)
+======
+### Global Variables
+
+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.
+
+Let's see an example on how a global variable is created in Python.
+
+#### Example 1: Create a Global Variable
+```
+x = "global"
+
+def foo():
+ print("x inside :", x)
+
+foo()
+print("x outside:", x)
+
+```
+
+When we run the code, the will output be:
+```
+x inside : global
+x outside: global
+
+```
+
+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.
+
+What if you want to change value of x inside a function?
+```
+x = "global"
+
+def foo():
+ x = x * 2
+ print(x)
+foo()
+
+```
+
+When we run the code, the will output be:
+```
+UnboundLocalError: local variable 'x' referenced before assignment
+
+```
+
+The output shows an error because Python treats x as a local variable and x is also not defined inside `foo()`.
+
+To make this work we use `global` keyword, to learn more visit [Python Global Keyword][1].
+
+### Local Variables
+
+A variable declared inside the function's body or in the local scope is known as local variable.
+
+#### Example 2: Accessing local variable outside the scope
+```
+def foo():
+ y = "local"
+
+foo()
+print(y)
+
+```
+
+When we run the code, the will output be:
+```
+NameError: name 'y' is not defined
+
+```
+
+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.
+
+Let's see an example on how a local variable is created in Python.
+
+#### Example 3: Create a Local Variable
+
+Normally, we declare a variable inside the function to create a local variable.
+```
+def foo():
+ y = "local"
+ print(y)
+
+foo()
+
+```
+
+When we run the code, it will output:
+```
+local
+
+```
+
+Let's take a look to the earlier problem where x was a global variable and we wanted to modify x inside `foo()`.
+
+### Global and local variables
+
+Here, we will show how to use global variables and local variables in the same code.
+
+#### Example 4: Using Global and Local variables in same code
+```
+x = "global"
+
+def foo():
+ global x
+ y = "local"
+ x = x * 2
+ print(x)
+ print(y)
+
+foo()
+
+```
+
+When we run the code, the will output be:
+```
+global global
+local
+
+```
+
+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.
+
+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`.
+
+#### Example 5: Global variable and Local variable with same name
+```
+x = 5
+
+def foo():
+ x = 10
+ print("local x:", x)
+
+foo()
+print("global x:", x)
+
+```
+
+When we run the code, the will output be:
+```
+local x: 10
+global x: 5
+
+```
+
+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()`.
+
+When we print the variable inside the `foo()` it outputs `local x: 10`, this is called local scope of variable.
+
+Similarly, when we print the variable outside the `foo()`, it outputs `global x: 5`, this is called global scope of variable.
+
+### Nonlocal Variables
+
+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.
+
+Let's see an example on how a global variable is created in Python.
+
+We use `nonlocal` keyword to create nonlocal variable.
+
+#### Example 6: Create a nonlocal variable
+```
+def outer():
+ x = "local"
+
+ def inner():
+   nonlocal x
+   x = "nonlocal"
+   print("inner:", x)
+
+ inner()
+ print("outer:", x)
+
+outer()
+
+```
+
+When we run the code, the will output be:
+```
+inner: nonlocal
+outer: nonlocal
+
+```
+
+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()`.
+
+Note : If we change value of nonlocal variable, the changes appears in the local variable.
+
+--------------------------------------------------------------------------------
+
+via: https://www.programiz.com/python-programming/global-local-nonlocal-variables
+
+作者：[programiz][a]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]:https://www.programiz.com/
+[1]:https://www.programiz.com/python-programming/global-keyword