iter() to get iterator from an iterable object in Python

We get an iterator ( output ) from an iterable object ( input ) by using iter function.

iter(iterable_obj,sentinel)

iterable_obj: Required, Object which can be iterated
sentinel: Optional, Value specifying the end of sequence

Example of iterable objects are : List, tuple, dictionary , string

my_str='plus2net'      # a string variable is declared 
my_str=iter(my_str)    # returns an iterator

print(next(my_str))    # p
print(next(my_str))    # l
print(next(my_str))    # u

next() returns the the next item from the iterator Example using one List

my_list=[4,3,5,1]       # declaring a List
my_list1=iter(my_list)  # returns iterator 

print(next(my_list1))   # 4
print(next(my_list1))   # 3
print(next(my_list1))   # 5
print(next(my_list1))   # 1

Type of Iterator

We can check the type of variable we get before and after using iter() function

my_str='plus2net'     # a string variable is declared
print(type(my_str))   # <class 'str'>  
my_str=iter(my_str)   # returns iterator
print(type(my_str))   # <class 'str_iterator'> 

my_list=[4,3,5,1,2]   # declaring a List
print(type(my_list))  # <class 'list'> 
my_list=iter(my_list) # returns iterator
print(type(my_list))  # <class 'list_iterator'>

Exception handling with iter()

What happens if we try to use iter() with a not iterable object as input? We will get TypeError

my_str=4356   # integer variable which is not iterable
my_str=iter(my_str)

Output is here

     my_str=iter(my_str)
TypeError: 'int' object is not iterable

We can use Try Except to handle such error

my_str=4356  # integer variable which is not iterable
try:
    my_str=iter(my_str)
except TypeError as my_msg:
    print ("This is a TypeError")
    print (my_msg)
except:
    print ("Some other error has occured")

Output is here

This is a TypeError
'int' object is not iterable

What happens when there is no item left to return with next() ? We will get StopIteration error

my_str='abc'  
my_str=iter(my_str)
try:
    print(next(my_str))   # a
    print(next(my_str))   # b
    print(next(my_str))   # c
    print(next(my_str))   # Error
except StopIteration:
    print ("This is a StopIteration error")
    
except:
    print ("Some other error has occured")

Output is here

a
b
c
This is a StopIteration error

We can use default value with next() to return when no value is left. In this case there will be no error as default vlaue is returned. In this example we have kept 'x' as default value to return if no value is left. After printing 'c' there is no value to return so 'x' is returned always.

my_str='abc'  
my_str=iter(my_str)
print(next(my_str))       # a
print(next(my_str))       # b
print(next(my_str, 'x'))  # c
print(next(my_str, 'x'))  # x
print(next(my_str, 'x'))  # x

We can create Iterable object having methods __iter__() and __next__().

class my_iter():

    def __iter__(self):
        self.x=1
        return self

    def __next__(self):
        x=self.x
        self.x=self.x+1
        return x

my_obj=my_iter()
my_iter=iter(my_obj)

print(next(my_iter))  # 1
print(next(my_iter))  # 2
print(next(my_iter))  # 3

We can create StopIteration exception by using raise

class my_iter():

    def __iter__(self):
        self.x=1
        return self

    def __next__(self):
        x=self.x
        self.x=self.x+1
        if(x<5):
         return x
        else:
         raise StopIteration    

my_obj=my_iter()
my_iter=iter(my_obj)

print(next(my_iter))  # 1
print(next(my_iter))  # 2
print(next(my_iter))  # 3
print(next(my_iter))  # 4
print(next(my_iter))  # Error : StopIteration

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iterator using iter()

Type of Iterator

Exception handling with iter()

Subhendu Mohapatra