Difference between revisions of "Python-3.4.3/C4/Advanced-Features-of-Functions/English"

Objectives

• Assign default values to arguments, when defining functions
• Define and call functions with keyword arguments
• Define and call functions with arbitrary arguments
• Learn some of the built-in functions available in Python standard library.

System Specifications

• Ubuntu Linux 16.04 operating system
• Python 3.4.3
• IPython 5.1.0

Pre-requisite slide

• run basic Python commands on the IPython console
• use functions.

If not, see the relevant Python tutorials on this website.

• Function arguments can have default values in Python.
• When we call a function without a value for an argument, its default value is used if available
• Otherwise it will give error.

Open the terminal

Open the terminal.

Type ipython3 and press Enter.

From here onwards, remember to press the Enter key after typing every command on the terminal.

def welcome(greet,name="World"):

print (greet, name)

Highlight welcome, greet, name

Highlight name

Type the code as shown.

Here,

• welcome is the function name
• greet is the argument with no default values and
• the name argument has a default value World.

In a function, all the arguments with default values should come after non-default arguments.

welcome("Hi", "Chandru")

Type,

welcome inside brackets inside double quotes Hi comma inside double quotes Chandru

welcome("Hello")

Highlight hello, world

Type,

welcome inside brackets inside double quotes Hello

We get the output as Hello World.

Here,

• “Hello” is passed to the parameter greet and
• “World” is the default value of the name parameter.

def welcome(greet,age = 23, name="World"):

print (greet, name,age)

Type,

welcome("Hello")

Now type, welcome inside brackets inside double quotes Hello

Here, welcome function still works since we have provided default value for age.

Default values allow us to add new parameters to an existing function.

It will not break the existing usage of the function.

<<PAUSE>>

Try this exercise and then resume the video.

Exercise 1

Place the name argument with its default value of "World" before the greet argument.

def welcome(name="World", greet):

print (greet, name)

Highlight SyntaxError

We get an error that reads SyntaxError: non-default argument follows default argument.

When defining a function all the arguments with default values should come at the end.

Try this exercise and then resume the video.

Exercise 2

Then, call the function without any arguments.

def welcome(greet="Hello", name="World"):

print (greet, name)

welcome()

Then type, welcome open and close brackets

As we can see, we get the output as Hello World.

Default values of both parameters are used since function is called without passing any value.

keyword arguments

• We no need to remember the order of arguments while calling functions by passing keyword arguments.
• Instead, we can use name of the argument to pass a value to it.

def marks(first,second,third):

print("first: %d second: %d and third: %d" %(first,second,third))

Type,

marks(34,23,45)

Type as shown.

Then we will call the marks function without specifying the keywords.

Type, marks inside brackets 34 comma 23 comma 45

We get the output as “first: 34 second: 23 and third: 45”

Here the values 34, 23 and 45 are passed according to the position.

marks(34,45,23)

Type,

marks inside brackets 34 comma 45 comma 23

We can see that the printed values are changed since they are passed according to the position.

marks(34,23,third=45)

Type,

marks inside brackets 34 comma 23 comma third is equal to 45

Here first two values are passed according to the position and the third as keyword argument.

But, the keyword arguments should be specified at the end.

marks(34,second=23,45)

marks inside brackets 34 comma second is equal to 23 comma 45

We get the SyntaxError, positional argument follows keyword argument.

This is because here the keyword argument is not specified at the end.

marks(second=34,first=23,third=45)

Type,

marks inside brackets second is equal to 34 comma first is equal to 23 comma third is equal to 45

Here even though the order of keyword is changed, we get the output as:

“first: 23 second: 34 and third: 45”

def marks(*, first, second, third):

print("first: %d second: %d and third: %d" %(first,second,third))

Type,

marks(second=34,first=23,third=45)

Type as shown.

Then to call the function, type,

marks inside brackets second is equal to 34 comma first is equal to 23 comma third is equal to 45

Note the asterisk symbol at the starting of parameters.

It restricts the function to accept keyword only arguments.

marks(45, 34, 23)

highlight TypeError in output

Type,

marks inside brackets 45 comma 34 comma 23

It gives a TypeError as marks() takes 0 positional arguments but 3 were given.

This way we can enforce usage of keyword only arguments without positional arguments.

• We may not always know in advance the number of arguments that will be passed into a function.
• Use an asterisk(*) before an argument name to denote arbitrary number of arguments.

def family(*names):

print(names)

Type,

family("Duryodhana", "Dushasana")

Type as shown.

Then type,

family inside brackets inside double quotes Duryodhana comma inside double quotes Dushasana

Here, we have called the function with multiple arguments.

These arguments get wrapped up into a tuple while passed into the function.

family("Duryodhana", "Dushasana", "Dushla", "Jalsandha")

As you can see, we can call the function family with more values passed to the names argument.

We have passed 2 values in the first call and 4 values in the second call to the function family.

In both cases these values are assigned to the parameter names.

def person(**attributes):

print(attributes)

Type,

person(name="John",age=34,height=182)

Highlight * and **

Type as shown.

Then to call the function type as shown.

The person function prints a dictionary of keyword arguments passed to it.

Note the double asterisk symbol at the beginning of the parameter names.

It enables to pass zero or more keyword arguments.

Note: Single asterisk (*) symbol is used to accept arbitrary number of positional arguments.

And the double asterisk (**) symbol is used to accept arbitrary number of keyword arguments.

person(name="Lisa",age=27,height=162,weight=58)

Here we have passed 3 keyword arguments in the first case and 4 in the second case.

The function prints a dictionary of keyword arguments with values.

Built-in functions

https://docs.python.org/3/library/functions.html

Some are

• abs()
• any()
• dir()
• help()

You can visit this link to get the full list of built-in functions and their usage.

Summary slide

In this tutorial, we have learned to define functions with,

• Default arguments
• keyword arguments
• Arbitrary arguments

Evaluation

• All the arguments of a function cannot have default values. - True or False?
• The following is a valid function definition. True or False?
• While calling a function, which one is correct in the following.

Solutions

1. False. All the arguments of a Python function can have default values.
2. False. All the parameters with default arguments should be defined at the end.
3. While calling a function, only keyword arguments can be in any order, but should be called at the end.

Fossee Forum

Acknowledgement

For more details, visit this website.