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# Python - Functions

A function is a block of organized, reusable code that is used to perform a single, related action. Functions provides better modularity for your application and a high degree of code reusing.

As you already know, Python gives you many built-in functions like print() etc. but you can also create your own functions. These functions are called user-defined functions.

# Defining a Function

You can define functions to provide the required functionality. Here are simple rules to define a function in Python:

- Function blocks begin with the keyword def followed by the function name and parentheses ( ( ) ).

- Any input parameters or arguments should be placed within these parentheses. You can also define parameters inside these parentheses.

- The first statement of a function can be an optional statement - the documentation string of the function or docstring.

- The code block within every function starts with a colon (:) and is indented.

- The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None.

## Syntax:

    def functionname( parameters ):
       "function_docstring"
       function_suite
       return [expression]

By default, parameters have a positional behavior, and you need to inform them in the same order that they were defined.

## Example:

Here is the simplest form of a Python function. This function takes a string as input parameter and prints it on standard screen.

    def printme( str ):
       "This prints a passed string into this function"
       print str
       return

# Calling a Function

Defining a function only gives it a name, specifies the parameters that are to be included in the function, and structures the blocks of code.

Once the basic structure of a function is finalized, you can execute it by calling it from another function or directly from the Python prompt.

Following is the example to call printme() function:

    #!/usr/bin/python

    # Function definition is here
    def printme( str ):
       "This prints a passed string into this function"
       print str;
       return;

    # Now you can call printme function
    printme("I'm first call to user defined function!");
    printme("Again second call to the same function");

This would produce following result:

    I'm first call to user defined function!
    Again second call to the same function

# Pass by reference vs value

All parameters (arguments) in the Python language are passed by reference. It means if you change what a parameter refers to within a function, the change also reflects back in the calling function. For example:

    #!/usr/bin/python

    # Function definition is here
    def changeme( mylist ):
       "This changes a passed list into this function"
       mylist.append([1,2,3,4]);
       print "Values inside the function: ", mylist
       return

    # Now you can call changeme function
    mylist = [10,20,30];
    changeme( mylist );
    print "Values outside the function: ", mylist

Here we are maintaining reference of the passed object and appending values in the same object. So this would produce following result:

    Values inside the function:  [10, 20, 30, [1, 2, 3, 4]]
    Values outside the function:  [10, 20, 30, [1, 2, 3, 4]]

There is one more example where argument is being passed by reference but inside the function, but the reference is being over-written.

    #!/usr/bin/python

    # Function definition is here
    def changeme( mylist ):
       "This changes a passed list into this function"
       mylist = [1,2,3,4]; # This would assig new reference in mylist
       print "Values inside the function: ", mylist
       return

    # Now you can call changeme function
    mylist = [10,20,30];
    changeme( mylist );
    print "Values outside the function: ", mylist

The parameter mylist is local to the function changeme. Changing mylist within the function does not affect mylist. The function accomplishes nothing and finally this would produce following result:

    Values inside the function:  [1, 2, 3, 4]
    Values outside the function:  [10, 20, 30]

# Function Arguments:

You can call a function by using the following types of formal arguments::

- Required arguments
- Keyword arguments
- Default arguments
- Variable-length arguments

## Required arguments:

Required arguments are the arguments passed to a function in correct positional order. Here the number of arguments in the function call should match exactly with the function definition.

To call the function printme() you definitely need to pass one argument otherwise it would give a syntax error as follows:

    #!/usr/bin/python

    # Function definition is here
    def printme( str ):
       "This prints a passed string into this function"
       print str;
       return;

    # Now you can call printme function
    printme();

This would produce following result:

    Traceback (most recent call last):
      File "test.py", line 11, in <module>
        printme();
    TypeError: printme() takes exactly 1 argument (0 given)

## Keyword arguments:

Keyword arguments are related to the function calls. When you use keyword arguments in a function call, the caller identifies the arguments by the parameter name.

This allows you to skip arguments or place them out of order because the Python interpreter is able to use the keywords provided to match the values with parameters.

You can also make keyword calls to the printme() function in the following ways:

    #!/usr/bin/python

    # Function definition is here
    def printme( str ):
       "This prints a passed string into this function"
       print str;
       return;

    # Now you can call printme function
    printme( str = "My string");

This would produce following result:

    My string

Following example gives more clear picture. Note, here order of the parameter does not matter:

    #!/usr/bin/python

    # Function definition is here
    def printinfo( name, age ):
       "This prints a passed info into this function"
       print "Name: ", name;
       print "Age ", age;
       return;

    # Now you can call printinfo function
    printinfo( age=50, name="miki" );

This would produce following result:

    Name:  miki
    Age  50

## Default arguments:

A default argument is an argument that assumes a default value if a value is not provided in the function call for that argument.

