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)) 

Sounds like a locale thing^(*). This prints 12,3456,7890 (Try it online!):

import locale

n = 1234567890

locale._override_localeconv["thousands_sep"] = ","
locale._override_localeconv["grouping"] = [4, 0]
print(locale.format_string('%d', n, grouping=True))

That's an I guess hackish way based on this answer. The other answer there talks about using babel, maybe that's a clean way to achieve it.

^(*) Quick googling found this talking about Chinese grouping four digits, and OP's name seems somewhat Chinese, so...

I'm surprised that no one has mentioned that you can do this with f-strings in Python 3.6+ as easy as this:

>>> num = 10000000
>>> print(f"{num:,}")
10,000,000

... where the part after the colon is the format specifier. The comma is the separator character you want, so f"{num:_}" uses underscores instead of a comma. Only "," and "_" is possible to use with this method.

This is equivalent of using format(num, ",") for older versions of python 3.

This might look like magic when you see it the first time, but it's not. It's just part of the language, and something that's commonly needed enough to have a shortcut available. To read more about it, have a look at the group subcomponent.

To be clear, a itself is not being changed into a string. There is a temporary, anonymous string object created by '{:,d}'.format(a) and then fed to "%s" %:

>>> a = 1234567
>>> "The number is %s but it's converted to a string" %'{:,d}'.format(a)
"The number is 1,234,567 but it's converted to a string"
>>> type(a)
<type 'int'>   
>>> a
1234567

So a itself has not changed at all similarly as your second example does not change the object b:

>>> b = 0.1234
>>> "The Norwegian Blue prefers kippin' on it's back! %0.2f%%" % (b*100)
"The Norwegian Blue prefers kippin' on it's back! 12.34%"
>>> type(b)
<type 'float'>
>>> b
0.1234

So the underlying values of a and b are not changing. The issue is that you are using a less than clear syntax to print a. Just do:

>>> "The number is {:,d} but it's NOT converted to a string".format(a)
"The number is 1,234,567 but it's NOT converted to a string"

There is no need to do two formatting steps as you have in your example.