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Python Programming – Scope
A scope defines the visibility of a name within a block. If a local variable is defined in a block, its scope includes that block. If the definition occurs in a function block, the scope extends to any blocks contained within the defining one. The scope of names defined in a class block is limited to the class block. If a name is bound in a block, it is a local variable of that block. If a name is bound at the module level, it is a global variable. The variables of the module code block are local and global.
In Python, variables that are only referenced inside a function are implicitly global. If a variable is ever assigned a new value inside the function, the variable is implicitly local, and the programmer need to explicitly declare it as global.
The scope is bit difficult to understand, the following examples might prove fruitful.
def f ( ) : print s s=" I hate spam " f ( )
The variable s is defined as the string “I hate spam”, before the function call f ( ). The only statement in f ( ) is the print statement. As there is no local variable s in f ( ), the value from the global s will be used. So the output will be the string “I hate spam”. The question is, what will happen, if the programmer changes the value of s inside of the function f ( ) ? Will it affect the global s as well? The test is in the following piece of code:
def f ( ) : s="Me too." print s s=" I hate spam." f ( ) print s
The output looks like the following. It can be observed that s in f ( ) is local variable of f ( ).
Me too. I hate spam.
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The following example tries to combine the previous two examples i.e. first access s and then assigning a value tp it in function
f ( ).
def f ( ) : print s s="Me too." print s s=" I hate spam." f ( ) print s
The code will raise an exception- UnboundLocalError: local variable ‘s’ referenced before assignment
Python assumes that a local variable is required due to the assignment to s anywhere inside f ( ), so the first print statement gives the error message. Any variable which is changed or created inside of a function is local, if it has not been declared as a global variable. To tell Python to recognize the variable as global, use the keyword global, as shown in the following example.
def f ( ) : global s print s s=" That's clear." print s s="Python is great ! " f ( ) print s
Now there is no ambiguity. The output is as follows:
Python is great ! That's clear. That's clear.
Local variables of functions cannot be accessed from outside the function code block.
def f ( ) : s=" I am globally not known" print s f ( ) print s
Executing the above code will give following error message- NameError : name ‘s’ is not defined