Python 3 – Functions, Iterators, List Comprehensions and Generators

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Python

Introduction

Here we will discuss about:

  • Functions
  • List Comprehensions
  • Iterators and Generators

Functions

  • A function is a piece of code, capable of performing a similar task repeatedly.
  • It is defined using def keyword in python.

Syntax of a function :

def <function_name>(<parameter1>, <parameter2>, ...):
     'Function documentation'
     function_body
     return <value>
  • Parameters, return expression and documentation string are optional.

Sample function – square

def square(x):
    'Returns square of a number.'
    return x**2
Common Built-in Functions
There are many built-in functions Python offers. Here are some of them listed.

  • len : Returns the length of an object.
    • e.g : len('hello') -> 5
  • type : Returns the type of an object.
    • e.g : type([2,3]) -> <type list>
  • range : Returns a iterator of a number sequence.
    • e.g : list(range(10, 13)) -> [10, 11, 12]
    • list(range(3)) -> [0, 1, 2]

Iterators

  • An Iterator is an object, which allows a programmer to traverse through all the elements of a collection, regardless of its specific implementation.
  • Values of an Iterator can be accessed only once and in sequential order.

Sample Iterator

x = [6, 3, 1]
s = iter(x)
print(next(s))      # -&gt; 6
print(next(s))      # -&gt; 3
print(next(s))      # -&gt; 1
print(next(s))      # -&gt; StopIteration Error

List Comprehensions

  • Alternative to for loops.
  • More concise, readable, efficient and mimic functional programming style.
  • Used to:
    • Apply a method to all or specific elements of a list, and
    • Filter elements of a list satisfying specific criteria.

Example

x = [6, 3, 1]
y = [ i**2 for i in x ]   # List Comprehension expression
print(y)                  # -&gt; [36, 9, 1]

Generators

  • A Generator object is an iterator, whose values are created at the time of accessing them.
  • A generator can be obtained either from a generator expression or a generator function.

Example

x = [6, 3, 1]
g = (i**2 for i in x)  # generator expression
print(next(g))         # -&gt; 36

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