Functions in python

Python

Function:- In any programming language we have a concept of function that actual definition of the function is, A function is a block of code, where we write our program in the parts and can be called multiple times.


   






Function Calls
We have already seen one example of a function call:

>>> type(42)
<class 'int'>

The name of the function types. The expression in parentheses is called the argument of
the function. The result, for this function, is the type of the argument.
It is common to say that a function “takes” an argument and “returns” a result. The result
is also called the return value.
Python provides functions that convert values from one type to another. The int function
takes any value and converts it to an integer, if it can, or complains otherwise:

>>> int('32')
32
>>> int('Hello')

ValueError: invalid literal for int(): Hello
int can convert floating-point values to integers, but it doesn’t round off; it chops off the
fraction part:

>>> int(3.99999)
3 >>> int(-2.3)
-2

float converts integers and strings to floating-point numbers:

>>> float(32)
32.0
>>> float('3.14159')
3.14159

Finally, str converts its argument to a string:

>>> str(32)
'32'
>>> str(3.14159)
'3.14159'

Math Functions

Python has a math module that provides most of the familiar mathematical functions. A
module is a file that contains a collection of related functions.
Before we can use the functions in a module, we have to import it with an import
statement:

>>> import math

This statement creates a module object named math. If you display the module object,
you get some information about it:

>>> math
<module 'math' (built-in)>

The module object contains the functions and variables defined in the module. To access
one of the functions, you have to specify the name of the module and the name of the
function, separated by a dot (also known as a period). This format is called dot notation.

>>> ratio = signal_power / noise_power
>>> decibels = 10 * math.log10(ratio)
>>> radians = 0.7
>>> height = math.sin(radians)

The first example uses math.log10 to compute a signal-to-noise ratio in decibels
(assuming that signal_power and noise_power are defined). The math module also
provides a log, which computes logarithms base e.

The second example finds the sine of radians. The name of the variable is a hint that sin
and the other trigonometric functions (cos, tan, etc.) take arguments in radians. To convert
from degrees to radians, divide by 180 and multiply by π:

>>> degrees = 45
>>> radians = degrees / 180.0 * math.pi
>>> math.sin(radians)
0.707106781187

The expression math.pi gets the variable pi from the math module. Its value is a floating-point
approximation of π, accurate to about 15 digits.
If you know trigonometry, you can check the previous result by comparing it to the square
the root of 2 divided by 2:

>>> math.sqrt(2) / 2.0
0.707106781187

Composition

So far, we have looked at the elements of a program — variables, expressions, and
statements — in isolation, without talking about how to combine them.
One of the most useful features of programming languages is their ability to take small
building blocks and compose them. For example, the argument of a function can be any
kind of expression, including arithmetic operators:

x = math.sin(degrees / 360.0 * 2 * math.pi)
And even function calls:
x = math.exp(math.log(x+1))

Almost anywhere you can put a value, you can put an arbitrary expression, with one
exception: the left side of an assignment statement has to be a variable name. Any other
expression on the left side is a syntax error (we will see exceptions to this rule later).

>>> minutes = hours * 60 # right
>>> hours * 60 = minutes # wrong!
SyntaxError: can't assign to operator

Adding New Functions

So far, we have only been using the functions that come with Python, but it is also
possible to add new functions. A function definition specifies the name of a new function
and the sequence of statements that run when the function is called.
Here is an example:

def print_lyrics():
print("I'm a lumberjack, and I'm okay.")
print("I sleep all night and I work all day.")

def is a keyword that indicates that this is a function definition. The name of the function
is print_lyrics. The rules for function names are the same as for variable names: letters,
numbers and underscore are legal, but the first character can’t be a number. You can’t use
a keyword as the name of a function, and you should avoid having a variable and a
function with the same name.

The empty parentheses after the name indicate that this function doesn’t take any
arguments.

The first line of the function definition is called the header; the rest is called the body.
The header has to end with a colon and the body has to be indented. By convention,
indentation is always four spaces. The body can contain any number of statements.
The strings in the print statements are enclosed in double quotes. Single quotes and double
quotes do the same thing; most people use single quotes except in cases like this where a
single quote (which is also an apostrophe) appears in the string.

All quotation marks (single and double) must be “straight quotes”, usually located next to
Enter on the keyboard. “Curly quotes”, like the ones in this sentence, are not legal in
Python.