Following example gives idea on default arguments, it would print default age if it is not passed:

    #!/usr/bin/python

    # Function definition is here
    def printinfo( name, age = 35 ):
       "This prints a passed info into this function"
       print "Name: ", name;
       print "Age ", age;
       return;

    # Now you can call printinfo function
    printinfo( age=50, name="miki" );
    printinfo( name="miki" );

This would produce following result:

    Name:  miki
    Age  50
    Name:  miki
    Age  35

## Variable-length arguments:

You may need to process a function for more arguments than you specified while defining the function. These arguments are called variable-length arguments and are not named in the function definition, unlike required and default arguments.

The general syntax for a function with non-keyword variable arguments is this:

    def functionname([formal_args,] *var_args_tuple ):
       "function_docstring"
       function_suite
       return [expression]

An asterisk (*) is placed before the variable name that will hold the values of all nonkeyword variable arguments. This tuple remains empty if no additional arguments are specified during the function call. For example:

    #!/usr/bin/python

    # Function definition is here
    def printinfo( arg1, *vartuple ):
       "This prints a variable passed arguments"
       print "Output is: "
       print arg1
       for var in vartuple:
          print var
       return;

    # Now you can call printinfo function
    printinfo( 10 );
    printinfo( 70, 60, 50 );

This would produce following result:

    Output is:
    10
    Output is:
    70
    60
    50

# The Anonymous Functions:

You can use the lambda keyword to create small anonymous functions. These functions are called anonymous because they are not declared in the standard manner by using the def keyword.

- Lambda forms can take any number of arguments but return just one value in the form of an expression. They cannot contain commands or multiple expressions.

- An anonymous function cannot be a direct call to print because lambda requires an expression.

- Lambda functions have their own local namespace and cannot access variables other than those in their parameter list and those in the global namespace.

- Although it appears that lambda's are a one-line version of a function, they are not equivalent to inline statements in C or C++, whose purpose is by passing function stack allocation during invocation for performance reasons.

## Syntax:

The syntax of lambda functions contains only a single statement, which is as follows:

    lambda [arg1 [,arg2,.....argn]]:expression

## Example:

Following is the example to show how lembda form of function works:

    #!/usr/bin/python

    # Function definition is here
    sum = lambda arg1, arg2: arg1 + arg2;

    # Now you can call sum as a function
    print "Value of total : ", sum( 10, 20 )
    print "Value of total : ", sum( 20, 20 )

This would produce following result:

    Value of total :  30
    Value of total :  40

# The return Statement:

The statement return [expression] exits a function, optionally passing back an expression to the caller. A return statement with no arguments is the same as return None

All the above examples are not returning any value, but if you like you can return a value from a function as follows:

    #!/usr/bin/python

    # Function definition is here
    def sum( arg1, arg2 ):
       # Add both the parameters and return them."
       total = arg1 + arg2
       print "Inside the function : ", total
       return total;

    # Now you can call sum function
    total = sum( 10, 20 );
    print "Outside the function : ", total

This would produce following result:

    Inside the function :  30
    Outside the function :  30

# Scope of Variables:

All variables in a program may not be accessible at all locations in that program. This depends on where you have declared a variable.

The scope of a variable determines the portion of the program where you can access a particular identifier. There are two basic scopes of variables in Python:

- Global variables
- Local variables

## Global vs. Local variables:

Variables that are defined inside a function body have a local scope, and those defined outside have a global scope.

This means that local variables can be accessed only inside the function in which they are declared whereas global variables can be accessed throughout the program body by all functions. When you call a function, the variables declared inside it are brought into scope.

## Example:

    #!/usr/bin/python

    total = 0; # This is global variable.
    # Function definition is here
    def sum( arg1, arg2 ):
       # Add both the parameters and return them."
       total = arg1 + arg2; # Here total is local variable.
       print "Inside the function local total : ", total
       return total;

    # Now you can call sum function
    sum( 10, 20 );
    print "Outside the function global total : ", total

This would produce following result:

    Inside the function local total :  30
    Outside the function global total :  0

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