In Python 2.7 and 3.x, you can use the format syntax :,

>>> total_amount = 10000
>>> print("{:,}".format(total_amount))
10,000

>>> print("Total cost is: ${:,.2f}".format(total_amount))
Total cost is: $10,000.00

This is documented in PEP 378 -- Format Specifier for Thousands Separator and has an example in the Official Docs "Using the comma as a thousands separator"

'{:20,.2f}'.format(TotalAmount)

You could use locale.currency if TotalAmount represents money. It works on Python <2.7 too:

>>> locale.setlocale(locale.LC_ALL, '')
'en_US.utf8'
>>> locale.currency(123456.789, symbol=False, grouping=True)
'123,456.79'

Note: it doesn't work with the C locale so you should set some other locale before calling it.

This is not particularly elegant but should work too :

a = "1000000.00"
e = list(a.split(".")[0])
for i in range(len(e))[::-3][1:]:
e.insert(i+1,",")
result = "".join(e)+"."+a.split(".")[1]

if you are using Python 3 or above, here is an easier way to insert a comma:

First way

value = -12345672
print (format (value, ',d'))

or another way

value = -12345672
print ('{:,}'.format(value))

A function that works in python2.7+ or python3.1+

def comma(num):
'''Add comma to every 3rd digit. Takes int or float and
returns string.'''
if type(num) == int:
return '{:,}'.format(num)
elif type(num) == float:
return '{:,.2f}'.format(num) # Rounds to 2 decimal places
else:
print("Need int or float as input to function comma()!")

The above answers are so much nicer than the code I was using in my (not-homework) project:

def commaize(number):
text = str(number)
parts = text.split(".")
ret = ""
if len(parts) > 1:
ret = "."
ret += parts[1] # Apparently commas aren't used to the right of the decimal point
# The -1 offsets to len() and 0 are because len() is 1 based but text[] is 0 based
for i in range(len(parts[0]) - 1,-1,-1):
# We can't just check (i % 3) because we're counting from right to left
#  and i is counting from left to right. We can overcome this by checking
#  len() - i, although it needs to be adjusted for the off-by-one with a -1
# We also make sure we aren't at the far-right (len() - 1) so we don't end
#  with a comma
if (len(parts[0]) - i - 1) % 3 == 0 and i != len(parts[0]) - 1:
ret = "," + ret
ret = parts[0][i] + ret
return ret

Started learning Python about 5 hours ago, but I think I came up with something for integers (sorry, couldn't figure out floats). I'm in high school, so big chance the code could be way more efficient; I just made something from scratch that made sense to me. If anyone has any ideas on how to improve with ample explanation of how it works, let me know!

# Inserts comma separators
def place_value(num):
perm_num = num  # Stores "num" to ensure it cannot be modified
lis_num = list(str(num))  # Makes "num" into a list of single-character strings since lists are easier to manipulate
if len(str(perm_num)) > 3:
index_offset = 0  # Every time a comma is added, the numbers are all shifted over one
for index in range(len(str(perm_num))):  # Converts "perm_num" to string so len() can count the length, then uses that for a range
mod_index = (index + 1) % 3  # Locates every 3 index
neg_index = -1 * (index + 1 + index_offset)  # Calculates the index that the comma will be inserted at
if mod_index == 0:  # If "index" is evenly divisible by 3
lis_num.insert(neg_index, ",")  # Adds comma place of negative index
index_offset += 1  # Every time a comma is added, the index of all items in list are increased by 1 from the back
str_num = "".join(lis_num)  # Joins list back together into string
else:  # If the number is less than or equal to 3 digits long, don't separate with commas
str_num = str(num)
return str_num

I feel comfortable using like this in python:

input_value=float(input())
print("{:,}".format(input_value))

Latest versions of python use f-strings. So you can do this:

print("Total cost: {total_amount:,}

As long as total_amount is a not a string. Otherwise you'd need to cast it to a number type first like so:

print("Total cost: {Decimal(total_amount):,}

another way very short is

value = -122212123.12
print(f"{value:,}")