If you type a function definition in interactive mode, the interpreter prints dots (...) to let
you know that the definition isn’t complete:

>>> def print_lyrics():
... print("I'm a lumberjack, and I'm okay.")
... print("I sleep all night and I work all day.")
...

To end the function, you have to enter an empty line.
Defining a function creates a function object, which has type function:

>>> print(print_lyrics)
<function print_lyrics at 0xb7e99e9c>
>>> type(print_lyrics)
<class 'function'>

The syntax for calling the new function is the same as for built-in functions:

>>> print_lyrics()
I'm a lumberjack, and I'm okay.
I sleep all night and I work all day.

Once you have defined a function, you can use it inside another function. For example, to
repeat the previous refrain, we could write a function called repeat_lyrics:

def repeat_lyrics():
print_lyrics()
print_lyrics()
And then call repeat_lyrics:
>>> repeat_lyrics()

I'm a lumberjack, and I'm okay.
I sleep all night and I work all day.
I'm a lumberjack, and I'm okay.
I sleep all night and I work all day.
But that’s not really how the song goes.
Definitions and Uses
Pulling together the code fragments from the previous section, the whole program looks
like this:

def print_lyrics():
print("I'm a lumberjack, and I'm okay.")
print("I sleep all night and I work all day.")
def repeat_lyrics():
print_lyrics()
print_lyrics()
repeat_lyrics()

This program contains two function definitions: print_lyrics and repeat_lyrics.
Function definitions get executed just like other statements, but the effect is to create
function objects. The statements inside the function do not run until the function is called,
and the function definition generates no output.
As you might expect, you have to create a function before you can run it. In other words,
the function definition has to run before the function gets called.
As an exercise, move the last line of this program to the top, so the function call appears
before the definitions. Run the program and see what error message you get.
Now move the function call back to the bottom and move the definition of print_lyrics
after the definition of repeat_lyrics. What happens when you run this program?
Flow of Execution

To ensure that a function is defined before its first use, you have to know the order
statements run in, which is called the flow of execution.
Execution always begins at the first statement of the program. Statements are run one at a
time, in order from top to bottom.

Function definitions do not alter the flow of execution of the program, but remember that
statements inside the function don’t run until the function is called.
A function call is like a detour in the flow of execution. Instead of going to the next
statement, the flow jumps to the body of the function, runs the statements there, and then
comes back to pick up where it left off.

That sounds simple enough, until you remember that one function can call another. While
in the middle of one function, the program might have to run the statements in another
function. Then, while running that new function, the program might have to run yet
another function!

Fortunately, Python is good at keeping track of where it is, so each time a function
completes, the program picks up where it left off in the function that called it. When it gets
to the end of the program, it terminates.
In summary, when you read a program, you don’t always want to read from top to bottom.
Sometimes it makes more sense if you follow the flow of execution.

Parameters and Arguments

Some of the functions we have seen require arguments. For example, when you call
math.sin you pass a number as an argument. Some functions take more than one
argument: math.pow takes two, the base and the exponent.
Inside the function, the arguments are assigned to variables called parameters. Here is a
definition for a function that takes an argument:

def print_twice(bruce):
print(bruce)
print(bruce)

This function assigns the argument to a parameter named bruce. When the function is
called, it prints the value of the parameter (whatever it is) twice.
This function works with any value that can be printed:

>>> print_twice('Spam')
Spam
Spam
>>> print_twice(42)
42
42
>>> print_twice(math.pi)
3.14159265359
3.14159265359

The same rules of composition that apply to built-in functions also apply to programmer-defined
functions, so we can use any kind of expression as an argument for print_twice:

>>> print_twice('Spam '*4)
Spam Spam Spam Spam
Spam Spam Spam Spam
>>> print_twice(math.cos(math.pi))
-1.0
-1.0

The argument is evaluated before the function is called, so in the examples the expressions
'Spam '*4 and math.cos(math.pi) are only evaluated once.
You can also use a variable as an argument:

>>> michael = 'Eric, the half a bee.'
>>> print_twice(michael)
Eric, the half a bee.
Eric, the half a bee.

The name of the variable we pass as an argument (Michael) has nothing to do with the
name of the parameter (bruce). It doesn’t matter what the value was called back home (in

the caller); here in print_twice, we call everybody bruce.