Convert an amount to Indian Notation
1. Style guides

leave two empty lines before each function (when you're not inside of a class)
Python is not C; don't use ;
put a space before and after operands. i&gt;=0 will become i &gt;= 0, s[i]=='.' will become s[i] == '.' and so on.
use the snake_case notation for function names: currencyInIndiaFormat(..) will become currency_in_indian_format(..)
use format() instead of + notation: e.g: print &quot;INR &quot; + currencyInIndiaFormat(n) might be print &quot;INR {}&quot;.format(currencyInIndiaFormat(n))
also, I'd recommend you use print('w/e you want here') so that your code can be python 3 compatible.
use 4 spaces for indentation instead of 2.
use augmented assignments where you can: res = res + s[i] can be written as res += s[i]
add docstrings to your functions to describe what their purpose are

If you want to read more about Python style guide, you should start from here.
Preview:
import decimal


def currency_in_indian_format(n):
    &quot;&quot;&quot; Convert a number (int / float) into indian formatting style &quot;&quot;&quot;
    d = decimal.Decimal(str(n))

    if d.as_tuple().exponent &lt; -2:
        s = str(n)
    else:
        s = '{0:.2f}'.format(n)

    l = len(s)
    i = l - 1

    res, flag, k = '', 0, 0
    while i &gt;= 0:
        if flag == 0:
            res += s[i]
            if s[i] == '.':
                flag = 1
        elif flag == 1:
            k += 1
            res += s[i]
            if k == 3 and i - 1 &gt;= 0:
                res += ','
                flag = 2
                k = 0
        else:
            k += 1
            res += s[i]
            if k == 2 and i - 1 &gt;= 0:
                res += ','
                flag = 2
                k = 0
        i -= 1

    return res[::-1]


def main():
    n = 100.52
    print(&quot;INR {}&quot;.format(currency_in_indian_format(n)))  # INR 100.52
    n = 1000.108
    print(&quot;INR {}&quot;.format(currency_in_indian_format(n)))  # INR 1,000.108
    n = 1200000
    print(&quot;INR {}&quot;.format(currency_in_indian_format(n)))  # INR 12,00,000.00


if __name__ == '__main__':
    main()

NOTE: I've also added if __name__ == '__main__'. By doing the main check, you can have that code only execute when you want to run the module as a program and not have it execute when someone just wants to import your module and call your functions themselves.
2. Algorithm
As I said in the comments, I would strongly recommend using the existing builtin module locale, which is doing exactly what you want:
Example code (it's working if you have the locale set on your machine - so that would be an ugly constraint):
&gt;&gt;&gt; import locale
&gt;&gt;&gt; locale.setlocale(locale.LC_ALL, 'en_IN.utf8')  # if this doesn't work, you might try as well: locale.setlocale(locale.LC_ALL, '')
'en_IN'
&gt;&gt;&gt; locale.format(&quot;%d&quot;, 1255000, grouping=True)
'12,55,000'

I'll let others to comment on your code regarding the efficiency of the algorithm you implemented and methods to make it shorter / better.