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Python - Scope

Python - Scope
- Scope

Scope

scope:
- A variable is only available from inside the region it is created.
A variable created inside a function belongs to the local scope of that function, and can only be used inside that function.

print("\n--------Scope--------\n")


def xfunc():
    x = 300   # x available only inside the function
    print(x)


xfunc()
# print(x)        # NameError: name 'x' is not defined

Rule: LEGB
- Local
- Enclosing Function local
- Global
- Built-in

Local Scope

Local: Names assigned in any way within a function (def or lambda), and not declared global in that function.

When a variable is called whithin a function, Python will look for it inside the function first.

When the variable is found, it is a local variable.
Local variable is accessable only within the function.

print("\n--------Scope: Local Scope--------\n")

# global variable
x = 200


def xfunc(x):
  # argument creates a local variable.
  # argument copies the value of referenced object.
  x = 300     # assign new value to local variable
  print(x)    # 300, call the local variable


xfunc(x)    # call function and pass an argument.
print(x)    # 200, here x is a global variable.

When the function has any argument, the argument will create a local variable and copy the value of the referenced object.

When the argument a literal value, its value within a function is a literal value.
- The value of global variable will not be affect.

print("\n--------Scope: Literal Value--------\n")

x = 200

def xFunc(x):
    x = 300
    print(x)    # 300

xFunc(x)
print(x)    # 200, the global value will be the same.

When the argument is an object, its value is the referenced address.
- If the function modifies the argument, it will affect the object outside of the function.

print("\n--------Scope: Reference Type--------\n")

x = [1, 2]

def xFunc(x):
    x[0] = 0
    print(x)    # [0, 2]

xFunc(x)
print(x)    # [0, 2], the object outside the function will be affected.

Enclosing Function Local

Enclosing Function local:
- Name in the local scope of any and all enclosing functions (def or lambda), from inner to outer.
Variable in the outside function can be access within the inside function.
When a variable is called whithin a function, Python will look for it inside the function first.
- When the variable is not found within the current function and the current function is a function inside a function, Python will look for the same name within the outside function.

print("\n--------Scope: function inside function--------\n")

def xfunc():
    x = 300     # local variable
    print(x)

    def yfunc():
        print(x)    # call variable outside of yfunc()

    yfunc()    # call the function inside function

xfunc()

Global

Global(module):
- Names assigned at the top-level of a module file, or declared global in a def within the file.
global variable
- A variable created in the main body of the Python code.
- belongs to the global scope.
- are available from within any scope, global and local.
When a variable is called whithin a function, Python will look for it inside the function first.
- If the no variable is found within the current function, Python then looks for the same name in the global variable.
```
print("\n--------Scope: Global--------\n")

x = 200


def xfunc():
    # no x inside the function, Python will call the global variable
    print(x)    # 200


xfunc()
```

`global` Keyword

global:
- to create a global variable in the local scope. The global keyword makes the variable global.
- to change the value of a global variable inside a function.

print("\n--------'global' Keyword--------\n")
x = 300


def xfunc():
    x = 200


def yfunc():
    global x        # declare that x in local scope is a global variable
    x = 200


xfunc()
print(x)            # 300 global variable does not change

yfunc()
print(x)            # 200 global variable does not change

Built-in

Built-in(Python):
- Names preassigned in the built-in names module : open,range,SyntaxError,…

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