I'm kinda' new to python, but I have already written many programs including some like download-managers, games and text-editors which require a lot of string manipulation.
For representing a string literal I use either single or double inverted commas.. whichever comes to my mind first at that time.
Although I haven't yet faced any trouble. My question: is there any purpose for which python allows both or is it just for compatibility, usability etc?
Just for compatibility and usability.
It is sometimes useful when you have one of them embedded in the string, so I would use "Who's there?" compared to 'He says: "Hello"'.
The other option is of course triple quoted strings, like
"""This is a long string that can contain single quotations like ' or ".
It can also span multiple lines"""
which is equal to
'''This is a long string that can contain single quotations like ' or ".
It can also span multiple lines'''
This SO answer will give you some tips on when to use which.
There is no difference between "string" and 'string' in Python, so, as you suggest, it's just for usability.
>>> 'string' == "string"
True
You can also use triple quotes for multiline strings:
>>> mystring = """Hello
... World!"""
>>> mystring
'Hello\nWorld!'
Another trick is that adjacent string literals are automatically concatenated:
>>> mystring = "Hello" 'World!'
>>> mystring
'HelloWorld!'
There are also string prefixes which you can read about in the documentation.
It means you can easily have a string with either single or double quotes in it without needing any escape characters. So you can do:
a = 'The knights who say "ni!"'
b = "We're knights of the Round Table, we dance whene'er we're able."