What are Functions in Python (User Defined Functions)
What are
Functions in Python (User Defined Functions)
We can define functions in three ways.
Simple Function: A function is a block of code which
only runs when it is called.
Function with Argument: You can pass data, known as
parameters, into a function.
Return Type: A function can return data as a result.
Creating a Function
In Python a function is defined using
the def keyword:
Example
def my_fun():
print("Hello
from a function")
Calling a Function
To call a function, use the function name followed by
parenthesis:
Example
def my_fun():
print("This is my Function")
my_fun()
Output:
This is my Function
Arguments
Information can be passed into functions as arguments.
Arguments are specified after the function name, inside the
parentheses. You can add as many arguments as you want, just separate them with
a comma.
The following example has a function with one argument
(fname). When the function is called, we pass along a first name, which is used
inside the function to print the full name:
Example
def my_function(fname):
print(fname
+ " Kumar")
my_function("Manoj")
my_function("Naresh")
my_function("Satish")
Output:
Manoj Kumar
Naresh Kumar
Satish Kumar
Parameters or Arguments?
The terms parameter and argument can be
used for the same thing: information that are passed into a function.
From a function's perspective:
A parameter is the variable listed inside the parentheses in
the function definition.
An argument is the value that is sent to the function when
it is called.
Number of Arguments
By default, a function must be called with the correct
number of arguments. Meaning that if your function expects 2 arguments, you
have to call the function with 2 arguments, not more, and not less.
Example
This function expects 2 arguments, and gets 2 arguments:
def my_function(fname,
lname):
print(fname
+ " " +
lname)
my_function("Manoj", "Kumar")
Output:
Manoj Kumar
Example
This function add of two values:
def my_function(val1,
val2):
print(val1+val2)
x=10
y=20
my_function(x,y)
Output:
30
Example
This function add of two values:
def my_function(val1,
val2):
print(val1+val2)
my_function(10,20)
Output:
30
Example
This function add of two values:
def my_function(val1,
val2=20):
print(val1+val2)
my_function(10)
Output:
30
Note: If you pass two value in function then its ignore the
value of “val2” argument.
Example
This function add of two values:
def my_function(val1,
val2=20):
print(val1+val2)
my_function(10,25)
Output:
35
If you try to call the function with 1 or 3 arguments, you
will get an error:
Example
This function expects 2 arguments, but gets only 1:
def my_function(fname,
lname):
print(fname
+ " " +
lname)
my_function("Manoj")
Output:
Traceback
(most recent call last):
File "demo_function_args_error.py",
line 4, in <module>
my_function("Manoj")
TypeError:
my_function() missing 1 required positional argument: 'lname'
Arbitrary Arguments, *args
If you do not know how many arguments that will be passed
into your function, add a * before the parameter name in the function
definition.
This way the function will receive a tuple of
arguments, and can access the items accordingly:
Example
If the number of arguments is unknown, add
a * before the parameter name:
def my_function(*kids):
print("The youngest child is " +
kids[2])
my_function("Manoj", "Monu", "Sonu")
Output:
The
youngest child is Sonu
Arbitrary Arguments are often shortened to *args in Python documentations.
Keyword Arguments
You can also send arguments with
the key = value syntax.
This way the order of the arguments does not matter.
Example
def my_fun(child3,
child2, child1):
print("The youngest child is " +
child3)
my_fun(child1 = "Manoj",
child2 = "Kumar",
child3 = "Sharma")
Output:
The
youngest child is Sharma
The phrase Keyword Arguments are often shortened to kwargs in Python documentations.
Arbitrary Keyword Arguments, **kwargs
If you do not know how many keyword arguments that will be
passed into your function, add two asterisk: ** before the parameter
name in the function definition.
This way the function will receive a dictionary of
arguments, and can access the items accordingly:
Example
If the number of keyword arguments is unknown, add a
double ** before the parameter name:
def my_function(**kid):
print("His
last name is " +
kid["lname"])
my_function(fname = "Manoj", lname = "Kumar")
Output:
His
last name is Kumar
Arbitrary Kword Arguments are often shortened to **kwargs in Python documentations.
Default Parameter Value
The following example shows how to use a default parameter
value.
If we call the function without argument, it uses the
default value:
Example
def my_function(country = "India"):
print("I am from " + country)
my_function("Nepal")
my_function("USA")
my_function()
my_function("Shri Lanka")
Output:
I
am from Nepal
I
am from USA
I
am from India
I
am from Shri Lanka
Passing a List as an Argument
You can send any data types of argument to a function
(string, number, list, dictionary etc.), and it will be treated as the same
data type inside the function.
E.g. if you send a List as an argument, it will still be a
List when it reaches the function:
Example
def my_function(food):
for x in food:
print(x)
fruits = ["apple", "banana", "orange"]
my_function(fruits)
Output:
Apple
Banana
ornage
Example
Passing a Tuple:
def my_function(food):
for x in food:
print(x)
fruits = ("apple", "banana", "orange")
my_function(fruits)
Output:
Apple
Banana
ornage
Example
Passing a set:
def my_function(food):
for x in food:
print(x)
fruits = {"apple", "banana", "orange"}
my_function(fruits)
Output:
Apple
Banana
ornage
Example
Passing a Dictionary:
def my_function(food):
for x in food:
print(x)
fruits = {"apple":100, "banana":65, "orange":50}
my_function(fruits)
Output:
Apple
Banana
ornage
Example
Passing a Dictionary and result with value
def my_function(food):
for x,y in food.items():
print(x,y)
fruits = {"apple":100, "banana":65, "orange":50}
my_function(fruits)
Output:
Apple 100
Banana 65
Ornage 50
Return Values
To let a function, return a value, use
the return statement:
Example
def my_function(x):
return 5 *
x
print(my_function(3))
print(my_function(5))
print(my_function(9))
Output:
15
25
45
Example
def my_fun(a,b):
return a+b
print(my_fun(3,5))
Output:
8
Example
def my_fun(a,b):
c=a+b
return c
x=my_fun(3,5)
print(x)
Output:
8
The pass Statement
function definitions cannot be empty, but if you for
some reason have a function definition with no content, put in
the pass statement to avoid getting an error.
Example
def myfunction():
pass
# having an empty function definition like this, would raise
an error without the pass statement
Recursion
Python also accepts function recursion, which means a
defined function can call itself.
Recursion is a common mathematical and programming concept.
It means that a function calls itself. This has the benefit of meaning that you
can loop through data to reach a result.
The developer should be very careful with recursion as it
can be quite easy to slip into writing a function which never terminates, or
one that uses excess amounts of memory or processor power. However, when
written correctly recursion can be a very efficient and mathematically-elegant
approach to programming.
In this example, tri_recursion() is a function
that we have defined to call itself ("recurse"). We use
the k variable as the data, which decrements (-1) every time we
recurse. The recursion ends when the condition is not greater than 0 (i.e. when
it is 0).
To a new developer it can take some time to work out how
exactly this works, best way to find out is by testing and modifying it.
Example
Recursion Example
def tri_rec(k):
if(k
> 0):
result = k + tri_rec(k - 1)
print(result)
else:
result = 0
return result
print("\n\nRecursion Example
Results")
tri_rec(6)
Output:
Recursion
Example Results
1
3
6
10
15
21
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